AU645860B2

AU645860B2 - Method of automatically adjusting side-tone in a telephone subset

Info

Publication number: AU645860B2
Application number: AU88264/91A
Authority: AU
Inventors: Ronald Christopher Shaw Fox
Original assignee: Alcatel Australia Ltd
Current assignee: Nokia Services Ltd
Priority date: 1990-12-18
Filing date: 1991-11-29
Publication date: 1994-01-27
Anticipated expiration: 2011-11-29
Also published as: AU8826491A

Description

.Avh r1m b4 P100/011 28/5/91 Regulation 3.2 0*ee 65 5 0 0 *5 S 5S S5 S S *5e5

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Titlc: "METHOD OF AUTOMATICALLY ADJUSTING SIDE-TONE IN A TELEPHIONE SUBSET" The following statcrrncnL. is a full dcscription of this invention, including the best method of performing it known to us::- S. S 05

SS

S S *0 S This invention relates to telephone su bsets and in particular to telephone subsets of the payphone type.

Payphone subsets are Frequently located in noisy locations such as, for example, railway stations and shopping malls. During a call, high levels of ambient noise activate the subset's transmitter diaphragm and it is transduced into the receiver circuit, from where it is converted into acoustic energy in the subset's receiver, masking normal acoustic energy from signals received from the line. This can prevent the user from hearing received speech clearly.

It is known to incorporate a cut-out key on the subset to selectively interrupt the subset's transmitter circuit during received speech when ambient noise is present.

This method is not entirely satisfactory as it makes spontaneous conversation difficult.

It is an object of the present invention to provide an arrangement which automatically adjusts the level of loudness of the received signal as the level of ambient 15 noise increases above a predetermined threshold, and at the same time to reduce the level of side-tone gain, which effectively causes the side-tone volume to be constant.

According to the present invention there is provided a method of automatically adjusting the level of received speech signals applied to the receiver of a telephone subset, and the level of side-tone signal produced by the subset's transmitter and applied to the said receiver when, in the absence of transmitted speech, ambient noise transduced by said transmitter exceeds a predetermined threshold, said method com- S prising the steps of: providing a hybrid means for coupling a two wire exchange line to a subset receive path having an adjustable receive gain means therein, and a 25 subset transmit path, respectively coupled to said receiver and said transmitter, said paths being coupled via a side-tone path which includes an adjustable sidetone gain means for providing a side-tone signal; monitoring signals produced by said transmitter to detect transmitted speech signals and ambient noise signals, and producing an enabling signal when, in the absence of transmitted speech signals, ambient noise signals exceed a predetermined threshold; enabling a gain control means to adjust said receive gain means and said side-tone gain means to increase the receive speech signal level and decrease the level of side-tone signal in a predetermined proportion; Preferably, echo cancelling adaptive filters are incorporated in the receive and transmit paths or in the receive path only.

Preferably, digital signal processing is utilised.

In order that the inventioi may be readily understood embodiments thereof will now be described in relation to the accompanying drawings, in which: Figure 1 shows a block schematic of a telephone subset incorporating a first embodiment of the present invention.

Figure 2 shows a block schematic of a telephone subset incorporating a second embodiment of the present invention.

Figure 3 shows a block schematic of a telephone subset incorporating a third embodiment of the present invention.

Referring to Figure 1 of the drawings, the telephone subset comprises a receiver a transmitter a hybrid circuit (HYB) coupling the transmitter and receiver to a telephone line, two analog-to-digital converters I and 2, and two digital-to-analog 15 converters 3 and 4 are respectively coupled inputs and outputs of a programmable 1 digital signal processor Digital signal processor 5 is programmed to perform acoustic and hybrid echo cancelling using adaptive filtering techniques, in a known manner, ambient noise detection, speech detection receive gain and side-tone gain control, and receive gain and side-tone gain summing. These various programmed functions are shown in block schematic. It will be understood, however, that analog signal processing may be used, in which case signal conversion is obviously not required.

Preferably, the adaptive filters arc in the form of finite impulse response filters using the Least Means Square algorithm. According to a known technique echo 0 25 cancellers generate an echo replica by applying a reference signal to a transversal filter. If the transfer function of the transver.sl filter is identical to that of the echo path, the echo replica will be identical to the echo, thus achieving total cancellation.

Since the transfer function of the echo path is not known in advance, the canceller adapts the coefficients of the transversal filter. To reduce error, the adaption algorithm infers from the cancellation the appropriate correction to the transversal filter coefficients.

The acoustic echo cancelling adaptive filter of digital signal processor 5 filters out, in the known manner, speech signals generated by the remote user that are fed from receiver into transmitter via the acoustic path provided by the handset.

Receive speech data at.the output of the receive gain multiplier is coupled to an input of the acoustic echo cancelling adaptive filter. This data represents analog receive signals applied to receiver R and from this data a duplicated echo signal is generated which is utilised in a known manner for echo cancellation in the adaptive filter. Adjustment of the acoustic ncho cancelling adaptive filter is only enabled when speech detector 7 detects the presence of speech from the remote user and speech detector 6 detects the absence of spccch from the local user, in order to prevent divergence of parameters of the adaptive filter.

The ambient noise det:ctor will detect noise signals transduced by transmitter that are present at the output of the acoustic echo cancelling adaptive filter. It does this by virtue of the fact that the signature of a noise signal is characterised in that it is a substantially constant signal of relatively low level having a wide frequency range and can be distinguished from a speech signal whose signature is characterised in that it is an interrupted signal of relatively high level having a narrow known fre- 15 quency band.

If the level of noise detected by the ambient noise detector exceeds a predetermined threshold, the detector will provide a control signal at its output which is applied to an input of the receive gain and side-tone gain control unit. The control unit will thereupon produce gain control signals for the receive gain multiplier and the side-tone gain multiplier, increasing the gain of the former and decreasing the gain of the latter in proportion to the level of ambient noise above the threshold. For example, normally the threshold level would be set to about 50-55 dBa. If this level is exceeded by, say, 20dB, then the ambient noise detector, and the receive gain/sidetone gain control are programmed to cause the receive gain to be increased by 25 above normal level, and the side-tone gain to be reduced by 20dB below normal level.

This effectively maintains the side tone volume constant for ambient noise levels 0. above the threshold.

If there are speech sihnals present at the output of the acoustic echo cancelling adaptive filter, speech detector 6 coupled thereto detects these signals and disables the adjustment of the side-tone gain and receive gain multipliers.

The hybrid echo cancelling adaptive filter removes any signal which leaks across the hybrid, such as ambient noise. This permits the gain multipliers to set the receive gain/side-tone gain without spurious side-tone signals caused by hybrid leakage interfering with the set levels. The adjustment of the hybrid echo cancelling adaptive filter is, however, only enabled when speech detector 6 detects the presence of speech from the local user, and speech detector 7 detects the absence of speech from the remote user, in order to prevent divergence of parameters of the adaptive filter.

The embodiment shown in Figure 2 is similar to that shown in Figure 1 except that the acoustic echo cancelling adaptive filter function, which reduces the level of side-tone that is returned via the hybrid to the remote user, is omitted together with the digital-to-analog converter 4. Referring to Figure 3, the embodiment shown therein is similar to that shown in Figure 2 except that digital signal processing is not used, which, of course, obviates the need for converters 1, 2 and 3 of the embodiment shown in Figure 2. Separate analog elements perform the functions performed by digital signal processor 5 in the other embodiments. The hybrid echo cancelling adaptive filter is replaced by a known adaptive balance circuit, such as, for example, a PLESSEY SL9009 with the adaption being enabled only when speech detector 6 detects the presence of speech from the local user and speech detector 7 indicates the absence of speech from the remote user.

15 The adaptive balance circuit is arranged to balance the balance impedance (BAL Z) of the hybrid in order to match the exchange line impedances to minimise leakage across the hybrid from the transmit circuit to the receive circuit. Any mis- S. match of the impedance would permit residue of the transmit signal to be added to the receive signal resulting in unwanted levels of side-tone.

The known adaptive balance circuit comprises two sections (not shown), a phase detector section and an impedance cell section. The phase detector section is used to S analyse an extracted signal from the receive side of the hybrid to determine how much of the residual transmit signal is present. Information obtained from the analysis, in the form of two voltages, is applied to the impedance cell section to adjust the balance 25 impedance presented by these cells to the hybrid. For clarity, the hybrid balance impedance is shown as a separate block coupled to the hybrid, whereas, in practice, it is part of the adaptive balance circuit.

While the present invention has been described with regard to many particulars, it is understood that equivalents may be readily substituted without departing from the scope of the invention.

Claims

1. A method of automatically adjusting the level of received speech signals ap- plied to the receiver of a telephone subset, and the level of side-tone signal produced by the subset's transmitter and applied to the said receiver when, in the at. nce of transmitted speech, ambient noise transduced by said transmitter exceeds a prede- termined threshold, said method comprising the steps of: providing a hybrid means for coupling a two wire exchange line to a subset receive path having an adjustable receive gain means therein, and a subset transmit path, respectively coupled to said receiver and said transmitter, said paths being coupled via a side-tone path which includes an adjustable side- tone gain means for providing a side-tone signal; monitoring signals produced by said transmitter to detect transmitted speech signals and ambient noise signals, and producing an enabling signal when, in the absence of transmitted speech signals, ambient noise signals .0' 15 exceed a predetermined threshold; enabling a gain control means to adjust said receive gain means and said side-tone gain means to increase the receive speech signal level and decrease S. the level of side-tone signal in a predetermined proportion.

2. A method of automatically adjusting the level of received speech signals ap- plied to tt receiver of a telephone subset, and the level of side-tone signal produced by the subset's transmitter and applied to the said receiver when, in the absence of i transmitted speech, ambient noise transduced by said transmitter exceeds a prede- termined threshold, said method comprising the steps of: providing a hybrid means for coupling a two wire exchange line 25 to a subset receive path having an adjustable receive gain means therein, and a subset transmit path, respectively coupled to s 'J receiver and said transmitter, said paths being coupled via a side-tone path which includes an adjustable side- tone gain means for providing a side-tone signal; converting analog signals produced by said transmitter into first digital signals; converting received analog speech signals coupled to said subset receive path by said hybrid means into second digital signals; monitoring said first digital signals to detect converted transmitted speech signals and converted ambient noise signals, and producing an enabling signal when, in the absence of transmitted speech signals, ambient noise signals exceed a predetermined threshold; enabling a gain control means to adjust said receive gain means and said side-tone gain means to increase the receive speech signal level and decrease the level of side-tone signal in a predetermined proportion; convert the adjusted receive speech signal into analog mode.

3. A method as claimed in claims I or 2, including the step of enabling an adap- tive hybrid leakage minimisation means when transmitted speech signals are present and receive speech signals are absent to cancel any transmitted speech signals and/or any ambient noise signals that leak from said transmission path to the receive path via said hybrid means.

4. A method as claimed in claim 3 as appended to claim 2, wherein the hybrid leakage minimisation means is a first adaptive filter means in said receive path. 15

5. A method as claimed in claims 2, 3, or 4, including the step of enabling a sec- ond adaptive filter means provided in said transmit path to cancel speech signals generated by a remote user that are introduced into said transmitter via an acoustic path formed by the subset's handset.

6. A method as claimed in claim 3 as appended to claim I, wherein the hybrid leakage minimisation means is an adaptive balance circuit.

7. An apparatus for carrying out the method as claimed in claim 1, comprising S. a receive path including an adjustable receive gain means, and a transmit path re- spectively coupling a receiver and a transmitter to a two wire exchange line via a hy- brid means, a side-tone path including an adjustable side-tone gain means coupling 25 said transmit path to said receive path, monitor means arranged to monitor trans- mitted speech signals and ambient noise signals, said monitor means producing a first enabling signal when in the absence of transmitted speech signals, ambient noise signals exceed a predetermined threshold, said first enabling signal being coupled to a gain control means which is arranged to adjust the-said receive gain control means and the side-tone gain control means such that the receive speech signal level is in- creased and the side-tone signal level is decreased in a predetermined proportion.

8. An apparatus for carrying out the method as claimed in claim 2, comprising a receive path including an adjustable receive gain means, and a transmit path re- spectively coupling a receiver and a transmitter to a two wire exchange line via a hy- brid means, a side-tone path including an adjustable side-tone gain means coupling said transmit path to said receive path, first converter means for converting analog signals produced by said transmitter and applied to said transmit path, and second converter means for converting analog receive signals derived from said hybrid means into second digital signals, wherein said apparatus further includes monitor means arranged to monitor said first digital signals to detect converted transmitted speech signals and converted ambient noise signals, said monitor means producing a first enabling signal when in the absence of transmitted speech signals, ambient noise signals exceed a predetermined threshold, said first enabling signal being coupled to a gain control means which is arranged to adjust the said receive gain control means and the side-tone gain control means such that the receive speech signal level is in- creased and the side-tone signal level is decreased in a predetermined proportion, and a third converter means for converting the adjusted receive speech signals into analog mode.

9. An apparatus as claimed in claim 7 or 8, wherein said monitor means com- 15 prises an ambient noise detector means and a first speech detector means whose re- g spective inputs are coupled to said transmit path and outputs of which are coupled S.O, to said gain control means. S.

10. An apparatus as claimed in claim 9, wherein said receive path includes a first adaptive filter means having a plurality of enabling input means respectively coupled to said first speech detector means, said transmit path and a second speech detector means whose input is coupled to said receive path, whereby when transmitted speech signals are present and receive speech signals are absent, said first adaptive filter means is enabled and cancels any transmittcd speech signals and/or any ambient noise signals that leak from said transmission path to the receive path via said hybrid 25 means,

11. An apparatus as claimed in claim 9 or 10, wherein said transmit path includes a second adaptive filter means having an iriput means to which is applied data rep- resentative of receive speech signals applied to said receiver, whereby any said receive speech signals that are introduced into said transmitter via the acoustic path formed by the subset's handset are cancelled by said second adaptive filter means.

12. An apparatus as claimed in claim 10 or 11, wherein said adaptive filter means are finite impulse response filters.

13. An apparatus as claimed in any one of claims 9 to 12, wherein said adjustable receive gain means, said adjustable side-tone gain means, said speech detector means, 9 said ambient noise detector means and said adaptive filter means are integrated within a single device.

14. An apparatus as claimed in claim 13, as appended to claim 6 wherein said single device is a programmable digital signal processor.

15. An apparatus as claimed in any one of claims 10 to claim 14, including an adaptive balance circuit having a first input means coupled to said receive path, a second input means coupled to said transmit path, a first enabling input means coupled to said first speech detector means, and a second enabling input means coupled to said second speech detector means whereby said adaptive balance circuit is enabled when transmitted speech signals are present and receive speech signals are absent, said adaptive balance circuit being arranged to analyse signals in said receive path to determine the presence therein of signals from the transmit path leaked via said hybrid means, and accordingly adjust a balance impedance means coupled to said hybrid means to obtain minimum leakage of signals from the transmit path to the receive path.

16. An apparatus as claimed in any one of claims 7 to 15, wherein said subset is of the payphone type,

17. An apparatus substantially as herein described with reference to Figures 1, 2 and 3 of the accompanying drawings. %:of 06* i DATED THIS TENTH DAY OF NOVEMBER 1993. ALCATEL AUSTRALIA LIMITED 5 S 0 4 Se ABSTRACT This invention discloses a method and apparatus for automatically adjusting side-tone in a telephone subset, particularly of the payphone type, in the presence of ambient noise. This is accomplished by coupling the receiver and the transmitter of the subset to an exchange line via a respective receive path and transmit path and a hybrid (HYB). The receive path includes an adjustable gain multiplier and the subset's side-tone path includes an adjustable side-tone gain multiplier. An ambient noise detector and a speech detector arc arranged to detect transmitted speech signals and ambient noise signals and to enable, in the absence of transmitted speech signal. and in the presence of ambient noise signals of a predetermined magnitude, a gain control means which thereupon adjusts the receive gain multiplier and the side- tone gain multiplier to increase the received speech signal level and decrease the level c: side-tone in a predetermined proportion. (Figure 1) S I