Signalling and channel loop test circuits for station carrier telephone system

A plural channel amplitude modulated station carrier telephone system and method wherein two a.c. signals are applied to modulate a called subscriber's channel carrier signal at a central office to signal an incoming call for the subscriber, wherein a phase locked loop is used at the subscriber terminal equipment to lock with one of the two a.c. signals following recovery of the two a.c. signals from the called subscriber's carrier signal, wherein the output of the phase locked loop is utilized to provide for the synchronous detection of the other of the two a.c. signals, and wherein a ring circuit responds to the synchronous detection of the other of the two a.c. signals to ring the called subscriber's telephone. Central office ringing signal frequency information is contained in the above-mentioned a.c. signal to develop a local ringing signal voltage having the same frequency as the central office ringing frequency for ringing the subscriber's telephone. Other features of this invention include a ring circuit with either a special transistor switching or relay switching arrangement for operating the ringer in the subscriber's telephone. Also disclosed is a synchronous detection circuit employing a special phase locked loop for synchronously detecting an a.c. signal.

FIELD OF INVENTION 
This invention relates to plural channel amplitude modulated station 
carrier telephone systems (also called subscriber carrier systems) and is 
particularly concerned with signalling circuits and channel loop testing 
circuits for such systems. 
BACKGROUND 
As is well known, a plural channel station carrier system provides for the 
simultaneous transmission of several conversations over the same 
transmission line between a central office and a plurality of remotely 
located subscriber telephone stations. Typically, the carrier equipment 
includes a central office transmitting and receiving channel terminal unit 
or circuit and a subscriber transmitting and receiving channel terminal 
unit or circuit for each of the several subscribers served by the carrier 
system. The central office channel terminal circuits are located at the 
telephone company's central office, and the subscriber channel terminal 
circuits are located remotely from the central office and are connected to 
the central office terminal circuits by the common transmission line. The 
subscriber channel terminal circuits form a part of what may collectively 
be referred to as the subscriber terminal equipment at the end of the 
transmission line remote from the central office. 
To signal incoming calls for the subscribers served by the carrier system 
it is a common cost-saving practice to use just one ringing signal 
generator in the subscriber terminal equipment to serve all of the 
subscribers rather than using a separate local ringing generator for each 
subscriber. 
When a call comes into the central office for one of the subscribers served 
by the carrier system, the central office-transmitted carrier signal 
assigned to the called subscriber will be modulated by an a.c. alerting 
signal. The local ringing signal generator at the subscribers' end of the 
transmission line responds to this modulation by applying an a.c. ringing 
signal voltage of relative low, fixed frequency (typically 20 Hz) to the 
called subscriber's telephone to operate the called subscriber's ringer. 
The frequency of the carrier-modulating alerting signal mentioned above is 
typically selected to be much higher than the low ringing signal frequency 
that is customarily used for operating the telephone ringers. For example 
the frequency of the carrier-modulating alerting signal may be 750 Hz. The 
higher modulating signal frequency provides greater noise immunity and 
requires less expensive filters for separating the modulating signal from 
the other components of carrier signal detection in the carrier signal 
receiver equipment at the subscribers' end of the transmission line. 
The foregoing type of station carrier system is sometimes installed in 
place of a party line (also referred to as a multiparty line) to furnish 
the party line subscribers with individual or single party service. The 
practice of equipping the carrier system with a single ringing generator 
to supply the same ringing signal for operating the ringers of all of the 
subscribers, however, leads to a problem where the party line to be 
converted uses a bridged frequency ringing system. 
Bridged frequency ringing is a frequency selective ringing or signalling 
system wherein a different central office ringing signal frequency is 
assigned to each subscriber on the multiparty line and wherein the 
telephone ringer for each subscriber is tuned (usually capacitively) to 
the assigned ringing signal frequency so that it responds only to the 
assigned ringing signal frequency. In converting this type of system to a 
carrier operation, it has been common practice to use the same ringing 
generator for all of the subscribers and to replace the tuned ringers with 
ones capable of operation by the same ringing frequency. Replacement of 
the tuned ringers is nevertheless an added cost and requires a service 
man's visit to the home of each subscriber. 
Another extra cost dealing with signalling arises where it is desired to 
use one of the carrier system's channels as a party line with bridged 
frequency ringing. To provide such a service one prior practice has 
involved the use of separate ringing signal generators which are located 
at the central office for supplying the different low ringing frequencies 
(e.g., 20 Hz, 30 Hz, 40 Hz) to which the subscribers' party line telephone 
ringers are tuned. In response to an incoming call for one of the party 
line subscribers the ringing signal frequency assigned to the called 
subscriber is applied to amplitude modulate a higher frequency tone 
(typically 900 Hz), and the modulated tone is then applied to amplitude 
modulate the carrier that is assigned to the party line channel. 
At the subscriber terminal equipment this modulated carrier is received and 
demodulated to recover the modulated 900 Hz tone. The 900 Hz tone is then 
demodulated to recover the modulating ringing frequency information for 
operating the called subscriber's tuned ringer. Such a signalling system 
is shown in FIGS. 5 and 6 of U.S. Pat. No. 3,904,833 which issued on Sept. 
9, 1975. 
The cost of the signalling system shown in U.S. Pat. No. 3,904,833 is 
increased by the additional circuits needed for first modulating the 900 
Hz tone at the central office terminal and then demodulating and filtering 
the modulated 900 Hz tone at the subscriber terminal to recover the 
desired ringing frequency information. 
Instead of modulating the 900 Hz tone with the low frequency central office 
ringing signal (e.g., 20 Hz), some carrier systems are equipped to 
individually modulate the carrier signal with the low frequency central 
office ringing signal and the 900 Hz tone to thereby mix the low frequency 
ringing signal with the 900 Hz tone at the central office. Upon 
demodulation of the modulated carrier at the subscriber terminal equipment 
the low frequency ringing signal and the 900 Hz tone are recovered. The 
recovered 900 Hz tone is then used as switching signal to apply an 
amplification of the low frequency ringing signal to ring the called 
subscriber's telephone. This signalling system also requires additional 
circuits at extra expense to provide bridged frequency ringing for a party 
line on one of the carrier system's channels. 
The present invention reduces the extra costs attendant with bridged 
frequency ringing as well as offering additional advantages as will become 
apparent from the following summary and detailed description. 
SUMMARY AND OBJECTS OF INVENTION 
As compared with prior station carrier equipment, the station carrier 
system of this invention may be installed in place of a multiparty line 
having bridged frequency ringing to provide individual or single party 
service for the party line subscribers without replacing the subscribers' 
tuned telephone ringers. Additionally, the present invention provides for 
the transmission of bridged frequency ringing information without using 
the central office ringing signal to amplitude modulate a carrier signal 
or a lower frequency alerting signal (e.g., 900 Hz) as is required in the 
previously discussed prior art carrier systems. 
Instead, the present invention provides a novel bridged frequency 
signalling arrangement whereby an alerting signal (e.g., a 900 Hz tone), 
rather than being amplitude modulated in the accepted sense, is 
periodically interrupted at whatever ringing rate or frequency the central 
office may supply. The interrupted alerting signal is applied to modulate 
the called subscriber's carrier signal which is transmitted from the 
central office. 
At the subscriber terminal equipment the modulated carrier signal is 
demodulated to recover the periodically interrupted alerting signal, and 
the recovered alerting signal is then detected and filtered to provide a 
local ringing signal whose frequency is the same as that of the central 
office ringing signal. The local ringing signal will therefore have the 
appropriate frequency for operating a telephone ringer that is tuned to an 
assigned one of several central office ringing frequencies. 
In the illustrated embodiment the interrupted alerting signal is 
advantageously synchronously detected to retrieve the ringing frequency 
information. Synchronous detection has the advantage of eliminating the 
need for special bandpass filtering to separate the alerting signal from 
the other frequency components that result from detection of the modulated 
carrier signal. 
Synchronous detection of a ringing signal has previously been proposed as 
shown in U.S. Pat. No. 4,081,609 which issued on Mar. 28, 1978. The 
synchronous ring detection arrangement described in this patent, however, 
is costly because it requires the generation of a special pilot carrier 
signal and a separate receiver for receiving the pilot carrier signal. 
Furthermore, the signalling system described in U.S. Pat. No. 4,081,609 is 
not effective to transmit central office ringing frequency information for 
bridged frequency ringing. 
In the present invention, the interrupted alerting signal is synchronously 
detected without using a separate pilot carrier signal and, consequently, 
a pilot carrier signal receiver. This is accomplished by using a phase 
locked loop to provide the synchronization. In the illustrated embodiment 
the called subscriber's central office-transmitted carrier signal is 
modulated with a harmonic of the alerting signal in addition to the 
alerting signal itself. Upon receiving and demodulating the modulated 
carrier signal in the receiver of the called subscriber's subscriber 
channel terminal circuit, the alerting signal and its harmonic are 
recovered and are applied to the called subscriber's ring detector as well 
as to the above-mentioned phase locked loop which is common to all of the 
subscribers served by the carrier system. 
The phase locked loop locks with the harmonic to supply an output which is 
then frequency divided to produce a synchronous signal. The synchronous 
signal is applied to the called subscriber's ring detector to 
synchronously detect the transmitted alerting signal. This results in the 
recovery of the desired ringing signal frequency which is used for 
operating the called subscriber's tuned ringer. 
In the illustrated embodiment the alerting signal's harmonic is also used 
with advantage in a channel loop test to check the continuity of the 
carrier and audio signal paths and audio signal receive level for any 
selected transmission channel in the carrier system. This is accomplished 
by first amplifying or otherwise adjusting the level of the harmonic 
signal at the central office, by then injecting the level-adjusted signal 
into the transmitter of a selected central office channel terminal circuit 
to modulate the carrier signal that is transmitted from the selected 
central office channel terminal circuit, by receiving and detecting the 
thusly modulated carrier signal in the companion subscriber terminal 
circuit to recover the level-adjusted harmonic signal, by using the output 
of the previously mentioned phase locked loop to synchronously detect the 
recovered level-adjusted harmonic signal in the companion subscriber 
channel terminal circuit, by sensing the level adjustment in the 
synchronously detected harmonic signal in the companion subscriber channel 
terminal circuit, by causing transmission of the subscriber-transmit 
carrier signal from the companion subscriber channel terminal circuit upon 
sensing the detection of the level-adjusted harmonic signal, by conducting 
the recovered harmonic signal through the receiver and transmitter of the 
companion subscriber channel terminal circuit to modulate the 
subscriber-transmit carrier signal, and by receiving and detecting the 
subscriber transmit carrier signal at the originating central office 
channel terminal circuit to complete the "loop-around" conduction of the 
harmonic signal. Test equipment, such as a level detector, at the central 
office is used to sense the return of the harmonic signal to the central 
office and to check the harmonic signal's level upon its return. 
In the illustrated embodiment, each transmission channel is equipped with a 
novel ring circuit for supplying the local ringing signal which is used to 
operate the subscriber's telephone ringer on the channel. In the ring 
circuit of this invention the local ring signal is derived by filtering 
the received alerting signal which is interrupted at the central office 
ringing signal rate. The ring circuit is also equipped to amplify the 
local ring signal for supplying sufficient power to drive the telephone 
ringer. 
In one embodiment of the ring circuit the amplification is accomplished by 
a special low cost transistor switching circuit having a built-in current 
limiting feature for protecting the switching transistors. In another 
embodiment of the ring circuit, a special relay switching arrangement is 
used for providing the local ringing signal with sufficient power to 
operate the telephone ringer. 
With the foregoing in mind a major object of this invention resides in the 
provision of a novel station carrier system that can be installed in place 
of a party line using bridged frequency ringing without requiring the 
replacement of the subscriber's tuned ringers. 
More particularly it is an object and purpose of this invention to provide 
a novel station carrier signalling system and method in which ringing 
frequency information is transmitted for operating tuned ringers. 
Another important object of this invention is the provision of a novel ring 
detector system and method for station carrier equipment. 
Still another important object of this invention is the provision of a 
novel channel loop testing system and method for station carrier 
equipment. 
A further object of this invention is the provision of a novel ring circuit 
for a station carrier system. 
Still another object of this invention is to provide a novel synchronous 
detection circuit capable of synchronously detecting the alerting signal 
mentioned above or other a.c. signal such as one of the carrier signals 
itself. According to this last object the a.c. signal to be detected is 
fed to a signal input of a synchronous detector, and a bandpass filter is 
tuned to the frequency of the a.c. signal to pass the signal to a 
synchronous input of the synchronous detector and thus provide for the 
synchronous detection of the signal at the detector's signal input. A 
special phase locked loop is included in the circuitry and has a 
controllable phase shifting circuit connected intermediate the bandpass 
filter and the synchronous input of the detector to provide a correction 
for any phase distortion or phase shift that may result in the passage of 
the a.c. signal through the bandpass filter. 
Further objects of this invention will appear as the description proceeds 
in connection with the appended claims and the below-described drawings.

DETAILED DESCRIPTION 
Referring to FIG. 1, one embodiment of a plural channel AM station carrier 
system (indicated at 20 in the drawings) incorporating the principles of 
this invention is shown to comprise a single two-conductor transmission 
line 22, a selected number of central office transmitting and receiving 
channel terminal circuits or units, and a corresponding number of 
subscriber transmitting and receiving channel circuits or units. Any 
suitable number of central office and subscriber channel terminal circuits 
may be employed depending upon the number of channels desired. For 
example, eight central office channel terminal circuits and eight 
subscriber channel terminal circuits are typically employed as shown to 
make up an eight-channel carrier system. 
Since the central office terminal circuits are alike, only three are shown 
in FIG. 1 and are indicated at COT1, COT2 and COT8. For the same reason 
only three of the subscriber terminal circuits, namely STU1, STU2 and 
STU8, are shown in FIG. 1. In the following description reference to all 
eight central office terminal circuits is made by the designation 
COT1-COT8 and reference to all eight of the subscriber terminal circuits 
is made by the designation STU1-STU8. 
As shown in FIG. 1, each of the central office terminal circuits COT1-COT8 
comprises a communication transmitter (indicated at 24 in FIG. 2) for 
transmitting a carrier signal of pre-selected frequency and a 
communication receiver (indicated at 26 in FIG. 2) tuned to receive a 
carrier signal from only a pre-selected one of the subscriber terminal 
circuits. Likewise, the subscriber terminal circuits STU1-STU8 each 
includes a communication receiver (indicated at 28 in FIG. 3) tuned to 
receive a carrier signal from only a pre-selected one of the central 
office terminal circuits and a transmitter (indicated at 30 in FIG. 3) for 
transmitting a carrier signal of pre-selected frequency. 
Each of the subscriber terminal circuits STU1-STU8 is paired with a 
different one of the central office terminal circuits COT1-COT8 to provide 
eight two-way transmission channels. It is understood that each of these 
transmission channels has two different allocated frequency bands to 
provide service for a subscriber, one band being for transmission in one 
direction from the central office terminal equipment to the subscriber 
terminal equipment, and the other band being for transmission in the 
opposite direction from the subscriber terminal equipment to the central 
office terminal equipment. In this example, the subscriber terminal 
circuits STU1-STU8 are paired with the central office terminal circuits 
COT1-COT8, respectively. The frequency spacing between adjacent carrier 
signals transmitted in either direction may be 8 kHz. 
The central office terminal circuits COT1-COT8 form a part of the central 
office terminal equipment and are located at a central office or central 
office station which is generally indicated at 33 in FIG. 1. The 
subscriber channel terminal circuits STU1-STU8 form a part of the 
subscriber terminal equipment and are located remotely from central office 
33 at the subscriber's end of the transmission line 22. 
The transmitters and receivers in each of the terminal circuits COT1-COT8 
are connected to line 22 by any suitable means such as a transformer or a 
central office group terminal unit or circuit 34. Group terminal circuit 
34 is also located at the central office. The transmitters and receivers 
in each of the subscriber terminal circuits STU1-STU8 are connected to 
line 22 remotely from central office 33 by way of any suitable means such 
as a transformer or a subscriber group terminal unit or circuit 36. 
The subscriber channel terminal circuits STU1-STU8 are separately connected 
to the telephones (indicated at 40 in FIG. 1) of eight different 
subscribers by suitable means such as subscriber drops 42. At the central 
office, the central office terminal circuits COT1-COT8 are separately 
connected by central office drops 44 to appropriate terminals in the 
central office exchange equipment which is indicated at 46 in FIG. 1. 
As is customary in telephone carrier systems, each of the central office 
terminal circuits COT1-COT8 transmits at a pre-selected carrier frequency 
that is different from the transmission carrier frequencies allocated to 
the remaining central office channel terminal circuits and also different 
from the carrier frequencies that are transmitted up the transmission line 
22 in the opposite direction from the subscriber channel terminal circuits 
STU1-STU8. Likewise, the subscriber terminal circuits STU1-STU8 transmit 
at pre-selected carrier frequencies that are different from each other and 
different from the transmit frequencies assigned to the central office 
terminal circuits COT1-COT8. The allocation of different carrier 
frequencies for the carriers on transmission line 22 is referred to and 
designated as frequency division multiplexing (FDM). 
In a typical FDM allocation scheme the carrier frequencies transmitted from 
the central channel terminal circuits COT1-COT8 are all contained in an 88 
kHz to 144 kHz frequency band and carrier frequencies that are transmitted 
from the subscriber channel terminal circuits STU1-STU8 are contained in a 
lower 8 kHz to 64 kHz band. 
As shown in FIG. 1, all of the central office terminal circuits COT1-COT8 
and unit 34 may advantageously be grouped together in a single central 
office terminal. Similarly, all of the subscriber terminal circuits 
STU1-STU8 and unit 36 may advantageously be grouped together in a single 
subscriber terminal. 
Still referring to FIG. 1 the central office group terminal unit 34 is a 
four-wire circuit providing separate transmit and receive signal paths or 
sections 80 and 82 which are coupled by a transformer 84 to the central 
office end of transmission line 22. The transmit signal path 80 feeds the 
central office-transmitted carrier signals from the transmitters of the 
central office channel terminal circuit COT1-COT8 to transmission line 22. 
The receiver signal path feeds the arriving subscriber transmitted carrier 
signals from transmission line 22 to the receivers in the central office 
channel terminal circuits COT1-COT8. 
Similar to group terminal unit 34, unit 36 is a four-wire circuit providing 
separate transmit and receive signal paths or sections 84 and 86 which are 
coupled by a transformer 88 to the end of transmission line 22 remote from 
central office 33. The receive signal path in group terminal unit 36 feeds 
the carrier signals arriving from the central office to the receivers in 
the subscriber channel terminal circuits STU1-STU8. The transmit signal 
path in unit 36 feeds the carrier signals from the transmitters in the 
subscriber channel terminal circuits STU1-STU8 to transmission line 22 for 
transmission up the line to the central office terminal equipment. 
Group terminal units 34 and 36 may be equipped with suitable amplifying and 
signal level adjusting circuitry such as that described in my copending 
application Ser. No. 932,706 filed on even dated herewith for Amplitude 
Modulated Telephone Carrier System and Terminal Equipment Therefor. The 
central office group terminal unit 34 is additionally equipped with the 
usual high and low pass directional filters (not shown) to provide for the 
separation of the high and low groups of central office and subscriber 
transmit carrier frequencies from each other. The high pass filter is 
connected in the transmit path of unit 34 for passing the high group of 
central office transmitted carrier frequencies (88 kHz-144 kHz) while 
rejecting the low group of subscriber-transmitted carrier frequencies (8 
kHz-64 kHz) to keep the low group of incoming carrier frequencies out of 
the transmit section 82. The low pass filter is connected in the receive 
path of unit 34 for passing the low group of incoming carrier frequencies 
while rejecting the high group of outgoing carrier frequencies to keep the 
latter out of receive section 84. Unit 36 is similarly equipped with high 
and low pass directional filters (not shown) for the same purpose. 
Since the central office channel terminal circuits COT1-COT8 are alike and 
since the subscriber channel terminal circuits STU1-STU8 are also alike, 
only the channel terminal circuits for one channel (namely, channel 
terminal circuits COT1 and STU1) will be described in greater detail. To 
this end, the transmitter of the central office terminal circuit COT1 
comprises a compressor 50, a low pass filter 52 and a modulator 54 all 
connected in series in the manner shown in FIG. 2. The receiver in the 
central office channel terminal circuit COT1 is equipped with a channel 
bandpass filter 62, a detector 64, a low pass filter 66, a fixed gain 
amplifier 68, an expandor 70, a hybrid 73 and an automatic gain control 
circuit 74. In this embodiment the AGC circuit 74 controls an attenuator 
75 or other variable gain circuit in the audio portion of receiver 26 to 
provide AGC action for the voice frequency signals that are passed by 
filter 66. 
The transmitter 30 and receiver 28 in the subscriber channel terminal 
circuit STU1 are the same as the construction thus far described for the 
transmitter and receiver in the central office terminal circuit COT1. To 
the extent that the terminal circuits STU1 and COT1 are the same, like 
reference numerals have been applied to designate the corresponding parts, 
except that the reference numerals applied to the parts of the subscriber 
terminal circuit STU1 have been suffixed by the letter "a" to distinguish 
them from the reference characters that are applied to designate the parts 
of the central office channel terminal circuit COT1. 
Voice frequency intelligence to be transmitted by way of terminal circuits 
COT1 and STU1 from the central office exchange equipment 46 to the 
subscriber's telephone 40 is fed by drop 44 and hybrid 73 to compressor 50 
in terminal circuit COT1. Compressor 50 compresses the dynamic range of 
the voice signals in the usual manner. 
From compressor 50 the compressed voice frequency signals are fed through 
filter 52 to modulator 54 where they amplitude modulate a sinusoidal 
carrier frequency signal from an oscillator 92 to produce a double 
sideband amplitude modulated carrier signal. According to one carrier 
frequency allocation scheme, the frequency of the sinusoidal wave produced 
by oscillator 92 may be 88 kHz and is different from the other central 
office-transmitted carrier frequencies and the subscriber-transmitted 
carrier frequencies as previously explained. 
The circuit of filter 52 may be of any suitable design for rejecting 
frequencies above approximately 3000 kHz. Filter 52 therefore passes only 
voice frequency information up to 3000 Hz and serves to keep the carrier 
and other high frequencies out of compressor 50. By limiting the upper 
frequency of the VF modulating signal to 3 kHz due to the filtering action 
of filter 52, the upper and lower sidebands of the modulated carrier 
signal will therefore extend only to a maximum of 3 kHz from the carrier 
frequency. 
The modulated carrier signal supplied at the output of modulator 54 is fed 
through the transmit section 80 of the central office group terminal unit 
34 and is coupled to transmission line 22 for transmission down the line 
to the subscriber group terminal unit 36 along with the other carrier 
signals that are transmitted from the central office channel terminal 
circuits. 
At the subscriber group terminal unit 36 the amplitude modulated carrier 
signal from the central office channel terminal circuit COT1 is fed along 
with the other central office-transmitted carriers through the receive 
section of terminal unit 36 to the channel bandpass filter 62a in the 
subscriber channel terminal circuit STU1 as well as to the corresponding 
channel bandpass filters in the remaining subscriber channel terminal 
circuits. 
The channel bandpass filters in the subscriber terminal circuits STU1-STU8 
are tuned to the different carrier frequencies that are allocated to their 
associated central office channel terminal circuits COT1-COT8. Each 
channel bandpass filter therefore passes with the least attenuation the 
incoming carrier frequency (together with its sidebands) that is allocated 
to its associated central office channel terminal circuit while rejecting 
the other carrier frequencies. Thus, for this example, the channel 
bandpass filter 62a in the subscriber terminal circuit STU1 will pass with 
the least attenuation the modulated carrier signal from the central office 
channel terminal circuit COT1. In a similar fashion the channel bandpass 
filter (not shown) in subscriber channel terminal circuit STU2 will pass 
the modulated carrier frequency from central office channel terminal 
circuit COT2, and so on. 
Upon passing through filter 62a, the modulated carrier signal from terminal 
circuit COT1 is applied to and detected by detector 64a which may be of 
any suitable type such as an envelope or synchronous detector. 
The components of detection are fed to filter 66a. Filter 66a is 
conventionally provided with an upper cutoff of about 3000 Hz to pass the 
VF components of detection while rejecting the carrier frequency component 
and other components higher than 3000 Hz. Filter 66a therefore separates 
the desired voice frequency intelligence from the other components of 
detection. 
The recovered VF signals which are passed by filter 66a are attenuated by 
attenuator 75a under the control of AGC circuit 74a and are then amplified 
by amplifier 68a. From amplifier 68a the VF signals are a.c. coupled to 
expandor 70a. Up to this point in the signal path, the dynamic range of 
the VF signals are still in their compressed state, having been compressed 
by the companion compressor 50 in the central office terminal circuit 
COT1. Expandor 70a operates to restore the VF signals to their original 
dynamic range. From expandor 70a the voice frequency signals are coupled 
to the receiver in telephone 40 by way of hybrid 73a and drop 42. 
Considering now the transmission of intelligence from the subscriber 
terminal equipment to the central office terminal equipment, voice 
frequency intelligence signals originating from the subscriber's telephone 
40 on the channel established by STU1 and COT1 are fed by way of hybrid 
73a to compressor 50a. Compressor 50a preforms the same function as 
compressor 50. 
From compressor 50a the compressed voice frequency signals are fed through 
filter 52a to modulator 54a where they modulate a carrier signal of 
pre-selected frequency from an oscillator 94 to develop a double sideband 
amplitude modulated carrier signal for transmission from the subscriber 
terminal circuit STU1. 
The purpose of filter 52a is the same as that described for filter 52. The 
frequency of the sinusoidal waveform produced by oscillator 94 is selected 
in accordance with the previously explained carrier frequency allocation 
scheme and may be 8 kHz for terminal circuit STU1. 
The carrier signal transmitted from the subscriber terminal circuit STU1 
and the carriers transmitted from the other subscriber terminal circuits 
are fed through the transmit section of the subscriber group terminal unit 
36 to the transmission line 22 for transmission to the central office 
group terminal unit 34. 
At the central office group terminal unit 34 the amplitude modulated 
carrier signal from the subscriber terminal circuit STU1 is coupled along 
with the other subscriber-transmitted carriers through the receive section 
of terminal unit 34 to the channel bandpass filter 62 in the central 
office channel terminal circuit COT1 as well as the corresponding channel 
bandpass filters in the remaining central office channel terminal 
circuits. 
Similar to the channel bandpass filters 62a in the subscriber channel 
terminal circuits, the channel bandpass filters in the central office 
terminal circuits COT1-COT8 are so designed that each filter is tuned to 
and hence passes with the least attenuation the transmit carrier frequency 
and associated sidebands coming in from its associated subscriber channel 
terminal circuit while rejecting the other incoming carrier frequencies. 
Thus, for this example, the bandpass filter 62 in the central office 
channel terminal circuit COT1 will pass the modulated carrier frequency 
from the subscriber terminal circuit STU1, the bandpass filter in terminal 
circuit COT2 will pass the modulated carrier frequency from terminal 
circuit STU2, and so on. 
Upon passing through filter 62 the modulated carrier signal is applied to 
and detected by detector 64. The resulting components of detection are 
applied to filter 66 which as an upper 3000 Hz cutoff. In performing the 
same function as filter 66a, filter 66 separates the desired voice 
frequency intelligence from the other components above 3000 Hz. 
The VF signals passed by filter 66 are attenuated at attenuator 75 under 
the control of the AGC circuit 74 and are then amplified by amplifier 68. 
From amplifier 68 the VF signals are a.c. coupled to expandor 70 which 
restores the VF signals to their original dynamic range in the same manner 
as described for expandor 70a. From expandor 70 the voice frequency 
signals are coupled to the central office exchange equipment 46 by way of 
hybrid 73 and drop 44. 
Channel terminal circuits COT2-COT8 and STU2-STU8 operate in the same 
manner as described above for transmitting intelligence in both directions 
over line 22. 
In order to signal incoming calls for the subscribers served by carrier 
system 20, each of the central office channel terminal circuits COT1-COT8 
is equipped with a pair of analog switches 100 and 102 (see FIG. 2), and 
each of the subscriber terminal circuits STU1-STU8 is equipped with a 
synchronous ring detector 104 and a ring circuit 106 as shown in FIG. 3. 
In addition, the central office terminal is equipped with an oscillator 
108, an amplifier 110, a comparator 111, a frequency divider 112 and a 
shaping circuit 114 as shown in FIGS. 1 and 2, and the subscriber terminal 
is equipped with a phase locked loop 116, a 90 degree phase shifter 117, 
and a frequency divider 118 as shown in FIGS. 1 and 3. Oscillator 108, 
amplifier 110, comparator 111, frequency divider 112 and shaping circuit 
114 form part of the equipment which is common to all of the central 
office channel terminal circuits COT1-COT8 at the central office, while 
the phase locked loop 116, phase shifter 117 and frequency divider 118 
form part of the equipment which is common to all of the subscriber 
channel terminal circuits STU1-STU8. 
Referring to FIGS. 1 and 2, oscillator 108 is connected to feed an a.c. 
sine wave signal of constant frequency to the input of amplifier 110 and 
also to the input of comparator 111. Comparator 111 converts the sine wave 
into an a.c. square wave of the same frequency and feeds it to frequency 
divider 112. The output of divider 112 is connected to shaping circuit 
114, and the output of shaping circuit 114 is connected to the switch 100 
in each of the central office terminal circuits by lines indicated at 120. 
The output of amplifier 110, on the other hand, is connected to the switch 
102 in each of the central office terminal circuits COT1-COT8 by lines 
indicated at 122. 
The a.c. square wave signal supplied by comparator 111 is frequency divided 
by divider 112, and the frequency-divided signal at the output of divider 
112 is used as the a.c. alerting signal to signal incoming calls for the 
subscribers. Following frequency division the alerting signal is applied 
to shaping circuit 114 which provides the alerting signal with a 
sawtoothed wave shape. From the shaping circuit 114 the alerting signal is 
applied to switch 100 in each of the central office channel terminal 
circuits COT1-COT8. 
In addition to being applied to comparator 111, the a.c. signal from 
oscillator 108 is amplified by amplifier 110 and applied to switch 102 in 
each of the central office terminal circuits COT1-COT8. 
The operating frequency of oscillator 108 may be any suitable value which 
is preferably in-band (e.g., less than 3000 Hz). For example, the 
frequency of the a.c. signal generated by oscillator 108 may be 1800 Hz. 
Likewise, the frequency of the frequency divided alerting signal at the 
output of divider 112 may be of any suitable value. In this embodiment 
divider 112 divides the incoming frequency by two, thus making the 
frequency of the outgoing, alerting signal 900 Hz where the incoming 
oscillator frequency is 1800 Hz. 
Thus, a 900 Hz alerting signal or tone is applied to the input of switch 
100 in each central office channel terminal circuit, and a 1800 Hz signal 
or tone is applied to the input of switch 102 in each central office 
channel terminal circuit. By deriving the 900 Hz alerting signal by 
frequency dividing the oscillator's 1800 Hz signal, the 900 Hz signal will 
have a fixed, predetermined phase relationship with the 1800 Hz signal. 
This phase relationship is utilized in a manner to be described later on. 
Both of the switches 100 and 102 are of the analog type that conduct or 
transmit a.c. signals without recitification upon being turned on. 
Switches 100 and 102 are each turned on to conduct the signals by applying 
a voltage of pre-determined polarity to their control electrodes. In the 
illustrated embodiment, positive polarity is used to turn the switches on. 
As shown in FIG. 2, each of the central office channel terminal circuits is 
equipped with a carrier detecting relay driver 124. Driver 124 is used to 
operate an off-hook relay RY1 and is also connected to the control 
electrode of switch 102 for controlling operation of the switch. As shown, 
relay driver 124 is connected to the output of amplifier 68 and is 
operative to turn switch 102 off in response to the presence of the d.c. 
voltage component that results from reception and detection of the 
subscriber carrier that is transmitted up transmission line 22 from the 
companion subscriber channel terminal circuit. Relay driver 124 is 
responsive to the absence of this d.c. voltage component to turn switch 
102 on. 
In the illustrated embodiment each of the subscriber channel terminal 
circuits STU1-STU8 will transmit its subscriber carrier signal only when 
its associated telephone 40 is off-hook. Therefore, no carrier signal will 
be transmitted from each of the subscriber channel terminal circuits when 
its associated telephone 40 is on-hook. 
From the foregoing it is clear that switch 102 will be turned on when the 
associated subscriber's telephone 40 is in its on-hook state and will be 
turned off when the telephone is brought off-hook. 
The output of switch 102 is connected to the compressor 50 in its central 
office channel terminal circuit so that when the switch 102 is on it 
conducts the amplified 1800 Hz oscillator signal from amplifier 110 to 
compressor 50. From there, the 1800 Hz oscillator signal is passed by 
filter 52 to modulator 54 to amplitude modulate the central 
office-transmit carrier signal supplied by oscillator 92. 
The switch 100 in each central office channel terminal circuit is turned on 
by the positive alternations of the central office ringing voltage which 
is applied to the ring terminal 126 by suitable central office ring 
generator equipment in the central office for signalling an incoming call. 
When switch 100 turns on it conducts the 900 Hz alerting signal to filter 
52. Filter 52 passes the 900 Hz alerting signal to modulator 54 where the 
alerting signal amplitude modulates the carrier signal that is supplied by 
oscillator 92. In response to an incoming call for a subscriber served by 
the carrier system, therefore, the subscriber's assigned central office 
transmit carrier signal will be modulated by the 900 Hz alerting signal as 
well as the 1800 Hz oscillator signal. 
The central office-transmit carrier signals, which are supplied by terminal 
circuits COT1-COT8, are continuously transmitted down line 22 in modulated 
or unmodulated form and are not interrupted for signalling purposes as are 
the subscriber-transmit carrier signals which are transmitted in the 
opposite direction from the subscriber channel terminal circuits 
STU1-STU8. These central office-transmit carrier signals are therefore 
continuously received and detected by their assigned subscriber channel 
terminal circuits. 
At the remote subscriber terminal equipment, the output of the amplifier 
68a in each of the subscriber channel terminal circuits STU1-STU8 is 
connected to the phase locked loop 116 which is common to all of the 
subscriber channel terminal circuits in the carrier system. Accordingly, 
all of modulation components recovered in each of the subscriber terminal 
circuits STU1-STU8 and having a low enough frequency to be passed by the 
low pass filter 66a in each subscriber channel terminal circuit will be 
applied to the phase locked loop 116. 
The phase locked loop 116, however, is set to acquire lock with only one 
pre-selected frequency, namely the 1800 Hz frequency component which is 
supplied by oscillator 108 at the central office terminal. The phase 
locked loop therefore supplies only one output frequency, namely the 1800 
Hz frequency if the 1800 Hz component is present at its input. 
The 1800 Hz component will be present at the input of loop 116 if any one 
or more of the subscribers served by system 20 is on-hook. In such a case 
the central office-transmit carrier signal assigned to each of the on-hook 
subscribers will be modulated by the 1800 Hz oscillator signal. This 1800 
Hz modulation is recovered in the appropriate subscriber channel terminal 
circuit and is fed to loop 116. 
Thus, if any one or more of the subscribers served by system 20 is on-hook, 
loop 116 will be locked with the recovered 1800 Hz oscillator signal to 
establish the 1800 Hz signal apart from the other applied components of 
detection. In the illustrated embodiment a phase locked loop output is 
used to provide the outgoing 1800 Hz signal with a triangular waveform as 
indicated at 127 in FIG. 6. The 1800 Hz signal 127 is phase shifted by 
phase shifter 117 and is then applied to frequency divider 118 which is 
also common to all of the subscriber channel terminal circuits STU1-STU8. 
At the output of loop 116 signal 127 will be shifted 90 degrees out of 
phase with respect to the original 1800 Hz sine wave signal at the input 
of loop 116. Phase shifter 117 compensates for this phase shift by 
shifting signal 127 90 degrees so that it will be either in phase or 
180.degree. out of phase with the 1800 Hz sine wave signal at the input of 
loop 116. 
As shown, the output of divider 118 is connected to the synchronous ring 
detector 104 in each of the subscriber channel terminal circuits 
STU1-STU8. Divider 118 divides the 1800 Hz frequency by two to feed a 900 
Hz signal (indicated at 129 in FIG. 6) to the ring detector 104 in each of 
the subscriber channel terminal circuits STU1-STU8. If the carrier signal 
received by one of the subscriber channel terminal circuits STU1-STU8 is 
modulated by the interrupted 900 Hz alerting signal, the alerting signal 
will therefore be synchronously detected by the ring detector 104 in the 
receiving subscriber channel terminal circuit, and the signal resulting 
from synchronous detection is applied to ring circuit 106 for ringing the 
called subscriber's telephone. 
Operation of the signalling equipment thus far described will now be 
considered in greater detail for the subscriber served by the channel 
terminal circuits COT1 and STU1. For convenience, this subscriber is 
referred to as the channel 1 subscriber, and the transmission channel 
established by terminal circuits COT1 and STU1 is identified as CH1 (see 
FIG. 1). In the embodiment shown in FIGS. 1-3, the transmission channel 
CH1 provides single party ringing. In a later embodiment, a bridged 
frequency ringing scheme is considered for a plurality of subscribers on a 
common, party line. 
Before the incoming call arrives at the central office for the channel 1 
subscriber, the switch 100 in central office channel terminal circuit COT1 
will be off, there being no ringing voltage applied to ring terminal 126. 
Switch 102, however, will be turned on, provided that the channel 1 
subscriber's telephone 40 is on-hook. The carrier signal transmitted from 
the channel terminal circuit will therefore be modulated by the 1800 Hz 
signal, but not the 900 Hz alerting tone. 
The modulated carrier signal transmitted from the central office terminal 
circuit COT1 is received and detected in the subscriber channel terminal 
circuit STU1 to recover the 1800 Hz modulation. The recovered 1800 Hz 
frequency is passed by filter 66a and fed through amplifier 68a to the 
phase locked loop 116. Loop 116 will therefore be locked with the 1800 Hz 
oscillator signal to generate the 1800 Hz signal 127 even if the channel 1 
subscriber is the only subscriber who is on-hook. 
As a result, divider 118 feeds the 900 Hz signal 129 to one input of the 
ring detector 104 in terminal circuit STU1. Before a call arrives at the 
central office for the channel 1 subscriber, however, the 900 Hz alerting 
signal will not be applied to the other input of the ring detector in 
terminal circuit STU1. As a result, no synchronous detection will take 
place, and ring circuit 106 will not operate to ring the channel 1 
subscriber's telephone even though the 900 Hz tone is present at the 
switching input of ring detector 104. 
When the incoming call arrives at the central office, a central office 
relay 130 (see FIG. 2) is operated to connect a central office ring 
generator 132 to the ring terminal 126 of the central office terminal 
circuit COT1. This central office ringing generator and relay equipment is 
conventional. In the embodiment shown in FIG. 2 generator 132 is used to 
supply the same central office ringing signal frequency for signalling 
incoming calls for all of the subscribers served by system 20. 
Generator 132 generates a typical central office a.c. ringing signal as 
indicated at 134 in FIG. 4. This central office ringing signal is applied 
to ring terminal 126 to signal the incoming call for the channel 1 
subscriber and is periodic or interrupted in that it has the usual ringing 
and silent intervals which alternate with each other. During the ringing 
interval the central office ringing signal is present, and during the 
silent interval the ringing signal is absent as shown. The duration of the 
ringing interval customarily is one second, while the duration of the 
silent interval customarily is two seconds. The fixed frequency of the 
central office ring signal 134 is typically 20 Hz. Alternatively, it may 
be some other relatively low frequency. 
As a result of applying the central office ringing signal 134 to ring 
terminal 126 to signal an incoming call for the channel 1 subscriber, 
switch 100 will be turned on by the positive alternations of ringing 
signal 134 and will be turned off by the negative alternations of the 
ringing signal. As a result, switch 100 will be alternately and cyclically 
turned on and off during the ringing interval of ringing signal 134. 
Additionally, switch 100 will be turned off throughout the silent 
interval, there being no ringing signal voltage applied during this 
period. For a central office ringing signal frequency of 20 Hz, switch 100 
will be on for 25 ms half the ringing signal cycle and off for the other 
25 ms half of the ringing signal cycle. 
The alerting signal will therefore be conducted to modulator 54 only during 
the positive alternations of the central office ringing signal 134 as 
shown in FIGS. 4 and 5 where the interrupted alerting signal is indicated 
at 136. 
As a result conduction of the 900 Hz alerting signal 136 will be 
intermittently interrupted at the central office ringing signal rate or 
frequency during the ringing intervals of ringing signal 134. The 
frequency of the alerting signal 136 (900 Hz in this example) is selected 
to be much higher than the frequency of the central office ringing signal 
134 so that a multiplicity of cycles of the alerting signal occur in each 
half cycle of the central office ringing signal. 
The 900 Hz alerting signal 136 will therefore be conducted in bursts to 
modulator 54, and the carrier signal supplied by oscillator 92 will 
therefore be amplitude modulated periodically by the interrupted alerting 
signal at the rate equal to the frequency of the central office ringing 
signal 134. The modulated carrier signal is indicated at 138 in FIG. 5. In 
response to an incoming call carrier signal 138 is therefore 
intermittently modulated by the 900 Hz signal 136 as well as being 
continuously moddulated by the 1800 Hz signal (indicated at 140 in FIG. 5) 
until the channel 1 subscriber's telephone is transferred off-hook. 
The modulated carrier signal 138 is received by the subscriber terminal 
circuit STU1, but not the other subscriber channel terminal circuits as 
indicated from the previous description. Thus, the modulated carrier 
signal 138 will be detected by the detector 64a in subscriber terminal 
circuit STU1 to produce the uninterrupted carrier frequency component, the 
interrupted 900 Hz alerting signal component and the continuous 1800 Hz 
signal component. 
Filter 66a passes the recovered 900 Hz alerting signal 136 and the 1800 Hz 
oscillator signal 140, but rejects the carrier frequency component as well 
as any other frequencies that are higher than 3000 Hz. The interrupted 900 
Hz alerting signal 136 will therefore be applied to the ring detector 104 
in the called subscriber's terminal circuit STU1. This application of the 
recovered 900 Hz alerting signal 136 to one signal input terminal of the 
ring detector 104 in terminal circuit STU1 occurs while the 900 Hz signal 
129 is being applied to the other signal input terminal of the ring 
detector. 
Because the interrupted 900 Hz alerting signal 136 is derived by frequency 
dividing the 1800 Hz oscillator signal 140, signal 136 will have a fixed, 
predetermined phase relationship with signal 140, and the zero crossovers 
of the 900 Hz alerting signal will be coincident with predetermined zero 
crossovers of the 1800 Hz oscillator signal. The zero crossovers of the 
interrupted alerting signal 136 will therefore be coincident with 
predetermined zero crossovers of the phase shifted 1800 Hz signal 127 at 
the output of phase shifter 117 as shown in FIG. 6. As a result, the zero 
crossovers of the interrupted 900 Hz alerting signal 136 will be 
coincident with the zero crossovers of the 900 Hz signal 129 at the output 
of frequency divider 118. Signals 136 and 129 will therefore be in phase 
or 180.degree. out of phase with each other. 
In either case, the result will be the synchronous detection of the 
interrupted alerting signal 136. In other words, synchronous detection of 
the alerting signal 136 occurs if the 900 Hz signal 129 is either in phase 
with signal 136.degree. or 180.degree. out of phase with signal 136. 
The recitifed or detected form of signal 136 appears at that output of ring 
detector 104 and is indicated at 144 in FIG. 6. In the illustrated 
embodiment signal 136 is half-wave recitifed by detector 104, but it could 
be full wave rectified if desired. 
The detected alerting signal 144 will have the same frequency as signal 136 
and will be periodically interrupted at the same rate as signal 136. The 
detected signal 144 will therefore be interrupted at the frequency of the 
central office ringing signal 134 as shown in FIG. 7. 
In the illustrated embodiment, ring detector 104 comprises an analog switch 
149 (see FIG. 8). Switch 149 is turned on by the positive alternations of 
signal 129 and is turned off by the negative alternations of signal 129 to 
thereby recitify the alerting signal 136. 
Although the recovered 1800 Hz signal is applied to ring detector 104 along 
with the 900 Hz alerting signals 136, the 1800 Hz frequency will not 
appear in the components of detection at the output of ring detector 104 
because the 1800 Hz signal has one positive alternation and one negative 
alternation for each half cycle of the 900 Hz synchronizing signal 129. 
The positive and negative alternations of the 1800 Hz signal cancel each 
other. As a result, neither the 1800 Hz frequency nor the beat frequency 
(i.e., the difference between the 900 Hz synchronizing frequency and 1800 
Hz) will be present at the output of ring detector 104. 
From ring detector 104, the detected alerting signal 144 is fed to a low 
pass filter 150 (see FIG. 8) which may be of the RC type. Filter 150 may 
form a part of ring circuit 106 as shown. Alternatively, it may be 
considered as being separate from ring circuit 106. 
In either case, filter 150 capacitively smooths and averages the detected 
alerting signal to develop a local square wave ringing signal as indicated 
at 152 in FIG. 7. The local ringing signal 152 is amplified or used as a 
switching signal in ring circuit 106 to develop an amplified square wave 
ringing signal 154 (see FIG. 7) whose voltage amplitude is large enough to 
operate the ringer (indicated at 155 in FIG. 3) in the called subscriber's 
telephone 40. Signal 154 is applied through ring side of drop 42 for 
signalling the incoming call. 
As previously explained, the 900 Hz alerting signal 136 will not be 
modulated onto the called subscriber's carrier signal throughout the 
silent interval of the central office ringing signal 134. As a result, 
local ringing signals 152 and 154 will not be present during the silent 
intervals of the central office ringing signal 134. Operation of ringer 
156 will therefore be intermittent, having the usual ringing and silent 
intervals corresponding to the ringing and silent intervals of the central 
office ringing signal 134. 
The frequency cutoff of filter 150 is selected to be just high enough to 
pass all of the ringing frequencies normally used in a bridged frequency 
ringing scheme. One suitable cutoff of filter 150 is about 100 Hz. Filter 
150 may be of any suitable circuit design. 
As shown in FIG. 3, each of the subscriber channel terminal circuits is 
equipped with an off-hook detector 156. Detector 156 receives an input 
from the ring side of telephone 40 to sense the off-hook condition of the 
telephone. 
Upon sensing the off-hook condition detector 156 operates a switch 157 
which turns on the modulator 54a in its subscriber channel terminal 
circuit. As a result, modulator 54a emits its subscriber-transmit carrier 
signal which is transmitted up line 22 for reception and detection in the 
companion central office channel terminal circuit. The d.c. voltage 
component resulting from detection of the received carrier signal is 
sensed by relay driver 124. When this happens, the relay 124 operates 
relay RY1 to close contacts RY1-1 (see FIG. 2) in the ring side of the 
drop to the central office equipment. This completes the circuit to the 
central office equipment 46 and signals the central office equipment to 
remove the central office ringing signal 134 from the line (i.e., to trip 
the ring). 
Considering the example of the incoming call for the channel 1 subscriber, 
the off-hook detector 156 in terminal circuit STU1 senses the off-hook 
condition when the channel 1 subscriber's telephone 40 is brought off-hook 
to answer the incoming call. Up to this time the modulator 54a in terminal 
circuit STU1 will be off, and no carrier signal is transmitted from 
terminal circuit STU1 as long as the channel 1 subscriber's telephone is 
on-hook. 
Upon sensing the off-hook state of the channel 1 subscriber's telephone 40, 
the off-hook detector 156 in terminal circuit STU1 operates switch 157 to 
thereby turn on the modulator 54a in terminal circuit STU1. 
Modulator 54a thereby produces the subscriber transmit carrier signal which 
is transmitted up line 22 to the central office terminal equipment where 
it is received and detected in the central office terminal circuit COT1. 
The d.c. voltage component which results from the detection of the received 
carrier operates the relay driver 124 in terminal circuit COT1, and 
operation of relay driver 124, in turn, results in operation of relay RY1, 
causing the closure of contacts RY1-1 in terminal circuit COT1. This 
completes the circuit connection of terminal circuit COT1 to the central 
office equipment 46 and signals the central office equipment to operate 
relay 130 for disconnecting the ring generator 132 from terminal COT1. 
When this happens the central office ringing signal 134 will be removed 
and the switch 100 in terminal circuit COT1 will be switched to its off 
state, thus removing the 900 Hz modulation from the carrier signal that is 
transmitted from the modulator 54 in terminal circuit COT1. 
In addition to operating relay RY1, the relay driver 124 also turns off 
switch 102 in terminal circuit COT1 upon sensing the d.c. voltage 
component that results from the detection of the carrier which is 
transmitted from terminal circuit STU1. As a result, the 1800 Hz 
oscillation signal will be removed from modulator 54 along with the 900 Hz 
signal when the channel 1 subscriber's telephone 40 is brought off-hook to 
answer the incoming call. 
From the foregoing description it is clear that the frequency of the 
amplified ringing signal 154 is the same as the frequency of the 
unamplified local ringing signal 152 which is supplied at the output of 
filter 150. The frequency of the local ringing signal 152, in turn, 
corresponds to the rate or frequency at which the 900 Hz alerting signal 
136 is interrupted by the central office ring signal 134. The frequency of 
the local ringing signal 152 and, hence, the amplified ringing signal 154 
will therefore be the same as the frequency of the central office ring 
signal 134. 
If the frequency of the central office ringing signal 134 is 20 Hz as 
previously mentioned, then the frequency of signals 152 and 154 will also 
be 20 Hz. If, on the other hand, the frequency of the central office 
ringing signal 134 is 30 Hz or is changed to 30 Hz then the frequency of 
the local ringing signals 152 and 154 will be 30 Hz. In summary, the 
frequency of the local ringing signals 152 and 154 will be the same as and 
will be determined by the frequency of the central office ringing signal 
134. The local ringing signal 154 will therefore have the appropriate 
frequency for operating a telephone ringer that is tuned to the central 
office ringing frequency, whatever that frequency may be. 
Accordingly, no modification to carrier system 20 is required and no change 
in telephone ringers is needed when it is desired to install the carrier 
system in place of a party line using bridged frequency ringing. Such a 
party line, by way of example, may serve two subscribers, one having a 
ringer tuned to 30 Hz and the other having a ringer tuned to 40 Hz. For 
party line service a separate central office ringing generator or ringing 
signal source is assigned to each of these subscribers, one supplying the 
30 Hz central office ringing signal and the other supplying the 40 Hz 
central office ringing signal. 
When installing carrier system 20 in place of the foregoing party line 
service the telephone of one of the two party line subscribers may be 
connected to one of the subscriber channel terminal circuits STU1-STU8, 
and the telephone of the other party line subscriber may be connected to a 
different one of the subscriber channel terminal circuits STU1-STU8 
without changing or replacing the tuned ringers in either of the 
telephones. For example, the telephone having the 30 Hz tuned ringer may 
be connected to the subscriber channel terminal circuit STU1, and the 
telephone having the 40 Hz tuned ringer may be connected to the subscriber 
channel terminal circuit STU2. In such a case the 30 Hz central office 
ringing generator will be connected to the switch 100 in the central 
office channel terminal circuit COT1, and the 40 Hz central office ringing 
generator will be connected to the switch 100 in central office channel 
terminal circuit COT2. This arrangement is shown in FIG. 9 where the 30 Hz 
central office ringing generator is indicated at 160, the 40 Hz central 
office ringing generator is indicated at 161, the telephone having the 30 
Hz tuned ringer is indicated at 40a and the telephone having the 40 Hz 
tuned ringer is indicated at 40b. The standard 20 Hz ringing generator 
(indicated at 132 in FIG. 2) may be used to supply the central office 
ringing signal (134) for the subscribers on remaining six transmission 
channels STU3-STU8 in carrier system 20. By this arrangement, individual 
service, rather than party line service, is supplied to the subscribers 
having telephones 40a and 40b. 
If a call arrives at the central office for the subscriber assigned to 
telephone 40a the 30 Hz central office ringing signal will be fed to 
switch 100 in central office terminal circuit COT1. When this happens the 
900 Hz alerting signal 136 will be applied to modulate the carrier signal 
sent out by central office terminal circuit COT1 and will be interrupted 
by the central office ringing signal from generator 160 at the generator's 
30 Hz rate. The local ringing signal 154 developed in subscriber terminal 
circuit STU1 will therefore have a frequency of 30 Hz for operating the 
tuned, 30 Hz ringer in telephone 40a to signal the incoming call. 
If a call arrives at the central office for the subscriber assigned to 
telephone 40b, the 40 Hz central office ringing signal will be fed to 
switch 100 in channel terminal circuit COT2. The 900 Hz alerting signal 
will therefore be applied to modulate the carrier signal transmitted from 
central office terminal circuit COT2 and will be interrupted by the 40 Hz 
central office ringing signal at the ringing signal's 40 Hz rate. The 
local ringing signal (154) developed in the subscriber channel terminal 
circuit STU2 will therefore be provided with a frequency of 40 Hz to 
operate the tuned, 40 Hz ringer in telephone 40b. 
The signalling system just described is also effective to accommodate party 
line service with bridged frequency ringing on any one or more of the 
carrier system's transmission channels. For example, party line service 
with bridged frequency ringing may be provided on transmission channel 1 
(terminal circuits COT1 and STU1) to serve two or more subscribers. To 
simplify the example assume that party line service is furnished to two 
subscribers on channel 1 as shown in FIG. 10. 
In FIG. 10, the telephone for one of the party line subscribers is 
indicated at 40c, and the telephone for the other of the party line 
subscribers is indicated at 40d. For this example assume that telephone 
40c is equipped with a 20 Hz tuned ringer and that telephone 40d is 
equipped with a 30 Hz tuned ringer. For a bridged frequency ringing scheme 
it will be noted that the ringer (not shown in FIG. 10) in each party line 
telephone is connected across the ring and tip sides of the subscriber 
drop 42. 
The terminal circuits STU1 and COT1 require no modification to provide the 
party line service and bridged frequency ringing operation for the 
subscribers' telephones 40c and 40d. Accordingly, all of the central 
office and subscriber channel terminal circuits are the same as that shown 
in FIGS. 1-3. 
To provide the different bridged frequency ringing frequencies (in this 
case 20 Hz and 30 Hz), two central office ringing generators 164 and 165 
are used in place of the single generator 132. Generators 165 and 164 form 
a part of the central office exchange equipment 46 and are individually 
connectable to the switch 100 in the central office terminal circuits COT1 
by operation of separate central office relays 166 and 167 as shown in 
FIG. 10. For the bridged frequency ringing scheme it will be appreciated 
that there will be as many central office ringing signal generators as 
there are subscribers on the party line. 
It is also understood that each of the generators 164 and 165 generates a 
central office ringing signal corresponding to signal 134. However, the 
frequencies of the signals supplied by generator 164 and 165 will be 
different and will be matched with the frequencies to which the ringers in 
telephones 40c and 40d are tuned. In this example, generator 164 supplies 
the 20 Hz central office ringing signal, and generator 165 supplies the 30 
Hz central office ringing signal. 
If a call comes in for the subscriber who is assigned to telephone 40c, 
relay 166 will be operated to connect generator 164 to the switch 100 in 
terminal circuit COT1. The carrier signal transmitted from terminal 
circuit COT1 will therefore be modulated with the 900 Hz alerting signal, 
and this 900 Hz modulating signal will be interrupted at the frequency of 
the ringing signal supplied by generator 164, namely 20 Hz. The frequency 
of the local ringing signal 154, which is applied to the ring side of drop 
42 to signal the incoming call, will therefore be 20 Hz. As a result, 
telephone 40c will be rung, but not telephone 40d. 
If a call comes in for the subscriber assigned to telephone 40c, relay 167 
will be operated to connect generator 165 to switch 100 instead of 
generator 164. As a result, the 900 Hz alerting signal 136, which is 
modulated onto the carrier signal coming out of terminal circuit COT1, 
will be interrupted at the 30 Hz rate which is the frequency of the 
central office ringing signal supplied by generator 165. The local ringing 
signal 154 will therefore be provided with a 30 Hz frequency so that it 
will ring telephone 40d, but not telephone 40c. 
Referring back to FIGS. 1-3, each of the transmission channels established 
in carrier system 20 may optionally be equipped to provide for a channel 
loop testing operation. This is accomplished by equipping each of the 
subscriber channel terminal circuits STU1-STU8 with a further synchronous 
detector 170 and a level detector 172 and by equipping the central office 
terminal with one additional amplifier 174 and a level or threshold 
detector 176. 
In each of the subscriber terminal circuits STU1-STU8 the output of 
amplifier 68a is connected to the signal input lead of synchronous 
detector 170, the output lead of detector 170 is connected to the input of 
level detector 172, and the output of level detector 170 is connected to 
the dial switch 157 (or off-hook switch as it is also called). The output 
of phase shifter 117, which is common to all of the subscriber terminal 
circuits STU1-STU8 is connected to the control input lead of the detector 
170 in each of the subscriber terminal circuits so that the local 1800 Hz 
signal 127 is fed to the detector 170 in each of the subscriber terminal 
circuits STU1-STU8 as long as the 1800 Hz oscillator signal 140 is present 
at the input of the phase locked loop 116 and the phase locked loop is 
locked with signal 140. 
At the central office the level detector 176 forms a part of the equipment 
that is common to the central office terminal circuits COT1-COT8 and is 
connected to the carrier-sensing relay driver 124 in each of the terminal 
circuits COT1-COT8. Amplifier 174 also forms a part of the equipment that 
is common to the central office terminal circuits COT1-COT8 as shown. In 
addition, a separate switch is provided for each of the central office 
channel terminal circuits COT1-COT8 for separately and selectively 
connecting the compressor 50 in each of the terminal circuits COT1-COT8 to 
the output of amplifier 174. The switches for the illustrated central 
office terminal circuits COT1, COT2 and COT8 are respectively indicated at 
SW1, SW2 and SW8 in FIG. 1. These switches are collectively referred to as 
SW1-SW8 for all eight of the central office terminal circuits COT1-COT8. 
For the illustrated part of the system, the output of amplifier 174 is 
connected by switch SW1 to the input of the compressor 50 in terminal 
circuit COT1, by switch SW2 to the input of the compressor 50 in terminal 
circuit, by switch SW8 to the input of the compressor 50 in terminal 
circuit COT8, and so on. 
If it is desired to perform a channel loop test on transmission channel 1, 
switch SW1 is closed. If it desired to perform a channel loop test on the 
transmission channel 2, which is established by terminal circuits COT2 and 
STU2, switch SW2 is closed, and so on. 
By connecting amplifier 174 to amplifier 110, amplifier 174 will amplify 
the 1800 Hz oscillator signal and will feed the amplified oscillator 
signal to the compressor 50 in any selected one of the central office 
terminal circuits COT1-COT8 upon closure of the appropriate one of 
switches SW1-SW8. Switches SW1-SW8 are normally open and are selectively 
closed one at a time only when it is desired to conduct a channel loop 
test on selected transmission channels in the carrier system. 
The voltage gains of amplifiers 110 and 174 are set so that there will be a 
pre-selected difference in signal levels at the outputs of the two 
amplifiers, with amplifier 174 supplying the larger amplitude signal as 
expected. The signal level difference resulting from these different 
oscillator signal strengths is sensed by the level detector 172 in the 
subscriber terminal circuit receiving the 1800 Hz signal. In response to 
receiving the stronger of the two 1800 Hz oscillator signals, the level 
detector 172 establish a condition for completion of the channel loop test 
in the manner described in detail below. 
The level difference between the 1800 Hz signal at the output of amplifier 
110 and the larger 1800 Hz signal at the output of amplifier 174 may be 
any suitable value that is sufficient to enable the level detector 172 to 
determine which of the two 1800 Hz signals is received. For example, the 
combined gain of amplifiers 110 and 175 may be set so that 1800 Hz 
oscillator signal at the output of amplifier 174 will produce sidebands 
that are 12 db down from the level of the carrier signal onto which the 
1800 Hz signal is modulated. The gain of amplifier 110 alone may be set to 
provide the 1800 Hz signal with a level that is 20 db down from the level 
of the 1800 Hz signal at the output of amplifier 174. 
This difference in the 1800 Hz signal levels at the outputs of amplifiers 
110 and 174 results in a signal level difference of 10 db at the output of 
compressor 50 for the typical 2:1 compression ratio in the compressor. 
Upon arriving at the output of the compressor 50 in each central office 
channel terminal circuit, the smaller of the two 1500 Hz signals (i.e., 
the 1800 Hz that is amplified by amplifier 110 alone) will be down 10 db 
from the level needed to make sidebands 12 db down from the level of the 
carrier signal. The same 10 db difference will occur in the levels of the 
larger and smaller 1800 Hz signals at the output of amplifier 68a in each 
of the subscriber terminal circuits receiving the 1800 Hz signals. 
The channel loop test operation for each of the eight transmission channels 
is the same. Accordingly, only channel loop test operation for channel 1 
will be explained in greater detail below, it being understood that the 
following explanation is applicable to the remaining transmission channels 
in carrier system 20. 
Switch SW1 is kept in its illustrated open position during normal operation 
when no channel loop test is conducted. As a result, the larger amplitude 
1800 Hz CLT signal will not be fed to the central office terminal circuit 
COT1 from amplifier 174. However, the smaller amplitude 1800 Hz oscillator 
signal 140 will be applied to the central office terminal circuit COT1 and 
will be conducted through switch 102--assuming that the channel 1 
subscriber's telephone 40 is on-hook--to modulate the central office 
transmit carrier signal 138, all in the manner previously described. 
Upon reception and detection of carrier signal 138 at the subscriber 
channel terminal circuit STU1, the smaller amplitude 1800 Hz oscillator 
signal 140 is recovered, separated by filter 66a from the other components 
of detection having frequencies greater than 3000 Hz and fed to the signal 
input lead of synchronous detector 170 which may be a MOS analog switch 
(indicated at 180 in FIG. 8) similar to switch 149. 
At this stage of operation, the recovered 1800 Hz signal will be the only 
component of detection having a frequency less than 3000 Hz so that only 
the 1800 Hz frequency will be present at the signal input of detector 170. 
The recovered 1800 Hz oscillator 140 will also be applied to phase locked 
loop 116 so that loop 116 will be locked with the 1800 Hz oscillator 
signal. The local 1800 Hz signal 127 will therefore be present at the 
output of loop 116, phase shifted 90.degree. by phase shifter 117 and 
applied to the control lead of synchronous detector 170 throughout the 
time in which the recovered 1800 Hz oscillator signal 140 is present at 
the signal input lead of detector 170. 
Because of the 90 degree phase shift, the local 1800 Hz signal 127 will be 
in phase with the recovered 1800 Hz oscillator signal 140 or 180 degrees 
out of phase with signal 140. The recovered oscillator signal 140 will 
therefore be synchronously detected by detector 170 with the result that 
the rectified form of the recovered 1800 Hz signal will be developed on 
the signal output lead of detector 170. The rectified 1800 Hz signal is 
indicated at 182 in FIG. 6. Half wave rectification is shown, but 
full-wave rectification of the recovered signal 140 may be established if 
desired. 
The detector switch 180 is turned on to conduct half cycles of the 
recovered 1800 Hz signal 140 by alternations of one selected polarity 
(e.g., positive polarity) of the locally generated 1800 Hz signal 127. 
Since signal 127 may be in phase with the recovered 1800 Hz signal 
140.degree. or 180.degree. with signal 140, then the detected signal 182 
may be either positive as shown or negative. Level detector 172, however, 
is insensitive to polarity as will be explained shortly. 
Level detector 172 may be of any suitable circuit design and is shown in 
the illustrated embodiment (see FIG. 8) to advantageously comprise a 
voltage divider 184 and a comparator 186. Voltage divider 184 is formed by 
a pair of resistors 188 and 189, the former being connected in the signal 
current path between the signal output lead of detector switch 180 and the 
negative input of comparator 186, and the latter being connected between 
the negative input of comparator 186 and the negative terminal of a 
suitable d.c. voltage source to divide down the incoming rectified signal 
182. 
As shown, the positive terminal of comparator 186 is connected to ground 
through a capacitor 190 so that the positive terminal is at a.c. ground to 
establish an a.c. ground potential reference. The rectified signal voltage 
developed at the junction between resistors 188 and 189 by the incoming 
signal 182 is compared with this reference. 
Since the voltage divider resistor 189 is connected to a negative source of 
d.c. voltage (e.g., -6 VDC), the voltage developed at the junction between 
the voltage dividing resistors 188 and 189 may be either negative or 
positive with respect to ground depending on the amplitude of the 
rectified signal 182. If the divided-down signal voltage at the negative 
input of comparator 186 is negative with respect to ground, the output 
voltage of comparator is pulled positive. If, on the other hand, the 
voltage at the negative input of comparator 186 is positive with respect 
to ground, the voltage at the output of comparator 186 will be pulled 
negative. 
For the given circuit design the difference in levels between the smaller 
amplitude 1800 Hz oscillator signal and the larger amplitude 1800 Hz CLT 
signal is such that the divided-down, signal voltage on the negative input 
of comparator 186 will be negative when only the smaller amplitude 1800 Hz 
signal 140 is synchronously detected by detector 170 and will be positive 
when the larger amplitude 1800 Hz signal (i.e., the one supplied from 
amplifier 174) is detected be detector 172. 
With switch SW1 open only the smaller amplitude 1800 Hz signal 140 will be 
present for detection by detector 170. The output of comparator 186 will 
therefore be positive when the channel loop test is not being conducted 
and will become negative when switch SW1 is closed to conduct the test. 
The negative voltage at the output of comparator 186 is used to cause the 
dial switch 157 to turn modulator 54a on while the channel 1 subscriber's 
telephone 40 is still on-hook. Dial switch 157 may be any suitable circuit 
design for this purpose and is normally under the control of the off-hook 
detector 156. However, when the output of comparator 186 is pulled 
negative, indicating detection of the stronger 1800 Hz signal which is 
used for the channel loop test, level detector 172 has the effect of 
overriding the control established by the off-hook detector 156 to cause 
switch 157 to turn on modulator 54a even though the subscriber's telephone 
is still on-hook. The positive voltage at the output of comparator 186 
will have no controlling effect on switch 157, thus leaving switch 157 
under the control of the off-hook detector 156 when only the weaker 1500 
Hz oscillator signal 140 is detected by detector 170. 
With switch SW1 open, therefore, dial switch 157 will remain under the 
control of the off-hook detector 156, thus keeping modulator 54a off while 
the channel 1 subscriber's telephone is on-hook. Accordingly, no carrier 
signal will be transmitted from the subscriber terminal circuit STU1 to 
feed the recovered 1800 Hz oscillator signal 140 back to the central 
office. In this regard, it will be noted that when the channel 1 
subscriber's telephone 40 is on-hook, the 1800 Hz oscillator signal 140, 
which is recovered in the receiver 28 of terminal circuit STU1, will be 
conducted to modulator 54a by way of hybrid 73a, compressor 50a and filter 
52a. However, the recovered 1800 Hz oscillator signal 140 will not be 
conducted back to the central office because modulator 54 will be held in 
its off condition by the operation of the off-hook detector 156 and dial 
switch 157. 
When the subscriber's telephone 40 is on-hook, the recovered 1800 Hz signal 
140, rather than being cancelled in hybrid 73a, will be conducted through 
the hybrid. The reason for this is that hybrid 73a will be terminated in 
the high impedance created by the open hook switch (indicated at 194) in 
telephone 40. As a result, the usual signal cancelling action will not 
occur in the autotransformer part of the hybrid on the subscriber's side 
of the hybrid. The recovered 1800 Hz oscillator signal 140 will therefore 
pass through the subscriber's side of hybrid 73a from the receiver to the 
transmitter without cancellation. 
When switch SW1 is closed to conduct a channel loop test on channel 1, the 
larger amplitude 1800 Hz signal is bypassed around switch 102 and is 
conducted to the input lead of compressor 50 in the central office 
terminal circuit COT1. From there, this larger amplitude 1800 Hz signal 
(hereinafter referred to as the 1800 Hz CLT signal) is conducted through 
the compressor and filter 52 to modulator 54 where it amplitude modulates 
the carrier signal (138) being transmitted from terminal circuit COT1. 
Upon reception and detection of the modulated carrier signal 138 in the 
subscriber terminal circuit STU1, the 1800 Hz CLT signal is recovered and 
fed to the synchronous detector 170 as well as the phase locked loop 116 
which is already locked with the 1800 Hz oscillator signal 140. 
As a result, the 1800 Hz CLT signal will be synchronously detected by 
detector 170. When this happens the amplitude of the detected or rectified 
1800 Hz signal (originally indicated at 182 in FIG. 6) will be increased 
by virtue of the 10 db difference between the 1800 Hz CLT signal and the 
weaker 1800 Hz oscillator signal (140) at the output of amplifier 68a. 
The increased amplitude of the resulting rectified 1800 Hz signal at the 
negative input of comparator 186 is sufficient to make the rectified 
signal more positive than the ground reference after it is divided down by 
divider 184 in the manner previously explained. 
Upon reception and detection of the stronger 1800 Hz CLT signal, therefore, 
the voltage at the output of comparator 186 will be pulled negative, 
thereby causing dial switch 157 to turn on modulator 54a. When this 
happens, the subscriber carrier signal will be transmitted from the 
subscriber terminal circuit STU1 and will be amplitude modulated by the 
1800 Hz CLT signal which is conducted through receiver 28, the 
subscriber's side of hybrid 73a, compressor 50a, and filter 52a. 
Upon transmitting the modulated carrier signal from terminal circuit STU1, 
it will be conducted up line 22 to the central office terminal equipment 
where it is received and detected in the companion central office terminal 
circuit COT1. The 1800 Hz CLT signal will therefore be recovered at the 
output of detector 64 and will be conducted through filter 66, attenuator 
75, amplifier 68 and the carrier-sensing relay driver 124. 
From relay driver 124, the recovered 1800 Hz CLT signal is conducted to the 
channel loop test level detector 176 where its level is compared with a 
reference or threshold voltage. If the 1800 Hz CLT signal has been 
conducted through circuits that are in satisfactory working order, its 
level will be above the reference level or voltage in detector 176. This 
satisfactory condition will be sensed by detector 176 which may be 
equipped with a signalling device (e.g., a lamp) to indicate the 
satisfactory condition. 
The level detector 176 will also sense the unsatisfactory condition in 
which the level of the returned 1800 Hz CLT signal is lower than the 
pre-selected threhold as well as the unsatisfactory condition in which the 
1800 Hz CLT signal is not returned to the CLT level detector. In such a 
case the CLT detector circuitry is such that it will prevent illumination 
of the signalling lamp mentioned above, and it may also be equipped with 
additional indicating lamps or other signalling devices that give an 
positive indication of a faulty condition. 
From the foregoing it will be appreciated that the channel loop test 
operation just described is effective to check the signal paths in the 
channel terminal circuits for continuity and proper audio level. If there 
is a break in the signal path that the 1800 Hz CLT signal follows, the 
1800 Hz signal will not be returned to the CLT level detector 176, and 
this faulty condition is sensed and indicated by detector 176 as explained 
above. Detector 176 will also sense and indicate an unsatisfactory audio 
level condition in which the level of the returned 1800 Hz CLT signal is 
too low. 
It will also be appreciated that the addition of a relative small number of 
inexpensive circuits and components affords an effective channel loop test 
in which the 1800 Hz CLT signal is conducted through the complete 
transmitter in the central office terminal circuit, through the 
transmission line 22 in the central office-to-subscriber direction, 
through the complete receiver in the companion subscriber channel terminal 
circuit, through the subscriber side of the hybrid in the subscriber 
channel terminal circuit, through the complete transmitter in the 
subscriber channel terminal circuit, through the transmission line in the 
subscriber-to-central office direction, and through the complete receiver 
in the receiving central office terminal circuit. Accordingly, the 
transmission line and all of the transmitters and receivers in the central 
office and subscriber channel terminal circuits will be checked with the 
singular exception of the hybrid in the central office terminal circuit. 
In addition to filter 150, ring circuit 106 includes an operational 
amplifier 200 and a pair of darlington circuits 201 and 202. In this 
embodiment, filter 150 comprises a capacitor 204 and a resistor 206 
connected in the manner shown. 
The filtered ringing signal 152 at the output of filter 150 is coupled by a 
capacitor 208 to amplifier 200 for amplification. Resistors 210 and 211 
set the gain of amplifier 200, and resistor 212 provides bias for the 
amplifier. Capacitor 208 has the effect of shifting the signal voltage 
waveform so that it will have negative and positive alternations rather 
than being all positive. The shifted waveform is indicated at 152' in FIG. 
7. The positive and negative peak voltages of signal 152' are of equal 
absolute magnitude as shown. 
The signal voltage 152" at the output of amplifier 200 will be the same as 
the input signal voltage 152' except that the former will be voltage 
amplified. The signal voltage at the output of amplifier 200 therefore has 
the same positive and negative alternations as the input signal voltage 
152'. 
From the output of amplifier 200 the amplified ringing signal 152" is 
coupled by a capacitor 214 to the base of the input transistor Q1 in 
darlington 201 and by a separate capacitor 216 to the base of the input 
transistor Q3 in Darlington 202. The output transistors for Darlingtons 
201 and 202 are indicated at Q2 and Q4, respectively. 
As shown, the collectors of the output transistors Q2 and Q4 are d.c. 
coupled together through a diode 218, and these coupled collectors develop 
the amplified local ringing signal 154 which is conducted through a 
resistor 220 to the ring side of drop 42 and hence to the ring terminal of 
telephone 40. 
Emitter bias for the transistors in darlington 201 is supplied from the 
negative terminal of a suitable d.c. voltage source VDC', and emitter bias 
for the transistors in Darlington 202 is supplied from the negative 
terminal of a separate d.c. voltage source VDC". 
The transistors in darlington 201 are of the PNP type and are therefore 
turned on by the negative alternations of signal 152" and turned off by 
the positive alternations of signal 152". Conversely, the transistors Q3 
and Q4 in darlington 202 are of NPN type and are consequently turned on by 
the positive alternations of signal 152" and turned off by the negative 
alternations of signal 152". Darlingtons 201 and 202 therefore conduct 
alternately in a complementry fashion. 
When transistor Q2 is on and transistor Q4 is off for the negative half 
cycle of signal 152", the output ringing signal voltage (indicated at 154 
in FIG. 7 and appearing at the junction 222 of the collector leads for 
transistors Q2 and Q4) will approach the value of the negative d.c. 
voltage -VDC' which is made significantly less negative than the negative 
d.c. -VDC". For example, -VDC' may be about -16 VDC and -VDC" may be about 
-200 VDC to provide the ringing signal voltage 154 with sufficient 
amplitude for operating the ringer 155 in telephone 40. 
When transistor Q4 is turned on and transistor Q2 is turned off for the 
positive half cycle of signal 152" the ringing signal voltage 154 will 
approach the value of the negative d.c. voltage -VDC" which may be about 
-200 volts as indicated above. The a.c. waveform of ringing signal voltage 
154 is therefore square, has the same frequency as signal voltages 152 and 
152' and has negative peak amplitude values approaching VDC' and VDC". The 
local ringing signal voltage 154 effectively represents a voltage 
amplification of signal voltage 152". In essence, the combined circuit of 
darlingtons 201 and 202 operates as an overdriven amplifier. 
From the foregoing description it is apparent that darlingtons 201 and 202 
combine to operate as a switching circuit in which the negative voltage 
sources -VDC' and VDC" are alternately and cyclically switched in to 
establish the local ringing voltage 154. 
Due to capacitor 208, ring circuit 106 is insensitive to the polarity of 
the detected alerting signal 144 which is fed into filter 150 from 
detector 104. 
One advantage of using the Q2 and Q4 collectors as outputs for Darlington 
circuits 201 and 202 is that there will be no collector current flow and 
consequently no waste of power during the ring circuit's idle condition 
when no alerting signal is detected. In this regard it will be noted that 
both of the Darlingtons will be turned off in absence of a detected signal 
at the input of filter 150. 
Characteristic of operational amplifiers, the peak voltage of signal 152" 
at the output of amplifier 200 will be limited by the magnitude of d.c. 
voltage supply which is applied for operating the amplifier. For example, 
the peak output voltage will be about 5 volts. 
By virtue of the circuit connections between amplifier 200 and the emitter 
of transistor Q4, the emitter voltage of transistor Q4 will, in turn, be 
limited by the voltage at the output of amplifier 200 and will be equal to 
the voltage at the output of amplifier 200 less the voltage drops 
occurring along the signal current path that passes through the 
base-emitter junction of transistor Q4. Thus, the emitter voltage of Q4 
will be about 1 volt less than the voltage at the output of amplifier 200 
owing mainly to the drop across the base-emitter junction of transistor 
Q4. 
By limiting the emitter voltage in this manner, the emitter current drawn 
by transistor Q4 will also be limited and will depend upon the selected 
value of the transistor's emitter resistor 224. By simply making the size 
of resistor 224 large enough the emitter current can be kept below an 
excessive value that might cause damage to the Darlington's transistor. In 
this way, the Darlington circuit 202 has a built-in current-limiting 
feature to permit relatively cheap bipolar transistors to be used in the 
Darlington circuit. It will be appreciated that Darlington 201, being of 
the same circuit design and having the same connections as Darlington 202, 
has the same built-in current limiting feature as the one just described 
for Darlington 202. The emitter resistor for Darlington 201 is indicated 
at 226 in FIG. 8. 
Still referring to FIG. 8, ring circuit 106 is provided with diodes 228 and 
230 which are connected in the manner shown to provide protection against 
current surges that result from lightning. Diodes 228 and 230 will become 
forward biased by these surges to conduct the surges to ground and thereby 
prevent them from forward biasing transistors Q2 and Q4. 
In particular, diode 228 will be forward biased by one polarity of the 
surge voltage to clamp junction 222 to the negative voltage of source 
-VDC". This keeps the collector of transistor Q4 from becoming forward 
biased to limit the excursion in one direction. 
At the same time, diode 228 will prevent the collector-emitter voltage of 
transistor Q4 from exceeding -200 volts or whatever is selected for -VDC". 
Diode 230 will be forward biased on the opposite polarity of the surge 
voltage, and when it becomes forward biased, it will clamp junction 222 to 
ground, thereby preventing the collector-emitter voltage from exceeding 
the voltage supply of -200 volts. 
When telephone 40 is brought off-hook to close hook switch 194, direct 
current will flow in drop 42 to cause diode 218 to become reverse biased. 
Reverse biasing of diode 218 prevents leakage of current from the ring 
side of the line, thereby preventing the occurrence of an unbalanced 
condition. 
In the illustrated embodiment, modulator 54a is shown in FIG. 8 to of the 
differential amplifier type having a pair of emitter coupled NPN 
transistors Q5 and Q6. The audio signal from filter 52a is fed to the 
coupled emitters of transistors Q5 and Q6 to modulate the carrier 
frequency oscillator carrier signal which is continuously and 
differentially applied to the bases of transistors Q5 and Q6 even when the 
modulator is turned off and not transmitting. The modulated carrier signal 
is taken from the collectors of transistors Q5 and Q6 as shown. 
Still referring to FIG. 8, the dial or off-hook switch 157 comprises a pair 
of transistors Q7 and Q8. The emitter of transistor Q7 is connected to the 
negative terminal of the d.c. voltage source VDC. The collector of 
transistor Q7 feeds the coupled emitters of the modulator's transistors Q5 
and Q6. Transistor Q7 operates as switch for controlling the supply of 
emitter current to transistors Q5 and Q6 and is turned on and off by 
transistor Q8. 
As shown, the collector of transistor Q8 feeds the base of transistor Q7. 
When transistor Q8 is turned off its collector voltage rises to turn on 
transistor Q7. When transistor Q7 is turned on it conducts current to the 
emitters of transistors Q5 and Q6, causing the carrier signal to be 
transmitted from the collectors of transistors Q5 and Q6. When transistor 
Q5 and Q6 are in this state, being fed with emitter current from 
transistor Q7, modulator 54a will turn on to transmit the carrier signal. 
When transistor Q8 is turned on, transistor Q7 will turn off, thus 
interrupting the supply of emitter current for transistors Q5 and Q6. 
Under this condition modulator 54a will be turned off in its 
non-transmitting state in which no transmission of the carrier signal 
occurs. 
As shown in FIG. 8, the emitter of transistor Q8 is also connected to the 
negative terminal of voltage source VDC. Transistor Q8 is turned on and 
off by changing its base voltage. 
Still referring to FIG. 8, the collector of a further transistor Q9 feeds 
the base of transistor Q8. Transistor Q9 forms a part of the off-hook 
detector 156. When hook switch 194 is closed by transferring telephone 40 
to its hook-off state, the circuit will be completed for conducting direct 
current through the ring and tip leads of drop 42. This direct current is 
supplied from the collector of a transistor Q10 which is connected in the 
manner shown to establish a current source 234. 
Emitter current for transistor Q10 is supplied by voltage source -VDC'. 
Upon closing hook switch 194 the d.c. collector current supplied by 
transistor Q10 will be conducted through a resistor 236, the lower coil of 
hybrid 73a on the subscriber's side of the line, the ring side of drop 42, 
hook switch 194, the tip side of drop 42 and the upper hybrid coil on the 
subscriber's side to ground. 
As shown the emitter of transistor Q9 is connected to the collector of 
transistor Q10 at the junction between one terminal of resistor 236 and 
the collector of transistor Q10. The other terminal resistor 236 is 
connected by a base biasing resistor 238 to the base of transistor Q9. 
Before hook switch 194 is closed by bringing telephone 40 off hook, 
transistor Q9 will be turned off. As a result no collector current will be 
supplied by transistor Q9 for conduction through diodes 239 and 240 and a 
resistor 242 to ground. Transistor Q8 will therefore be in its conductive 
state (which is its normal condition) in which base current is conducted 
through a further diode 244. 
With transistor Q8 turned on, transistor Q7 will be turned off. Modulator 
54a will therefore be turned off to prevent the transmission of the 
carrier signal from the subscriber channel terminal circuit. 
When hook switch 194 is closed by bringing telephone 40 off-hook, the 
circuit is completed for conducting direct current from source 234 through 
the ring and tip-sides of drop 42 as previously explained. Transistor Q9 
will therefore be biased on, and since the collector current of transistor 
Q10 is the emitter current for transistor Q9, the voltage on the collector 
of transistor Q9 will be pulled more negative than the d.c. voltage that 
is applied by source VDC to the emitter of transistor Q8. In this regard 
it will be noted that the -VDC' voltage is more negative than the negative 
voltage of source VDC. For example, the former may be -16 VDC while the 
latter may be -6 VDC. 
When transistor Q9 is turned on, therefore, the base voltage for transistor 
Q8 will be pulled more negative than the emitter voltage of transistor Q8. 
When this happens, transistor Q8 will turn off, causing transistor Q7 to 
turn on. As a result modulator 54a turns on to transmit its carrier signal 
back to the central office. From this description it will be appreciated 
that transistor Q9 operates as a current detector for detecting or sensing 
the flow of direct current (also called loop current) in drop 42. 
As shown in FIG. 8, the collector of transistor Q9 is connected through a 
resistor 245 to the cathodes of two additional diodes 246 and 248, and the 
anodes of diodes 246 and 248 are respectively connected to the control 
leads of detector switches 180 and 149. These diodes are used in 
conjunction with resistor 245 and two additional resistors 250 and 252 to 
keep detector switches 149 and 180 from conducting when telephone 40 is 
brought off hook. This operation provides a local ring trip to remove the 
local ring signal voltage 154 and also the 1800 Hz signal upon 
transferring telephone 40 to its off hook state as will now be described. 
When telephone 40 is on-hook to place transistors Q9 in its non-conducting 
state, a capacitor 251 (FIG. 8) will be discharged to a value that reverse 
biases diode 246 and 248. As a result, the on-off operation of detector 
switch 180 will be under the control of the locally generated 1800 Hz 
signal voltage 127 and the operation of detector switch 149 will be under 
the control of the locally generated 900 Hz signal voltage 129. The peak 
voltages of these two signals are typically .+-.5 volts. 
When telephone 40 is brought off-hook, causing transistor Q9 to conduct, 
the collector voltage of transistor Q9 will be sufficiently negative to 
cause diode 246 to become forward biased on the positive alternations of 
the locally generated 1800 Hz detector switching signal and to also cause 
diode 248 to become forward biased on the positive alternations of the 
local 900 Hz switching signal. When diodes 246 and 248 become forward 
biased a voltage divider 263 (see FIG. 8) will be established by resistor 
245 and the parallel combination input resistors 254 and 255, the former 
being in the current path to the control lead of detector switch 149, and 
the latter being in the current path to the control lead of detector 
switch 180. 
With diodes 246 and 248 forward biased, the voltage divider 253 will 
operate to hold the voltage on the control leads of detector switches 149 
and 180 negative at about the value which is set by divider 253 at 
junction 257. This negative voltage on the control leads of switches 149 
and 180 prevents the switches from conducting, it being recalled that 
switch 149 and 180 will conduct only when the control lead voltages become 
positive. 
By holding detector switch 149 off, the local ring signal voltage 154 will 
be removed to provide the local ring trip. By holding detector switch 180 
off, any chance of the 1800 Hz signal being heard by the interconnected 
parties is avoided. 
As shown in FIG. 8, diodes 258, 242 and 244 are connected in series between 
the output of comparator 186 and the base of transistor Q8 in dial switch 
157. When level detector 172 fails to sense the stronger 1800 Hz CLT 
signal, the voltage at the output of comparator 186 will be positive to 
reverse bias diode 258. Accordingly, the output of comparator 186 will 
have no effect on the base voltage of transistor Q8. 
When level detector 172 senses the stronger 1800 Hz CLT signal the voltage 
at the output of comparator 186 is pulled negative to forward bias diode 
258. This negative voltage at the output of comparator 186 will be about 
-5.4 volts for a 6 volts power supply. When diode 258 becomes forward 
biased, therefore, the voltage at the anode of diode 240 will become 
clamped to approximately -4.8 volts, for the -5.4 volts comparator output 
mentioned above. 
The -4.8 volts at the anode of diode 240 is not positive enough to keep 
transistor Q8 in conduction. As a result, transistor Q8 will turn off when 
the stronger 1800 Hz CLT signal is received. 
When transistor Q8 is turned off, transistor Q7 turns on to turn on 
modulator 54a as previously explained. The carrier signal will therefore 
be transmitted from the subscriber terminal circuit, and 1800 Hz CLT 
signal, which is received and conducted to modulator 54a by way of hybrid 
73a, will be applied to modulate the transmitted carrier. 
When the larger amplitude 1800 Hz CLT signal is removed by opening switch 
SW1 at the end of the channel loop test, the voltage at the output of 
comparator 186 will resume its positive value, thus reverse biasing diode 
258 and allowing transistor Q8 to turn on again, provided that transistor 
Q9 is non-conducting. 
In the embodiment shown in FIG. 11, the darlington circuits 201 and 202 are 
replaced by a relay switching circuit 270. The two ring circuits shown in 
FIGS. 8 and 11 are otherwise the same, and to the extent that they are, 
like reference numerals have been applied to designate like components. 
As shown in FIG. 11, the relay switching circuit 270 comprises a pair of 
relays RY2 and RY3 which combine to form an A type relay. The windings for 
relays RY2 and RY3 are each connected between the output of amplifier 200 
and ground. Relay RY2 has a set of normally open contacts RY2-1, and relay 
RY3 also has a set of normally open contacts RY3-1 as shown. 
A diode 272 in series with the winding of relay RY2 allows current flow in 
only one direction through the winding of relay RY2. Diode 272 is poled so 
that relay RY2 is energized only by the positive alternations of the 
signal voltage 152" at the output of amplifier 200. 
Another diode 274 connected in series with the winding of relay RY3 is 
poled oppositely with respect to diode 272. Relay RY3 will therefore be 
energized only on the negative alternations of the signal voltage 152" at 
the output of amplifier 200. Relays RY2 and RY3 therefore operate in a 
complementry fashion to alternately close contacts RY2-1 and RY3-1. 
As shown, the movable elements of contacts RY2-1 and RY3-1 are connected 
together to feed a common pin or junction 276 which is connected through a 
resistor to the ring side of drop 42. 
When contacts RY2-1 close, they connect the -VDC" voltage source to 
junction 276 to apply a suitable negative d.c. voltage such as -200 volts 
to junction 276. When contacts RY3-1 close, they connect the -VDC' voltage 
source to junction 276 to apply the less negative voltage (e.g., -16VDC) 
to the junction. The alternate switching of the two d.c. voltage sources 
establishes the local ringing signal voltage 154 which is applied to 
operate the ringer 155 in telephone 42. Diodes 278 and 280, which are 
connected in the manner shown, provide arc and lightning supression. 
If telephone 40 is brought off-hook during the ring cycle (i.e., when 
contacts RY2-1 are closed) excessive current may be drawn from the source 
VDC" due to the low impedance load that is established by closure of the 
hook switch 194. This high current may cause damage to and premature 
failure of contacts RY2-1. 
To avoid this objectionable condition a special current limiter 282 is 
connected between the negative terminal of the d.c. voltage source VDC" 
and the power input pin or terminal 284 of the relay switching circuit 280 
as shown. Pin 284 connects to the stationary contact element of the 
contact set RY2-1 to apply the -200 volts. Limiter 282 operates to limit 
the direct current that can be conducted through contacts RY2-1 and the 
load established by telephone 40. 
Current limiter 282 is common to all of the subscriber terminal circuits 
STU1-STU8 and is used to supply direct current to all of the relay 
switching circuits (270) in terminal circuits STU1-STU8. 
As shown in FIG. 11, current limiter 282 has a simplified inexpensive 
circuit design and mainly comprises a bipolar NPN transistor Q11 and a 
pair of diodes 288 and 290. The negative terminal of souce VDC" is 
connected to the emitter of transistor Q11 through an emitter resistor 
292. Diodes 288 and 290 are connected in series between the base of 
transistor Q11 and the negative terminal of source VDC". A resistor 294 
connected between ground and the base of transistor Q11 feeds the 
transistor's base with biasing current. 
The collector of transistor Q11 is connected to and feeds the input 
terminal 284 of the relay switching circuit 270 as shown. Transistor Q11 
is normally conducting to feed collector current to contacts RY2-1 as long 
as the transistor's emitter current is below a predetermined value. 
For the circuit connections shown, the voltage on the base of transistor 
Q11 will normally be about 0.6 volts less negative than the voltage on the 
emitter of the transistor Q11. The transistor's emitter voltage is 
determined by the drop across the emitter 292 and hence by the size of 
resistor 292 and the amount of emitter current drawn by the transistor. 
The amount of emitter current will depend upon the amount of collector 
current, and the amount of collector current in turn will vary inversely 
with the impedance of the load established in telephone 40. The 
transistor's collector current and emitter current may be regarded as 
approximately equal. 
As the amount of emitter current drawn by transistor Q11 increases, the 
voltage drop across emitter 292 increases, thus making the transistor's 
base voltage less negative. If, for example, the emitter resistor voltage 
drop increases from 0.1 volt to 0.2 volt due to an increase in the 
transistor's emitter-collector current, then the transistor's base voltage 
will become less negative changing from -199.3 volts to -199.2 volts. 
Because of the connections of diodes 288 and 290, however, the difference 
between the voltage at the base of transistor and the fixed voltage of 
-200 volts at source VDC" cannot exceed approximately 1.2 volts, there 
being a virtually fixed drop of about 0.6 volt across each diode. In other 
words, the transistor's base voltage cannot become less negative than 
about -198.8 volts because of diodes 288 and 290. 
As the sum of the transistors base-emitter voltage drop of about 0.6 volts 
and the emitter resistor voltage drop approaches the maximum 1.2 volt drop 
allowed by diodes 288 and 290, transistor Q11 will conduct less and will 
begin to turn off. As a result, the collector current pulled by transistor 
Q11 will reach a limit that cannot be exceeded. 
Assume that telephone 40 is brought off hook to close hook switch 194 at 
the moment contacts RY2-1 are closed. The collector-emitter current of 
transistor Q11 will be conducted through the closed hook switch and the 
relatively light load may tend to draw a large enough current that would 
produce a 0.8 volt drop across resistor 292. The difference left over 
between 0.8 volt and the 1.2 volt maximum is not enough base-emitter 
forward bias for causing transistor Q11 to conduct. 
In the foregoing manner limiter 282 operates to limit the magnitude of 
current that the telephone load can draw and prevents the flow of 
excessive current and consequently damage to the relay contacts. 
Because of the circuit design for limiter 282, the collector of transistor 
Q11 may be connected to each of the corresponding power input terminals 
(284) in the other subscriber terminal circuits STU2-STU8 to limit the 
supply of current to the relay switching circuits (270) in the other 
subscriber terminal circuts. Current limiter 282 also has the effect of 
saving power. For this purpose a corresponding current limiter (not shown) 
may be connected to supply current to the -16VDC power input terminal 298 
in the relay switching circuit 270 for each of the subscriber terminal 
circuits. 
From the previous description of the signalling circuitry it will be noted 
that if the telephone 40 for one of the subscriber's is off-hook at the 
time that a call for him arrives at the central office, the calling party 
will receive a busy signal and the central office equipment will not 
operate to connect the central office ringing generator (e.g., generator 
132) to the called subscriber's central office channel terminal circuit. 
As a result the switch 100 in the called subscriber's central office 
channel terminal circuit will not conduct the 900 Hz signal. 
When the subscriber is off-hook at the time a call for him comes into the 
central office, therefore, neither the 1800 Hz signal nor the 900 Hz will 
be applied to modulate the carrier signal that is transmitted from the 
off-hook subscriber's central office channel terminal circuit. 
Accordingly, no 900 Hz signal will be recovered in the off-hook 
subscriber's subscriber channel terminal circuit, so that no 900 Hz 
synchronous detection occurs. Furthermore, the previously described local 
ring trip action will keep the off-hook subscriber's ring detector 104 
from synchronously detecting the 900 Hz if for some reason it happens to 
be present. In either case, ringing of a subscriber's telephone 40 will 
not take place if the telephone is off-hook. 
It also will be appreciated that the purpose of the phase locked loop 116 
is to establish the recovered 1800 Hz signal apart from the other 
components that result from demodulation of the central office carriers. 
In this way, only the 900 Hz frequency will be applied to the control lead 
of each detector switch (149 and 180) in each of the subscriber channel 
terminal circuits to properly facilitate the synchronous detection of the 
desired signals. 
It also will be noted that each pair of companion central office and 
subscriber terminal circuits (e.g., COT1 and STU1, COT2 and STU2, and so 
on) and the common transmission line 22 combine to represent a separate 
carrier circuit over which intelligence and other signals (e.g., the 
oscillator and alerting signals 140 and 136) are transmitted by means of a 
carrier wave. 
From the foregoing description it will also be appreciated that one of the 
important features of the embodiment shown in FIGS. 1-8 is a signalling 
operation whereby two signals are applied to modulate just one carrier 
signal, namely the central office-transmit carrier signal that is assigned 
to the called subscriber when a call arrives for him at the central 
office. 
In the illustrated embodiment, these modulating signals are of different 
pre-selected frequencies, one frequency being an integral multiple of the 
other. Alternatively, two signals of the same pre-selected frequency could 
be used to modulate the called subscriber's central office-transmit 
carrier signal. This alternate signalling mode may be accomplished by 
making the levels of the two modulating signals different from each other 
when a call for the subscriber arrives at the central office and by 
sensing the level difference at the called subscriber's subscriber channel 
terminal circuit following synchronous detection of one of the two signals 
similar to the previously described channel loop test operation. 
In the embodiment shown in FIGS. 12 and 13 only one ringing or alerting 
signal is utilized to alert a called subscriber to an incoming call rather 
than using two signals to signal the incoming call as disclosed in the 
preceding embodiment of FIGS. 1-8. To accomplish this a narrow bandpass 
filter 310 (see FIG. 13) is added to each of the subscriber terminal 
circuits STU1-STU8, and the frequency divider 118 is eliminated from the 
group terminal that is common to the subscriber terminal circuits. 
Additionally, the central office terminal equipment is modified by 
eliminating the switch 102 from each of the central office terminal 
circuits COT1-COT8 and by using just one amplifier 312 (see FIG. 12) in 
place of the two amplifiers 110 and 174 that are used in the preceding 
embodiment. Apart from these changes the central office and subscriber 
terminal equipment for the embodiment of FIGS. 12 and 13 is the same as 
that shown in the embodiment of FIGS. 1-8. 
To the extent that the embodiment of FIGS. 12 and 13 is the same as the 
preceding embodiment shown in FIGS. 1-8 like reference numerals have been 
applied to designate like circuits and components. 
As shown in FIG. 13, filter 310 may be of the active type having an 
amplifier and a suitable reactive feedback 312. The output of the AGC 
amplifier 68a is connected to the input of filter 310. Filter 310 is tuned 
to pass only the 900 Hz alerting signal. 
The output of filter 310 is connected to both the signal input lead and the 
switching control lead of the synchronous ring detector 104 to thereby 
apply the same signal to both leads. In the embodiment of FIG. 13 detector 
104 may be a FET (field effect transistor) switch 314 rather than the 
particular type of analog switch described in the preceding embodiment. 
As shown, the output of filter 310 is connected to both the gate and drain 
electrodes of FET 314 to apply the same 900 Hz signal. A single signal 
voltage, rather than two separate signals, is therefore applied to the 
drain and gate of the FET synchronous detector. The circuit may be 
designed in such a manner that FET 314 is normally non-conducting and is 
turned on only by the positive alternations of the 900 Hz alerting 
signals. Alternatively, FET 314 may be of the type that is turned on only 
by the negative alternations of the 900 Hz alerting signal. In either case 
the 900 Hz signal will synchronously detect itself. 
In the circuit shown in FIG. 13, the detected or rectified 900 Hz signal 
appears on the source of FET 314 and is fed to the ring circuit 106 for 
operating the ring circuit 106 in the previously described manner to 
generate the a.c. voltage that is used to ring the called subscriber's 
telephone. 
The signalling operation for the embodiment shown in FIGS. 12 and 13 will 
now be considered, using the example of the channel 1 subscriber who is 
served by the channel terminal circuits COT1 and STU1. 
When an incoming call arrives at the central office for the subscriber 
served by the channel terminal circuits COT1 and STU1 the central office 
relay 130 is operated as previously described to connect ring generator 
132 to the ring terminal 126 of the central office terminal circuit COT1. 
The central office ringing signal supplied by ring generator 132 will 
therefore be applied to switch 100, causing switch 100 to be turned on by 
the positive alternations of the ringing signal 134 and to be turned off 
by the ringing signal's negative alternations as previously explained. 
The 900 Hz alerting signal will therefore be conducted to modulator 54 only 
during the positive alternations of the central office ringing signal 134. 
As a result, conduction of the 900 Hz alerting signal will be periodically 
interrupted at the central office ringing signal rate or frequency during 
the ringing intervals of ringing signal 134. The channel 1 carrier signal 
will therefore be amplitude modulated periodically by the interrupted 
alerting signal at the rate equal to the frequency of the central office 
ringing signal 134. 
In the embodiment of FIGS. 12 and 13 the interrupted 900 Hz alerting signal 
will be the only signal applied to modulate the called subscriber's 
carrier signal to alert the subscriber to an incoming call. This 
signalling operation is in contrast to the one described in the embodiment 
of FIGS. 1-8 where the called subscriber's carrier signal is modulated by 
two signals to alert the called subscriber to an incoming call. In the 
embodiment shown in FIGS. 12 and 13 the 1800 Hz oscillator signal will 
only be applied when one or more of the switches SW1-SW8 is selectively 
closed to conduct a channel loop test. 
Upon receiving and detecting the called subscriber's modulated carrier 
signal in terminal circuit STU1, the interrupted 900 Hz alerting will be 
recovered and passed by filter 310 which acts to separate the 900 Hz 
signal from the other VF components of detection that pass through the low 
pass filter 66a. The 900 Hz signal will therefore be applied to FET 314 to 
synchronously detect itself. 
The waveform of the resulting rectification will be the same as the 
detected signal 144 and will be applied to ring circuit 106 for developing 
the ringing voltage 154 for operating the called subscriber's ringer. 
As shown in FIGS. 12 and 13 the circuitry for conducting the channel loop 
test may be the same as that shown in the embodiment of FIGS. 1-8. 
Operation of this circuitry is also the same as that previously described 
and is initiated by closing any selected one of the switches SW1-SW8, 
depending upon the channel to be tested. 
As shown in FIG. 13 the output of the off-hook detector 156 is connected to 
the gate of FET 314 to prevent FET 314 from conducting and to thus squelch 
the interrupted alerting signal when the subscriber's telephone is brought 
off-hook. 
The embodiment shown in FIG. 14 is the same as that shown in FIG. 13 except 
that the sychronous detector 104 is replaced by a envelope detector 319 
and a separate ring squelch 322. Detector 319 comprises a diode 320 
connected in the signal current path at the output of filter 310 to detect 
the recovered 900 Hz alerting signal and thereby provide a rectification 
that is the same as the one indicated at 144 in FIG. 7. 
Ring squelch 322 is connected between diode 320 and ring circuit 106 and 
may comprise a FET 324. As shown the source and drain electrodes of FET 
324 are connected in series between diode 320 and the input of ring 
circuit 106. The output of the off-hook detector 156 is connected to the 
gate electrode of FET 324. 
When the subscriber's telephone is on-hook the voltage applied to the gate 
of FET 325 will be such to cause the FET 325 to conduct, thus providing 
for the conduction of the detected or rectified 900 Hz alerting signal 144 
to ring circuit 106. When the off-hook detector 156 senses the off-hook 
condition of the subscriber's telephone it causes the voltage on the gate 
of FET 324 to go negative sufficiently to turn off FET 324. When this 
happens the circuit between diode 320 and ring circuit 106 will open, thus 
squelching the 900 Hz alerting signal and thereby preventing its 
application to ring circuit 106. 
In all three of the embodiments shown in FIGS. 3, 13 and 14 will be 
appreciated that the central office ringing frequency information will be 
transmitted to provide the ringing signal voltage 154 with a frequency 
which is the same as that of the central office ringing signal 134, 
whatever the frequency of signal 134 may be. 
Like the systems shown in FIGS. 12-14, only one signal, namely the 
interrupted alerting signal, is used in the embodiment of FIGS. 15 and 16 
to alert a subscriber to an incoming call. In the system shown in FIGS. 15 
and 16, the phase locked loop and frequency divider 118 are not used to 
supply the synchronous signal for synchronously detecting the transmitted 
alerting signal as described in the first embodiment of FIGS. 1-8. 
Instead, the subscriber terminal equipment shown in FIGS. 15 and 16 is 
equipped with a narrow bandpass filter 330, a special phase-correcting 
type of phase locked loop 332 and a 90.degree. phase shifter 334 for 
supplying the synchronous signal. To the extent that the embodiments of 
FIGS. 1-8 and FIGS. 15 and 16 are alike, like reference characters have 
been applied to designate like circuits and components. 
As shown, filter 330, loop 332 and phase shifter 334 all form a part of the 
subscriber group terminal equipment which is common to all of the 
subscriber terminal circuits STU1-STU8. Filter 330 may advantageously be 
of the active type such as filter 310 and is tuned to the frequency of the 
alerting signal that is used to signal an incoming call. The alerting 
signal frequency separated by filter 330 is used to operate the 
synchronous detectors 104 in the subscriber terminal circuits as will be 
described in greater detail shortly. 
At resonance (i.e., at the alerting signal frequency) the circuit of filter 
330 is essentially resistive. However, for off-resonant frequencies filter 
330 will either be capacitive or inductive. In situations where the 
incoming alerting signal is not precisely or closely at the resonant 
frequency of filter 330, it will therefore undergo a phase shift upon 
being conducted through the filter 330. The phase-correcting loop 332 is 
used to automatically correct or adjust for this undesirable phase shift. 
Loop 332 differs from an ordinary phase locked loop in that is has no 
oscillator and instead uses a controllable phase shifting circuit 335 in 
place of an oscillator. In this regard it will be noted parenthetically 
that a conventional phase locked loop typically includes a voltage 
controlled oscillator. 
In addition to the controllable phase shifting circuit 335 loop 332 
includes a phase detector 336 and a low pass filter 338 which may be of 
the active type. Phase detector 336 and filter 338 may be the same as or 
similar to the type of phase detector and low pass filter that are found 
in the ordinary type of phase locked loop having a voltage controlled 
oscillator. 
The outputs of the AGC amplifier 68a in all of the subscriber terminal 
circuits STU1-STU8 are OR'D or otherwise combined together to feed the 
input of filter 330 and also the signal input 341 of phase detector 336. 
When the alerting signal is received and recovered at any one or more of 
the subscriber terminal circuits STU1-STU8 it will therefore be fed to 
filter 330 and input 341 of detector 336. Filter 330 passes the alerting 
signal while rejecting or attenuating to frequency dependent degrees 
signals of all other frequencies. The output of filter 330 feeds the 
signal input of the phase shifting circuit 335 as shown. 
The output of the controllable phase shifting circuit 335 is fed to phase 
shifter 334 and from there to the synchronous signal input (indicated at 
343 in FIG. 16) of the ring detector 104 in each of the subscriber 
terminal circuits STU1-STU8. In addition, the signal at the output of the 
phase shifting circuit 335 is fed back in loop 332 to a further input of 
the phase detector 336 which is separate from the signal input 341. Phase 
detector 336 may be a FET (a field effect transistor) and compares the 
phase of the signal voltage at the output of the phase shifting circuit 
335 with the signal voltage at the signal input 341 to develop an error 
correction voltage. The error correction voltage is a measure of the phase 
difference between the signal voltage at the output of the phase shifting 
circuit 335 and the signal voltage at input 341 and hence at the output of 
the AGC amplifier 68a. The error correction signal at the output of phase 
detector 336 is applied to filter 338 which filters off undesired 
frequency components and makes the error correction signal a d.c. voltage. 
The filtered error correction signal is fed from filter 338 to the control 
input of the phase shifting circuit 335 to control or vary the delay or 
phase shift of the incoming alerting signal as a function of the phase 
difference that is detected or sensed by phase detector 336. 
The phase corrected signal voltage at the output of the phase shifting 
circuit 335 will be 90.degree. out of phase with the signal voltage 
applied to the signal input 341 of phase detector 336. This phase 
difference is due to the operation of phase detector 336 and is also 
encountered in the operation of an ordinary phase locked loop using a 
voltage controlled oscillator. A 90.degree. phase shift is therefore 
needed at some point in the system to put the output of the phase shifting 
circuit 335 into phase with the received alerting signal at the output of 
the AGC amplifier 68a in any one or more of the subscriber terminal 
circuits STU1-STU8. 
In the illustrated embodiment this is accomplished by locating the 
90.degree. phase shifter 334 at the output of the controllable phase 
shifting circuit 335 to shift the phase corrected signal voltage by the 
required 90.degree. before applying the phase corrected signal to the 
synchronous inputs of the ring detectors 104 in the subscriber terminal 
circuits STU1-STU8. Alternatively, the 90.degree. phase shifter 334, which 
may be of the LC type, may be connected to phase shift the incoming signal 
before it is applied to the signal input 341 of phase detector 336. 
Phase shifting circuit 335 may be a controllable time delay line in which 
the signal at the output of circuit 335 is variably delayed with respect 
to the signal at the input of circuit 335 by a magnitude that is 
determined by the error correction signal mentioned above. 
For example, the phase shifting circuit 335 may be a monostable 
multivibrator (also called a one-shot multivibrator). The time delay is 
obtained by using the monostable multivibrator circuit to provide a 
trigger pulse output at some variable and controllable prescribed time 
interval after the application of a pulse input to the circuit. The error 
correction signal voltage mentioned above may be utilized to control or 
adjust an appropriate resistance in the monostable multivibrator to vary 
the time delay between the application of a signal pulse at the input of 
the multivibrator circuit and the trigger pulse at the output of the 
multivibrator circuit. This variable resistance may take the form of a 
field effect transistor (FET). If a monostable multivibrator is used, then 
a shaping circuit (not shown) will be connected between the output of 
filter 330 and the input of circuit 335 to transform the filtered a.c. 
alerting signal waveform into suitable pulses at the alerting signal 
frequency for operating the one-shot multivibrator. 
The signalling operation of the circuitry thus far described for the 
embodiment of FIGS. 15 and 16 will now be considered, using the example of 
the channel 1 subscriber who is served by the channel terminal circuits 
COT1 and STU1. 
When an incoming call arrives at the central office for the subscriber 
served by the channel terminal circuits COT1 and STU1 the central office 
relay 130 is operated as previously described to connect the ring 
generator 132 to the ring terminal 126 in the central office terminal 
circuit COT1. The central office ringing supplied by ring generator 132 
will therefore be applied to switch 100, causing switch 100 to be turned 
on by the positive alternations of the ringing signal 134 and to be turned 
off by the ringing signal's negative alternations as previously described. 
The alerting signal will therefore be conducted to modulator 54 only during 
the positive alternations of the central office ringing signal 134. As a 
result, conduction of the alerting signal will be interrupted periodically 
at the central office ringing signal rate or frequency during the ringing 
intervals of the ringing signal 134. The channel 1 carrier signal will 
therefore be amplitude modulated periodically by the interrupted alerting 
signal at a rate equal to the frequency of the central office ringing 
signal 134. 
Upon receiving and detecting the called subscriber's modulated carrier 
signal in terminal circuit STU1, the interrupted alerting signal will be 
recovered and will be present at the output of the AGC amplifier 68a in 
terminal circuit STU1. From amplifier 68a the recovered alerting signal 
will be fed to the bandpass filter 330 and also to the signal input 341 of 
phase detector 336. Additionally, the recovered alerting signal at the 
output of amplifier 68a will be fed to the signal input (indicated at 345 
in FIG. 16) of the synchronous detector 104 in the called subscriber's 
subscriber terminal circuit STU1. 
The alerting signal fed to bandpass filter 330 will be passed by the filter 
to the controllable phase shifting circuit 335. If the alerting signal at 
the output of circuit 335 is not 90.degree. out of phase with the alerting 
signal at the output of the AGC amplifier 68a, phase detector 336 will 
develop the previously mentioned error correction signal which causes 
circuit 335 to automatically correct the phase of the alerting signal. 
The phase corrected alerting signal at the output of circuit 335 is then 
phase shifted 90.degree. by phase shifter 334 so that the signal at the 
output of phase shifter 334 and hence at the synchronous input 343 of ring 
detector 104 will be synchronized with the alerting signal which is fed 
directly from the output of amplifier 68a to the signal input 345 of the 
synchronous ring detector 104. As a result, the phase corrected alerting 
signal at the synchronous input 343 of ring detector 104 will act as a 
synchronous signal to cause ring detector 104 to synchronously detect the 
alerting signal which is fed to the signal input 345. The synchronously 
detected alerting signal at the output of ring detector 104 is fed to the 
ring circuit 106 in the called subscriber's subscriber terminal circuit 
STU1 which operates in the manner previously described to ring the called 
subscriber's telephone. 
It will be appreciated that the alerting signal frequency at the 
synchronous signal input 343 will be the same as the alerting signal 
frequency at the signal input 345. In effect the interrupted alerting 
signal will appear at two places or terminals, one being at input 343 and 
the other being at the input 345. The alerting signal at input 343 is 
considered to be synchronized with its counterpart at input 345 if it is 
either in phase or 180.degree. out of phase with the alerting signal at 
input 345. In either case it will have the effect of synchronously 
detecting the alerting signal at input 345. 
It will be noted that the waveform of the alerting signal at input 343 does 
not have to be the same as the waveform of its counterpart at input 345 as 
long as the zero cross-overs of the two waveforms occur at or 
approximately at the same time. Thus, the phase-corrected alerting signal 
fed to input 343 may be a square wave while the alerting signal which is 
fed to input 345 for detection may be a sine wave. Use of a one-shot 
multivibrator as the phase-shifting circuit 334 provides a square wave. 
The phase locked loop 116 and phase shifter 117 may be used to supply the 
synchronous signal to the channel loop test detector 170 in each of the 
subscriber terminal circuits STU1-STU8 as previously described. 
Alternatively, loop 116 and phase shifter 117 may be replaced by a 
frequency divider 350 (see FIG. 16) which is connected to the output of 
the phase shifting circuit to frequency divide the signal at the output of 
circuit 335 by a suitable interger such as two. 
The frequency-divided output of divider 350 is fed to the synchronous input 
(indicated at 352 in FIG. 6) of the channel loop test detector 170 in each 
of the subscriber terminal circuits. For this arrangement the 1800 Hz 
frequency supplied by oscillator 108 my be used as the alerting signal to 
alert subscribers to incoming calls and the frequency divided signal of 
900 Hz may be used for the channel loop test operation. This frequency 
allocation obviously necessitates an interchange of signal input 
connections to switches 100 and 102 in the central office terminal 
circuits COT1-COT8 such that the 1800 Hz signal is fed to switch 100 and 
the 900 Hz signal is fed to switch 102. 
The particular circuit design of loop 332 may also be used to synchronously 
detect other signals, such as the carrier signal itself. If it is desired 
to synchronously detect the carrier signal in this manner, for example, a 
special phase locked loop corresponding to loop 332 is added to each of 
the subscriber terminal circuits in the manner shown in FIG. 16. In FIG. 
16 the loop used for synchronously detecting the incoming carrier signal 
is indicated at 360. In addition to loop 360 the subscriber terminal 
circuit is equipped with a bandpass filter 330a, and the regular detector 
64a is replaced by a synchronous detector 362. 
If desired, the channel bandpass filter 62a may be eliminated as 
illustrated in FIG. 16. Alternatively, it may be replaced by a channel 
bandpass filter of the type described in my previously identified 
copending application. In this embodiment the group terminal circuit 36 is 
advantageously equipped with a 90.degree. phase shifter 364 which is 
connected in the receive section 86. 
Since loop 360 is the same as loop 332 like reference numerals suffixed by 
the letter a have been applied to designate the corresponding parts of 
loop 360. 
As shown in FIGS. 15 and 16 the composite of the incoming carrier signals 
is fed to the input of phase shifter 364. Additionally, this composite of 
the incoming carrier signals is fed to the signal input 363 of the 
synchronous detector 362 and to the phase detector's signal input 341a in 
each of the subscriber terminal circuits. 
The 90.degree. phase shifter 364 in the group terminal circuit shifts each 
of the incoming carrier signals by the required 90.degree.. The composite 
of the phase shifted carrier signals at the output of phase shifter 364 is 
fed to the input of the bandpass filter 330a in each of the subscriber 
terminal circuits as shown. 
Bandpass filter 330a may be of the active type like filter 330 and is tuned 
to the particular carrier frequency to be received in its associated 
subscriber terminal circuit. Thus, the bandpass filter 330a in terminal 
circuit STU1 will be tuned to the frequency of the carrier signal 
transmitted by terminal circuit COT1, the bandpass filter 330a in terminal 
circuit STU2 will be tuned to the frequency of the carrier signal 
transmitted by terminal circuit COT2, and so on. The output of filter 330a 
feeds the input of the controllable phase shift circuit 335a, and the 
output of the controllable phase shifting circuit 335a is connected to and 
feeds the synchronous input 365 of the synchronous detector 362. Phase 
shifter 364 performs the same function as phase shifter 334. 
From the foregoing description it will be appreciated that bandpass filter 
330a will pass only the desired carrier signal and will reject all of the 
other carrier signals which are transmitted down transmission line 22 from 
the central office terminal equipment. The carrier signal passed by filter 
330a is fed to the controllable phase shifting circuit 335a in loop 360. 
Circuit 335a will provide the previously described phase correction of the 
incoming carrier signal such that the phase of the carrier signal at the 
synchronous input 365 of detector 362 will either be in phase or 
180.degree. out of phase with the unshifted carrier signal of the same 
frequency at input 363. In either case the phase shifted carrier signal at 
the synchronous input 365 will have the effect of synchronously detecting 
the unshifted carrier signal of the same frequency at input 363. 
When the phase-corrected or phase-adjusted carrier signal at the 
synchronous input 365 and the desired incoming carrier signal at input 363 
are combined in detector 362, the resulting beat or difference frequency 
will be zero, and the detector output will contain an audio signal 
reproduction of the transmitted modulation (i.e., the VF signal 
intelligence which was applied to modulate the carrier at the 
corresponding central office terminal circuit.) 
For example, if a 1 kHz tone is applied to modulate the carrier signal that 
is transmitted from the central office channel terminal circuit COT1, then 
the modulated carrier signal will domodulate as a 1 kHz signal at the 
output of the synchronous detector 362 in the subscriber channel terminal 
circuit STU1. The beat or difference frequency between the applied 
synchronous signal at input 365 and each of the remaining unwanted carrier 
signals which may appear at input 363 will be significantly above the 3000 
Hz cutoff of the low pass filter 66a. Likewise, the beat or difference 
frequency between the applied synchronous signal at input 365 and the 
upper and lower side frequencies of each of these unwanted carrier signals 
from the other channels will also be significantly above the 3000 Hz 
cutoff of filter 66a. As a result these undesired frequency components 
will be filtered off by filter 66a so that only the desired voice 
frequency intelligence will appear at the output of filter 66a. 
From the foregoing description it will be appreciated that each of the 
controllable phase shifting circuits 335 and 335a merely shifts the phase 
of the input signal that is fed to its input port and does not change the 
frequency of the phase-corrected signal. Following phase correction the 
signal out of each of the phase shifting circuits 335 and 335a will 
therefore be the same as the input signal frequency. 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments are therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all changes 
which come within the meaning and range of equivalency of the claims are 
therefore intended to be embraced therein.