Digital duplex transmission system

A digital duplex transmission system for use in a local area uses a modified dipulse code from the exchange terminal to the subscriber terminal and either a different modified dipulse code or a modified AMI (Alternate Mark Inversion) code in the other direction to achieve time separation of signals at each terminal. Incorporation of a hybrid separator in each terminal further improves the performance of the system.

BACKGROUND OF THE INVENTION 
This invention relates to a digital duplex transmission system, such as may 
be used in a telephone local exchange area. 
A recent development in telephony is the employment of digital transmission 
of signals between subscribers and the exchange in the local area using 
pulse code modulation techniques. Such systems can employ traditional 
terminating sets to realize bi-directional transmission over a single pair 
of wires connecting the exchange and the subscriber's set, hereinafter 
both referred to as terminals. However, the performance of systems using 
terminating sets for separation of digital signals is governed by the 
degree to which the interconnecting 2-wire transmission line can be 
matched. This is a disadvantage when a standard subscriber's terminal with 
a fixed matching impedance is required to be used for a range of line 
characteristics. 
Another technique that can be employed to realize bi-directional 
transmission is time separation. This technique inherently implies the use 
of shorter duration signals per bit of information communicated. The 
necessary transmission period is governed by both the number of bits in 
the time separated blocks and the distance between the terminals. The 
number of bits in a block is in turn governed by the information 
transmission delay which it introduces and the need for storage. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a digital duplex 
transmission system which is more tolerant to line impedance variations 
than that provided by simple terminating set separation, but which 
requires a lower frequency band than the established time separation 
method and does not have the disadvantages of block methods of operation. 
A feature of the present invention is the provision of a digital duplex 
transmission system comprising: two terminals interconnected by a 2-wire 
transmissin line; first means disposed in one of the terminals coupled to 
the line for transmitting isochronous digital signals as pulse encoded 
signals of one type having a duty factor not exceeding 50% over the line; 
second means disposed in the other of the terminals coupled to the line 
for transmitting isochronous digital signals as pulse encoded signals of a 
different type having a duty factor not exceeding 50% over the line; the 
one type of encoded signals and the different type of encoded signals 
having the same digit repetition rate; third means disposed in the one of 
the terminals coupled to the line to receive the different type of encoded 
signals from the line; and fourth means disposed in the other of the 
terminals coupled to the line to receive the one type of encoded signals 
from the line; the timing of one of the one type of encoded signals and 
the different type of encoded signal transmitted from one of the one of 
the terminals and the other of the terminals having a predetermined fixed 
relationship with the other of the one type of encoded signals and the 
different type of encoded signals received at the one of the one of the 
terminals and the other of the terminals such that the periods of zero 
signal in the one of the one type of encoded signals and the different 
type of encoded signals coincide with the significant portions of the 
other of one type of encoded signals and the different type of encoded 
signals.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Consider the system shown in FIG. 1 which comprises an exchange terminal 1, 
a subscriber's terminal 2 and an interconnecting 2-wire transmission link 
3. Each terminal is provided with a digital modulator 11 and 21, 
respectively, a digital demodulator 12 and 22, respectively, and 
interconnecting devices 13 and 23, respectively, which may be terminating 
sets for connecting the modulators and demodulators to link 3. Exchange 
terminal 1 emits a continuous stream of digital signals corresponding to 
binary "1"'s and "0"'s of a message to be communicated. These signals must 
contain an adequate clock component so that a synchronized clock can be 
derived at subscriber terminal 2 for demodulating the incoming signals. It 
is evident that the same clock can be used for transmission by 
subscriber's terminal 2 and that, provided the incoming digit signals are 
suitably spaced, the receive and transmit digit periods can be time 
separated. In the systems being considered herein, however, the idle time 
corresponding to the propagation delays in the two directions of the known 
time separation technique is not included. 
The corresponding phasing between the received and transmitted signals at 
the exchange terminal is arbitrary and is conditioned by the propagation 
delay. In the absence of an idle period, it is not possible to achieve 
complete time separation at both terminals. The two embodiments of the 
invention to be described below have been evolved for dealing with this 
situation at the exchange terminal. Both embodiments employ the same 
transmission code in one direction but they use different codes and 
different decoding principles in the other direction. 
In the first embodiment the basis of the operation at the exchange terminal 
is a combination of terminating set separation and sufficient time 
separation so that a suitable segment of each received digit signal for 
decoding is always present outside the significant segment of each 
transmitted digit signal. Terminating set separation is not fundamental to 
the decoding operation, which is effectively that of time separation, but 
provides for a considerable improvement in performance. 
The first embodiment is implemented by the use of two different variants of 
the dipulse modulation code for line transmission, one for each direction. 
The basic dipulse code is illustrated in FIG. 2(a). In the basic dipulse 
code each digit is represented by a transition from a negative to a 
positive voltage level, or vice versa, in the center of the digit period. 
This results in what is termed a balanced line signal if the amplitudes of 
the voltage levels are equal. In the case of a 100% duty factor code, the 
relevant voltage levels are each maintained for the complete half digit 
period. FIG. 2(b) shows a 50% duty factor variant of the basic dipulse 
code, in which each voltage level is maintained for only one quarter of 
the digit period. Thus, the first and last quarters of the digit period 
now provide what might be termed "pseudo idle periods", the significant 
part of the period being confined to the segment comprising the second and 
third quarters. FIG. 2(c) shows another 50% duty factor variant of the 
basic dipulse code, only in this case the voltage levels are now 
maintained in the first and third quarters while the pseudo-idle time is 
in the second and fourth quarters. Other 50% duty factor variants of the 
basic dipulse code can readily be envisaged. Thus, a dipulse encoded 
signal having a 50% duty factor is defined as a dipulse signal in which 
for the duration of a digit period the positive and negative voltage 
levels are each maintained for only one quarter of the total digit period. 
Reverting now to the system of FIG. 1, modulator 11 at the exchange 
terminal 1 is arranged to transmit one form of 50% duty factor dipulse 
signal, e.g. the variant shown in FIG. 2(b) while modulator 21 in 
subscriber's terminal 2 is arranged to transmit another variant, e.g. that 
shown in FIG. 2(c). At subscriber's terminal 2 a clock is extracted (by 
means not shown) from the zero crossings of the received signals as shown 
in FIG. 3. This clock is used to control not only the sampling of the 
received signals to extract the information in demodulator 22, but also 
the operation of modulator 21. The arrangement is that at least one of the 
significant quarters of the transmit signal digit period in modulator 21 
coincides with one of the pseudo-idle time quarters of the received signal 
digit period (depending on the choice of the two 50% duty factor variants 
employed). FIG. 3 shows the fixed timing relationship between the received 
and transmitted signals at subscriber's terminal 2. As an alternative to 
extracting the clock from the transition within the dipulse signal of FIG. 
2(b), the clock can be extracted from the front edge of the signal 
occurring in the second quarter of the digit period. As in the case of 
exchange terminal 1 separation by use of a terminating set is not 
fundamental, but does add to the performance of the terminal. 
At exchange terminal 1 the decoding clock can be extracted (by means not 
shown) from the incoming signals. This can be made easier by the 
deliberate introduction in each direction of an idle transmit period at 
intervals, which do not coincide with one another. The alternative and 
preferred method is to use the terminal's master clock for decoding, the 
appropriate phase being selected as part of a synchronization procedure 
and/or from the signal received during a pseudo-idle transmit period, as 
shown in FIG. 4. The shape of the received signals for decoding at each 
terminal can be improved by modification of the transmitted signals. This 
is equivalent to some degree of line pre-equalization. One suitable 
modification is to reduce the amplitude of the line voltage in the second 
of two successive voltage levels of the same polarity, as shown in FIGS. 3 
and 4. By arranging for the amount of the reduction to be made dependent 
on the line attenuation variation of pre-equalization can be effected as 
required. Note that consecutive voltage levels of reduced amplitude are of 
opposite polarity so that a balanced signal waveform is maintained. 
The second embodiment utilizes selection of a suitable balanced transmit 
signal from exchange terminal 1 with an unbalanced transmit signal (for 
the duration of a digit period only) from the subscriber's terminal 2, 
coupled with integration over a digit period or less as a means of 
decoding to eliminate the unwanted terminating set unbalance. In this case 
a 50% duty factor dipulse code is transmitted from exchange terminal 1 and 
a 50% duty factor AMI code variant is transmitted from subscriber's 
terminal 2. FIG. 5(a) shows an appropriate form of 50% duty factor dipulse 
code, in which the significant voltage levels are confined to the second 
and third quarters of the digit period. FIG. 5(b) shows a 50% duty factor 
AMI code variant. Note that for digits of one binary significance, e.g. 
binary "0", no voltage level is transmitted for the whole digit period 
while for digits of the other significance, e.g. binary "1", a positive 
voltage is normally generated during the first half of the digit period 
but for alternate "1"'s this is inverted to a negative voltage for 
transmission. Thus, the 50% duty factor AMI is balanced (over a digit 
period) only when "0"'s are being transmitted. When "1"'s are transmitted 
it is an unbalanced signal, although over a number of digit periods the 
line signals will have an accumulated balanced condition. 
FIG. 6 illustrates the signals transmitted by subscriber's terminal 2 and 
the received signals at terminal 2 and shows the fixed timing relationship 
between the two. FIG. 7 illustrates the operations at exchange terminal 1 
where the phase relationship, i.e. the timing, between the transmitted and 
received signals is arbitrary. As stated above, the received signal is 
separated from the terminating set unbalance signal by integration over 
one digit period. To effect separation it is necessary for the sum of the 
significant portions of the transmit digit period and the major portion of 
the signficant received digit signal period to be less than the digit 
period. Additionally, the significant portions of the transmit signal and 
the significant portion of the received signal must be contained within 
the particular digit period used for integration. To effect this it is 
both necessary and convenient for the integration period to be defined by 
the start of the transmit digit period, provided the start of the received 
digit signal occurs within the transmit period. Otherwise it is necessary 
and convenient for the integration period to be defined by the start of 
the received digit signal. Typical examples of both cases are shown in 
FIG. 7. 
At subscriber's terminal 2 the clock is extracted by the preferred method 
from the transition within the received signal. The use of terminating set 
separation is not fundamental, but provides a greatly improved 
performance. 
At exchange terminal 1 the clock can be derived from the incoming signal 
provided the AMI code is changed to a High Density Bipolar code (HDB3) to 
offset the effect of consecutive spaces. Initial synchronization of the 
integration operation can be effected by inhibiting transmission for a 
period. An alternative and preferred method is to use the exchange 
terminal clock and to select an appropriate phase again by a 
sychronization procedure. 
The shape of the received signal at exchange terminal 1 for the purpose of 
decoding can be improved by modifying the transmit signal from the 
subscriber's terminal 2, as indicated by the dotted extension to the 
signal shown in FIG. 6. The amplitude of the complementary pulse can be 
adjusted to be dependent on the line attenuation as indicated by signal 
amplitude. The method provides a simple first order pre-equalization for 
the line necessary to avoid excessive pulse spreading. 
Equalization as described in the first embodiment above, for the direction 
exchange to subscriber is not applicable due to the fact that it would 
mitigate integration as a means of elimination of interference from the 
transmit signal. 
While reference has only been made to the use of dipulse and/or AMI encoded 
signals, it is to be noted that other signal formats can be envisaged 
which can also be utilized, provided that they conform to the 50% duty 
factor limit. One such signal format is that known as the Top Hat code, in 
which each digit is represented by a signal level of one polarity which 
has a duration of half the significant portion of the digit period, i.e. 
one quarter of the total digit period in a 50% duty cycle code, 
immediately preceded and followed by levels of the opposite polarity each 
of half the duration of the level of the one polarity, i.e. one eighth of 
the total digit period. Conveniently, the duration of the significant 
signal levels is confined to the center portion of the digit period, i.e. 
the digit period begins and ends with two portions of zero signal each one 
quarter of the total digit period. 
While we have described above the principles of our invention in connection 
with specific apparatus it is to be clearly understood that this 
description is made only by way of example and not as a limitation to the 
scope of our invention as set forth in the objects thereof and in the 
accompanying claims.