GB2073555A

GB2073555A - Hands-free telephone circuit

Info

Publication number: GB2073555A
Application number: GB8102669A
Authority: GB
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1980-02-29
Filing date: 1981-01-29
Publication date: 1981-10-14
Also published as: DE3007632A1; NZ196279A; AU6753481A; AU535815B2

Abstract

In a hands-free telephone, or a telephone with an additional loudspeaker difficulty may be experienced due to acoustic feedback from the loudspeaker to the microphone, which can cause the system to howl. This is overcome by the use of two electronic switches (SES, EES) arranged in the transmitting channel (SK) and the receiving channel (EK). These are controlled by a clock pulse generator (RG) which generates pulses at twice the maximum frequency to be transmitted, the pulses controlling the switches alternately. In certain cases it may be desired to so control the switches that in each clock pulse cycle there is a short period in which both channels are open. <IMAGE>

Description

SPECIFICATION Hands4ree telephone circuit This invention relates to circuits for suppressing acoustic feedback in hands-free telephones and/or telephone ones with an additional loudspeaker.

Such circuits are known e.g. from German DE-OS Nos. 2436 956; 2439 740; 2440 604; 2445 420; 26 59 028 as well as the technical journal "Elektrisches Nachrichtenwesen", 1978, pp. 288 to 293. All such circuit arrangements relate to so-called voice-controlled hands-free telephones. In these circuits, two attenuators, one in the transmitting channel and one in the receiving channel, are controlled inversely by the analog or digital processing of the signals in these two channels respectively, thus suppressing feedback howl. The conventional circuits are expensive even if they use pure digital circuits which are well suited to monolithic integration.

It is an object of the invention to produce circuits for hands-free telephones or telephones with additional loudspeakers which are simpler than the conventional circuits.

According to the invention there is provided a circuit for suppressing acoustic feedback suppression in hands-free telephones and/or telephones with additional loudspeakers, which includes a transmitting channel including the microphone, the microphone amplifier and the transmitting amplifier, a receiving channel including the receiving amplifier, terminal amplifier and loudspeaker are connected, via a hybrid circuit, with one another and with the subscriber's line, in which one electronic switch each is arranged in the receiving channel and another electronic switch is arranged in the transmitting channel, in which the two switches are so controlled by a square wave clock pulse generator that signal flow does not appear simultaneously in both channels, in which the clock frequency of the clock pulse generator is at least twice as high as the highest frequency to be transmitted, in which a first low-pass filter is arranged between the incoming line and the hybrid, in which a second low-pass filterfollowsthe electronic switch in the receiving channel, and in which the upper limit frequency of said two filters is defined by the highest frequency to be transmitted.

This is based on the application ofthe scanning theorem to the present problem, thus achieving good and sufficient feedback suppression. This gives the advantage that the voluminous and expensive circuitry needed according to the prior art are no longer needed and that in fact only the controlling square wave clock pulse generator and the two electronic switches as set forth above are needed.

An embodiment of the invention will now be explained with reference to the accompanying drawings, in which: Fig. is a block diagram of a hands-free telephone, Fig. 2 is a block diagram of a hands-free telephone embodying the invention, and Fig. 3 shows the waveform of the two clock signals transmitted by the clock pulse generator in the circuit of Fig. 2.

In Fig. 1, both the transmitting channel SK and the receiving channel EK are connected via a hybrid circuit GS, to the incoming office line AL. Starting from the microphone M, the transmitting channel SK contains the microphone amplifier MV and the transmitting amplifier SV while the receiving channel EK, starting from thehybrid GS, contains the receiving amplifier EV, the power amplifier LV and the loudspeaker L. The dashlined loop indicates that in this circuit acoustic feedback may occur, especially if the microphone and the loudspeaker are so located that a sufficient portion of the sound energy from the loudspeaker L, can be taken up by the microphone M.

In Fig. 2, we see a first low-pass filter ATP between the incoming office line AL and the hybrid GS. The transmitting channel SK includes, between the amplifier SV and the amplifier MV an electronic switch SES, while in the receiving channel EK, following the receiving amplifier there is an electronic switch EES.

In addition, there is another low-pass filter ETP between the switch EES and the amplifier LV. The upper limiting (cut-off) frequency of the filters ATP, ETP is defined by the highest frequency to be transmitted.

The electronic switches are so controlled by either the output A or the output B of a square wave generator RG, that signal flow never appears at the same time in both channels, i.e. that always only one of the two switches SES, EES is closed. Thus, the two switches are always opened and closed in opposite senses. The clock frequency 1IT of the square wave generator RG, owing to the principles of the scanning theorem, must be at least twice as high as the highest frequency to be transmitted. We have found that this simple insertion of the two switches and the square wave generator RG, which is correspondingly dimensioned in its clock frequency, can solve the feedback problem.

The low-pass effect, which is usually unwanted, but produced by the hybrid GS, the amplifier SV and the amplifier LV, can be rendered ineffective by opening switches SES, EES simultaneously in each clock period for one short interval t. This is illustrated with reference to Fig. 3. Fig. 3a shows the output signal atthe output A, and in Fig. 3b shows the output signal at the output B of the generator RG. Upon occurrence of the amplitude indicated by the reference numeral lithe respective switch is assumed to be closed, i.e. switch SES during the closing time TEK, see Fig. 3a, and switch EES during the closing time Tusk, see Fig. 3b. Thus, both switches are momentarily opened during the time interval t.The upper limiting frequency of the combinations of the hybrid GS, the amplifier (SV) and the amplifier EV, indicated by the hatchings in Fig. 2, is such as to at least equal the reciprocal value l/tofthe interval time t.

The two clock signals as shown in Figs. 3a or 3b can be produced in a simple way with which any person skilled in the art is familiar, from a square wave oscillation by adding corresponding logic circuits or else with the aid of counters, from a higherfrequency clock signal. It is also possible, however, to produce the clock signals with the aid of a microprocessor provided for serving other purposes in the telephone set, i.e. by correspondingly programming the same, because also for operating this microp rocessor, there is likewise required at least one clock signal from which then, with the aid of the microp rocessor, there can be derived the square wave signals for controlling the electronic switches SES, EES.

Claims

1. A circuit for suppressing acoustic feedback suppression in hands-free telephones and/ortele phones with additional loudspeakers, which includes a transmitting channel including the microphone, the microphone amplifier and the transmitting amplifier, a receiving channel including the receiving amplifier, terminal amplifier and loudspeaker are connected, via a hybrid circuit, with one another and with the subscriber's line, in which one electronic switch each is arranged in the receiving channel and another electronic switch is arranged in the transmitting channel, in which the two switches are so controlled by a square wave clock pulse generator that signal flow does not appearsimultaneously in both channels, in which the clock frequency of the clock pulse generator is at least twice as high as the highestfrequencyto be transmitted, in which a first low-pass filter is arranged between the incoming line and the hybrid, in which a second low-pass filter follows the electronic switch in the receiving channel, and in which the upper limit frequency of said two filters is defined by the highest frequency to be transmitted.

2. A circuit as claimed in claim 1, and wherein said two-electronic switches are both opened during one short interval per clock period.

3. Acircuit as claimed in claim 1 or2, and wherein the upper limiting (cut-off) frequency of the partial circuit composed of said hybrid and the transmitting amplifier and the receiving amplifier is at least equal-to the reciprocal value of the interval time of the clock pulses.

4. A circuit for suppressing acoustic-feedback in a hands-free telephone or in a telephone with a loudspeaker, substantially as described with reference to tne accompanying drawing.