Terminal station for telecommunication by wire

A telephone set includes an anti-sidetone circuit formed by a feedback loop having a filter in the feedback path. The feedback loop output is substantially equal to the signal produced at the set connection terminals by the microphone. Preferably the filter is composed of two sub-filters connected in cascade, formed of switched capacitors in an integrated circuit. An EEPROM control circuit can be included to permit easy programming during manufacture, to match a selected transmission line impedance.

BACKGROUND OF THE INVENTION 
The invention relates to a terminal station for telecommunication by wire, 
comprising at least two connection terminals for connecting the terminal 
station to a transmission line of a telecommunication network, and also 
comprising an anti-sidetone circuit coupled to the connection terminals. 
A terminal station of this kind is generally known as an electronic 
telephone as described in the Philips Data Handbook "ICs for Telecom 
Subscriber Sets, Cordless Telephones, Mobile/Cellular Radio Pagers CA 3089 
to PC 4413", Data Handbooks IC03a and IC03b, 1991. 
Page 851 of the handbook IC03a shows a block diagram of such a terminal 
station, comprising connection terminals a/b, b/a for connecting the 
terminal station to a telephone line. The terminal station comprises a 
transmission circuit, for example as realised in the IC type 1067, and a 
control device in the form of a microcontroller designed for telephony 
purposes, for example the IC type PCD 3349. Also shown are a keypad 
coupled to the microcontroller and a microphone and telephone audio output 
device such as an earpiece or loudspeaker coupled to the transmission 
circuit. 
FIG. 5 on page 1465 of the handbook IC03b shows an anti-sidetone circuit, 
forming part of the transmission circuit, in the form of a bridge circuit. 
This bridge circuit, including the line impedance, comprises a balance 
impedance and two discrete impedances which are included in the branches 
of the bridge. As is described on page 1464, the line impedance varies 
substantially as a function of the type and the length of the line. To 
this end, a value which is optimum for the mean line length of the 
transmission line is usually chosen for the balance impedance, and the 
further discrete impedances also have permanently adjusted values. 
Integration of these components on a telephone set IC is not very feasible 
because of the high values of the real and the complex parts of these 
impedances. Moreover, generally speaking, accurate impedances in an 
absolute sense cannot be realised on an IC. A further problem is that 
different countries impose different requirements regarding the line 
impedance. The prior art solution to this problem has involved using 
external components connected to the IC. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a terminal station for 
telecommunication by wire which comprises an anti-sidetone circuit which 
can be integrated in a simple manner. 
Another object of the invention is to eliminate the need for external 
components to adapt to various national requirements, by providing a 
universal terminal station which can be readily programmed during 
manufacturing. 
To achieve the first object, a terminal station in accordance with the 
invention is characterized in that the anti-sidetone circuit comprises a 
filter which is included in a feedback loop. The feedback loop generates 
from a microphone signal, a signal which is substantially equal to a 
signal generated across the connection terminals by the microphone signal. 
Preferably, the filter is included in the feedback path of the loop. 
The invention is based on the recognition of the fact that the 
anti-sidetone circuit can be simply realised by means of a filter included 
in a feedback circuit. 
An embodiment of a terminal station in accordance with the invention is 
characterized in that the filter consists of the cascade connection of two 
normalized sub-filters. 
This offers the advantage that the realisation of the anti-sidetone circuit 
in integrated form is further simplified. 
A further embodiment of a terminal station in accordance with the invention 
is characterized in that the filter is adjustable, and the terminal set 
includes a control unit which is connected to the filter in order to 
compensate for the effect of differences in the value of the transmission 
line impedance with the filter. 
This offers the additional advantage that compensation is achieved not only 
for the various line impedances specified by the authorities in various 
countries, but also for the actual line length. 
A further embodiment of a terminal station in accordance with the invention 
is characterized in that the filter is constructed by means of switched 
capacitances. 
Because an accurate filter, or the sub-filters, can thus be simply 
realised, integration of an anti-sidetone circuit adaptable to the line 
length and the requirements imposed by authorities is thus simplified. 
It is to be noted that the reproducibility and the accuracy of the filter 
to be realised do not depend on the reproducibility and the absolute 
accuracies of impedances constituting the filter or the sub-filters, but 
on the ratio of impedances. These ratios can be reproducibly and 
accurately realised during integration processes.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows diagrammatically a terminal station 1 for communication by 
wire. The terminal station 1 comprises connection terminals 2 and 3, 
denoted by the references a/b and b/a as usual, for connecting the 
terminal station to a transmission line, for example a subscriber line of 
a telecommunication network. When the terminal station is connected to the 
line, the terminal station sees the line impedance Z.sub.L. The terminal 
station 1 comprises a transmission circuit 4 which is connected via a 
bridge circuit 7. The transmission circuit comprises a line termination 
impedance Z.sub.s. The transmission circuit 4 couples a telephone 
loudspeaker (or earpiece) and a microphone, usually accommodated in a 
telephone receiver (not shown), to the subscriber line for receiving and 
transmitting speech signals from and to the line, respectively. For 
control of the transmission circuit 4 there is provided a control device 8 
whereto a keypad 9 is connected. The control device may be a 
microcontroller specifically designed for telephony purposes. The 
microcontroller 8 inter alia serves to convert, during dialling, the 
signals supplied by the keypad into either pulse signals for the 
transmission circuit 4 in order to interrupt the line current in a 
pulse-like manner when the terminal station 1 is arranged for pulse 
dialling, or to convert these signals into tone dialling signals for the 
transmission circuit if the terminal station is arranged for tone 
signalling. The terminal station also comprises a ringing device 10 which, 
in the on-hook condition, is connected to the line, via the position of a 
cradle contact 11, in order to ring the subscriber in the event of an 
incoming call. 
Because the signals originating from the network may not be reflected by 
the terminal station 1 when the latter is connected to the 
telecommunication network, the impedance Z.sub.s should be matched as well 
as possible with the line impedance Z.sub.L. In order to obtain suitable 
line matching, the termination impedance Z.sub.s should satisfy the 
requirements imposed by network management authorities. 
The required termination impedances may differ from one country to another 
and are also dependent on the length of the line. The realisation of such 
a termination impedance Z.sub.s does not form part of the present 
invention. 
A signal originating from the line forms a signal voltage, denoted by the 
reference V.sub.LN in FIG. 1, across the termination impedance Z.sub.s. 
Via an input terminal 13-1, the signal voltage is applied to the 
anti-sidetone circuit 13 which outputs the signal, possibly after 
amplification, to the telephone loudspeaker 7. It is to be noted that the 
termination impedance Z.sub.s is formed by means of an electronic circuit 
15 which is not described in detail, so that it is virtually grounded for 
the signal voltage V.sub.LN on the other side, subsequent to the line 
terminations Z.sub.s. 
A signal supplied by the microphone 6, being a signal voltage V.sub.T 
symbolically represented in the Figure by an alternating voltage source 14 
with a signal voltage V.sub.T which is connected between virtual ground 
and the line termination impedance Z.sub.s, is applied, via the 
termination impedance Z.sub.s, to the transmission line connected to the 
connection terminals 2 and 3. The sub-voltage formed across the line 
impedance Z.sub.L of the transmission line, however, is also applied to 
the input terminal 13-1 of the anti-sidetone circuit 13. In order to 
prevent this signal from reaching the telephone loudspeaker 7, the signal 
voltage V.sub.T supplied by the microphone is applied directly to the 
input terminal 13-2 of the anti-sidetone circuit 13. 
In accordance with the invention, the anti-sidetone circuit 13 comprises a 
filter 16 which is included in the feedback path of a feedback loop or 
circuit 19. More specifically, the feedback circuit 19 also comprises an 
adder device 17 and the anti-sidetone circuit comprises a differential 
amplifier 18 which receives the signal voltage present for the line 
connection terminals 2 and 3 as well as the signal voltage supplied by the 
feedback circuit. 
The operation of the anti-sidetone circuit 13 will be described in detail 
hereinafter with reference to FIG. 2 which shows the equivalent diagram of 
the part of the transmission circuit 4 shown in FIG. 1. 
In said equivalent diagram the signal source present at the other end of 
the transmission line is denoted by the reference 2-3 and the signal 
voltage supplied by said source 2-3 is denoted by the reference V.sub.c. 
As has already been stated, the signal voltage V.sub.c supplied by the 
signal source 2-3 produces a line alternating voltage V.sub.LN at the 
input terminal 13-1, said voltage being applied to the differential 
amplifier 18. Because of the virtual ground of the input terminal 13-2, 
this input terminal will not carry a signal voltage originating from the 
source 2-3. Consequently, the entire line alternating voltage V.sub.LN is 
applied, possibly after amplification by the differential amplifier 18, to 
the telephone loudspeaker 7. 
A voltage signal V.sub.T supplied by the signal source 14 produces a signal 
voltage at the connection terminal 13-1 which amounts to: 
EQU V.sub.T.Z.sub.L /(Z.sub.L +Z.sub.s) (1) 
The signal voltage supplied by the signal source 14, via the connection 
terminal 13-2, to the other input terminal of the differential amplifier 
is denoted by the reference V.sub.i. 
For V.sub.i it holds that 
EQU V.sub.i =V.sub.T -F V.sub.i, 
where F is a transfer function of the filter 16. 
Consequently, 
EQU V.sub.i =V.sub.T /(1+F) (2) 
The differential amplifier 18 functions as a comparison circuit whose 
output is applied to the telephone loudspeaker 7. In order to ensure that 
the signal from the signal source 14 is not applied to the telephone 
loudspeaker 7, the voltages represented in the equations 1 and 2 should be 
made equal. This means that 
##EQU1## 
This condition can be satisfied by choosing the transfer function F to be 
equal to 
EQU Z.sub.s /Z.sub.L (3) 
This embodiment offers the major advantage that merely the ratio of the 
impedances Z.sub.s and Z.sub.L need be absolutely accurate, and not the 
individual impedances. In this respect an anti-sidetone circuit 13 
constructed so as to include a filter 16 provided in the feedback path of 
a feedback loop 19 is particularly suitable for integration. 
Because the filter 16 contains the ratio of the line termination impedance 
Z.sub.s and the line impedance Z.sub.L, a further simplification can be 
obtained by subdividing the filter 16 into two cascade-connected 
sub-filters 20 and 21. 
The transfer function F' of the filter 20 can be chosen equal to Z.sub.s 
/R.sub.N, the transfer function F" of the filter 21 then being equal to 
R.sub.N /Z.sub.L, and the resistance value of R.sub.N being chosen at 
random. The subdivision of the filter 16 into two sub-filters 20 and 21 
and the standardization of the line termination impedance Z.sub.s and the 
line impedance Z.sub.L with an arbitrary value of R.sub.N enables a 
further simplification of the filter 16 with the transfer function F in 
integrated form. 
This will be described in detail hereinafter with reference to the transfer 
characteristics of the filter 16 and the sub-filters 20 and 21 shown in 
FIG. 3. 
The transfer function F to be realised by the filter 16 is represented by a 
solid line in FIG. 3. By forming this transfer function as the sum of the 
transfer functions F', F", denoted by the dashed line and the dash-dot 
line, respectively, a simpler realisation is possible because the 
sub-filter 20 has only one pole at f.sub.1 and a zero point at f.sub.3, 
the sub-filter 21 having a zero point at f.sub.2 and a pole at f.sub.4. 
FIG. 4 shows an example of a further simplified embodiment of the filter 
16. The sub-filter 21 comprises an auxiliary filter 23 which is included 
in the feedback path of an additional feedback loop 22 and which has a 
transfer characteristic F.sub.i " which, as will be demonstrated 
hereinafter, is the inverse of the transfer function F" of the sub-filter 
21. More specifically, the sub-filter 21 comprises a further adder device 
24 and an amplifier which is included in the forward branch of the 
additional feedback loop 22 and which has a gain factor A. 
It can be simply demonstrated that the transfer function of this sub-filter 
is 
##EQU2## 
If A is large, the transfer function is substantially equal to 
##EQU3## 
The transfer function of this sub-filter was chosen to be equal to R.sub.N 
/Z.sub.L, which means that the transfer function F.sub.i " of the 
auxiliary filter 23 equals Z.sub.L /R.sub.N. 
This embodiment offers the advantage that the auxiliary filter 23 can be 
realised in the same way as the transfer function Z.sub.s /R.sub.N for the 
sub-filter 20. 
The advantage of the subdivision of the filter 16 into the sub-filters 20 
and 21 resides in the fact that the sub-filter 20 is composed of the 
normalized line termination impedance Z.sub.s. 
This means that upon realisation of this filter, the line termination 
impedance Z.sub.s already realised in integrated form is to be doubled on 
the chip. The filter 20 can then also be realised as a duplicate of the 
line termination impedance Z.sub.s normalized to R.sub.N. This advantage 
exists notably when the line termination impedance Z.sub.s is 
controllable, for example, when it can be matched with a predetermined 
line impedance Z.sub.L specified by the network authorities. The auxiliary 
filter 23 of the sub-filter 21 can be adjusted in a corresponding manner. 
To this end, as is shown in FIG. 1, the control device 8 comprises not only 
the customary devices (not shown) such as a processor, a RAM, etc., but 
also a programmable read-only device 27 which is coupled to a bus 28. The 
programmable read-only device may be a so-called EEPROM (Electrically 
Erasable Programmable Read-Only Memory) in which control information for 
adjustment of the line termination impedance Z.sub.s, and hence also of 
the transfer function F' of the sub-filter 20 and of the transfer function 
F.sub.i " of the auxiliary filter 23, is stored. 
When the terminal station 1 is switched on, the microcontroller derives the 
control signals ctl.sub.1 and ctl.sub.2 from the EEPROM information and 
applies these signals, via a coupling circuit (interface) 29, to the 
transmission circuit 4. The interface 29 may be a buffer, the output of 
which carries the control signals ctl.sub.1 and ctl.sub.2 in binary form 
after activation of the terminal station 1. 
Realisation of the filter 16 in analog integrated form is virtually 
impossible because of the low pole and zero point frequencies of the 
filter specified by the line impedance Z.sub.s ; this would require an 
excessively large chip surface area and, moreover, could not be realised 
with the desired accuracy. Therefore, in an attractive embodiment of the 
invention the filter 16 is constructed as a filter with switched 
capacitances or switched currents. The pole and zero point pattern of 
filters with switched capacitances is defined by capacitance ratios in the 
filter and the clock frequency of the filter. Because of the fact that 
filter properties are defined by capacitance ratios, no absolutely 
accurate capacitances are required. For a more detailed description of 
filters with switched currents, reference is made to a variety of 
literature, for example the handbook "Analog MOS Integrated Circuits for 
Signal Processing", R. Gregorian et al, Wiley 1986. An embodiment of the 
filter 16 in the form of a filter with switched capacitances can also be 
convened into a filter with switched currents. For such conversion 
reference is made to the article "Switched current filters", J. B. Hughes 
et al, IEEE Proceedings, Vol. 137, Pt. G, No. 2, April 1990, pp. 156-162. 
When the filter 16 is constructed-as a filter with switched capacitances, 
the filter 16 also comprises a clock input for receiving a clock signal 
cl. The clock signal cl, whose frequency is adjustable, can be supplied by 
the control circuit 15. The other components are also integrated as much 
as possible. In the embodiment shown, the terminal set 1 functionally 
consists essentially of two integrated circuits, because separate ICs for 
the transmission circuit and for control are still demanded by OEMs. 
Ultimately, as integration progresses the transmission circuit 4 and the 
control device 8 will be integrated in a single IC. 
FIG. 5 shows an embodiment of the filter 16 in accordance with the 
invention, using a first-order filter with switched capacitances; this 
filter may be the auxiliary filter 23 as well as the sub-filter 20. The 
filter 20/23 comprises an operational amplifier 30, an inverting input 31 
of which receives, via a switched input capacitance C.sub.in and via an 
amplifier 32, the voltage supplied by the adder device 17 and the 
amplifier 25, respectively. The filter 20/23 furthermore comprises a 
switched, fixed feedback capacitance C.sub.t and a number of integration 
capacitances C.sub.i1, C.sub.i2 and C.sub.i3 which can be switched on via 
multiplexing. The number of integration capacitances shown is not 
restricted to three, extension being simply possible. The integration 
capacitances can be connected, via a logic circuit 33, between an output 
37 of the operational amplifier 30 and the input 31 by switches 34, 35 and 
36, for example integrated MOS-FETs. The logic circuit 33, being driven by 
the binary control signal ctl.sub.1 /ctl.sub.2 from the control device 5, 
supplies control signals for the switches 34, 35 and 36. The output 37 
also constitutes the output of the filter and serves to couple the filter 
to the adder circuit 24 shown in FIG. 4. The first-order transfer of the 
filters 20/23 comprises a pole and a zero point. The position of the pole 
is a function of the ratio of the feedback capacitance C.sub.t to the 
integration capacitance C.sub.i, and also of the clock frequency cl. The 
frequency behaviour of the filter, therefore, can be simply adjusted. When 
the clock signal is absent, a non-inverting input 40 of the operational 
amplifier 30 is coupled at the output side to the differential amplifier 
32 via a voltage divider consisting of the resistors 41 and 42. During 
operation, the clock signal cl switches the capacitances C.sub.in and 
C.sub.t by means of the switches 43, 44 and 45. When the clock signal 
fails, a resistive termination of given value can thus be activated. A 
tapping point 46 of the resistors 41 and 42 is connected, as shown, to the 
input 40 of the operational amplifier 30, but can also be connected 
directly to the adder circuit 17/24. Furthermore, the input capacitance 
C.sub.in and at least one of the resistors 41 and 42 may be constructed so 
as to be variable and digitally adjustable, like the integration 
capacitance C.sub.i ; however, this is not shown here. As a result, the 
gain of the filter with switched capacitances 20/23, being a function of 
the ratio of the capacitances C.sub.in and C.sub.t, is also variable. In 
order to shift back a zero point which has been shifted due to adjustment 
of the gain, the resistance value of at least one of the resistors is 
varied. 
When information relating to the magnitude of the line current is applied 
to the microcontroller 8, the filter transfer functions F' and F" can be 
varied as a function of the line length and the anti-sidetone circuit 13 
can be compensated with respect to the length of the line. This 
compensation can be performed in the same way as described above for 
matching the filters with the input impedance of the terminal station 
specified by the authorities. In other words, this can be performed by 
means of additional tables of values of control signals, stored in the 
microcontroller 8, for adjustment of the filters 20/23 in dependence on 
the length of the transmission line.