GB2131255A - Full-duplex speaker phone - Google Patents

Abstract

A speaker phone terminal in which a microphone (12) and a speaker (18) are connected to a telephone line pair (14) through a hybrid circuit (16) which, in itself, is insufficient to prevent oscillation due to feedback through audio coupling between the microphone and the speaker. Feedback oscillation is suppressed by converting the audio signal from the microphone to a higher frequency range by single- sideband modulation. The higher frequency signal is passed through a clipping circuit (42) which, in effect, is a switch which only closes when the absolute amplitude of the signal exceeds a predetermined threshold level. The output of the clipping circuit is then converted back down to the original audio range and coupled to the telephone line and the local speaker through the hybrid circuit. <IMAGE>

Description

SPECIFICATION Full-duplex speaker phone This invention relates to speaker phone systems, and more particularly, to a full-duplex speaker phone including a clipping ci rcuit for suppressing feedback oscillation.

Afull-duplextelephone system is one in which both parties can talkand be heard atthesametime.

Conventional telephone hand sets provide full-duplex operation. However, in systems where the conventional telephone hand set is replaced by a speaker phone, full-duplex operation has not been possible because of the high level of audio coupling between the speaker and the microphone which provides sufficient feed back to produce oscillation due to the gain in the transmission loop linking the microphone to the speaker. It has therefore been the practice in speaker phones to use a voice actuated switch which causes the speaker to be cutoffwhile the person is talking. While this arrangement solves the feedback problem, it is annoying because it does notallowthe party atthe other end ofthe line to interrupt orto be heard until the other party stops talking.

The present invention is directed to an improved circuitarrangementfor preventing or suppressing oscillation due to the closed loop formed by audio coupling between the local speaker and the microphone. Since the feedback signal starts out at a very low level and builds up with successive passes around the loop, itis possible to preventorsuppressthe build-up of a feedback by a centre-clipping circuit which, in effect, is a switch which only closes when the absolute amplitude ofthe signal exceeds a predetermined threshold level. Thus the centre-clipping circuit prevents the feedback signal from getting started since the signal is initially belowthe threshold level at which the switch is closed.

One problem with conventional centre-clipping circuits is thatthe switching action, which takes place twice each half cycle as the signal amplitude of an audio signal rises above and then falls back below the threshold level, introduces odd harmonics ofthe audio signal. Since the telephone audio band extends approximately from 300 Hzto 3300 the odd harmonics of signals below 1100 Hz are still within the telephone audio band. These added harmonics cause severe distortion ofthe audio signal.

The present invention is directed to an improved centre-clipping ci rcu it which can be used to provide a full-duplex speaker phone system without noticeable distortion orfeedbackoscillation.

This is accomplished, in brief, by providing a speaker phone system in which the audio output from the microphone is first converted to a higherfrequen cybyasingle-sideband modulation. The higher frequency signal is then applied to a centre-clipping ci rcuit which provides substantially zero gain for signals having amplitude levels lessthan a threshold voltage while providing unity gain for signal voltages in excess ofthe threshold voltage. Harmonic distortion is prevented by first converting the frequencies of any audio signal received from the microphoneto higherfrequencies by single-sideband modulation.

The sideband is then applied to a centre-clipping circuit which switches off the output for all absolute amplitude values below a predetermined threshold voltage. The output of the centre-clipping circuit is then reconverted to the frequencies ofthe initial audio signal by a balanced modulator and bandpass filter.

The output of the bandpass filter is coupled to the telephone line and to the speakerthrough a conventional hybrid circuit.

The present invention isafull-duplexspeakerphone terminal comprising a microphone, a hybrid circuit, means including a centre-clipping circuit connecting the output ofthe microphone to the hybrid circuit, a speakerconnected to the hybrid circuit.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, wherein:- Fig. is a schematic block diagram of a full-duplex speaker phone terminal according to the present invention; Fig. 2 isa schematic circuit diagram ofthecentreclipping circuit; and Fig. 3 is a waveform used in explaining the operation of the centre-clipping circuit.

Referring to the drawings in detail, the numeral 10 indicates generally a speaker phone system which includes a microphone 12 that produces an audio signal that is amplified and coupled to a remote phone over a phone line 14through a conventional hybrid circuit 16. The hybrid circuit 16 also couples the signals received on the phone line 14to a speakerl8.

While the hybrid circuit 16 provides some isolation between the microphone 12 and the speaker 18,the transhybrid loss is approximately6dbwhich is insufficientto prevent oscillation in the closed loop including audio coupling from the speaker 18 back to the microphone 12.

To providefull-duplex operation with communica tion taking place over the phone line 14 in both directions simultaneously, the present invention provides a feedback suppression circuit in the transmission portion of the speaker phone terminal between the microphone 12 and the hybrid circuit 16. This is accomplished by applying the audio signal from the microphone through an audio pre-amp 20 to a pair of sideband phase shifter circuits 22 and 24. The phase shifter circuits provide a difference in phase between the outputsignalsfrom the two phase shifters of substantially 90" over the audio frequency spectrum of 300 Hzto 3300 Hz.

The outputs ofthe respective phase shifters 22 and 24 are appiied to the inputs of a pair of balanced modulator circuits 26 and 28. The modulator circuits moduiate carrier signals, which are also in phase quadrature, derived from a carrier generator 30 that is synchronized with a 1 Megahertz crystal oscillator 32.

The carrier frequency is preferably 20 KHz. The balanced modulators 26 and 28 provide double sideband suppressed carriers modulation. By summing the outputs from the two balanced modulators, as indicated at 34, only the lower sideband is retained, having a frequency range of 16.7 KHz to 19.7 KHz.

The lower sideband output from the summing circuit 34 is applied to a centre-clipping circuit, indicated generally at 36. The centre-clipping circuit includes a fullwave rectifier 38 which provides an output voltage that varies with the absolute amplitude ofthe signal from the summing circuit34. This absolute amplitude is compared with a threshold voltageVT in an amplitude comparatorcircuit40. The comparator 40 provides a binary output which changes state wheneverthevoltage atthe output of the rectifier 38 drops below or rises above the threshold level.The output level from the comparator 40 is used to control a solid state switching circuit 42 which connects the signal from the output ofthe so miming circuit 34 to the output of the centre-ciipping circuit. The switch 42 is turned on when the instantaneous absolute amplitude of the alternating signal from the output of the summing circuit 34 exceeds the level of the threshold voltage VT. The switch 42 is turned off when the absolute amplitude drops below the level of the threshold voltage VT. The resulting output from the switch 42 for a particular sine wave on the input is shown by the waveform in Fig. 3. A detailed schematic of a preferred embodimentofthe centre-clipping circuit 36 is shown in Fig. 2.

The output from the centre-clipping circuit is applied to a balanced modulator 34, togetherwith the carrier generator 30. The modulator produces two side-bands, one of which is in the frequency range of 300-3300 Hz. This lower sideband is isolated by a bandpass filter 46 having a frequency pass band of 300-3300 Hz. The output ofthe bandpass filter 46 is coupled to the hybrid circuit 16through a power amplifier 48 for transmission on the phone line 14.

In operation, the centre-clipping circuit 36 has the effect of preventing feedback oscillation from building up around the closed loop formed by the audio coupling between the speaker 18 and microphone 12.

At the same time, the frequency conversion and reconversion eliminates distortion due to generation of odd harmonics by the switching action ofthe centre-clipping circuit 36. For example, the third harmonic of the input signal to the centre-clipping circuitwill be in the range of 50,100 to 59,100 Hz. On reconversion to the audio range, these harmonics will be completely out of the passband of the filter 46.

Claims (9)

1. Afull-duplex speaker phone terminal comprising a microphone, a hybrid circuit, means including a centre-clipping circuit connecting the output of the microphone to the hybrid circuit, and a speaker connected to the hybrid circuit.

2. A phone terminal as claimed in claim 1, wherein said means including the centre-clipping circuit comprises means for converting the frequency ofthe microphone output to a higherfrequency at the input of the centre-clipping circuit, and means converting the frequency of the output ofthe clipping circuit back to the initial frequency atthe inputto the hybrid circuit.

3. Aphoneterminal as claimed in claim 2,wherein said means converting to a higherfrequency includes a single-sideband modulator, and said means reconverting backto the initial frequency includes a balanced modulator and bandpass filter.

4. Afull-duplexspeaker phoneterminal comprising a microphone for generating an audio signal, a single-sideband modulator coupled to the mic rophone for converting the audio signal to a singlesideband signal at a higherfrequency range, centreclipping means coupled to the output of the modulator for switching off the single-sideband signal when the absolute voltage amplitude of the signal is below a predetermined threshold, means for reconverting the outputofthecentre-clipping means backtothe original audio signal frequency, a speaker, a hybrid circuit connecting the output from said means for recoverting the output of the centre-clipping means to a telephone line, and means connecting the hybrid circuit to the speaker.

5. A phone terminal as claimed in claim 4, wherein the centre-clipping means includes a full-wave rectifier, and amplitude comparatorfor comparing the voltage amplitude ofthe output of the rectifier with a threshold voltage, and a switch operated bythe output of the amplitude comparator.

6. A low distortion centre-clipping circuit for attenuating low amplitude signal components, comprising a single-sideband modulatorforconverting an audio signal to a higherfrequency, balanced modulator and filter means for converting the higherfrequency back to an audio signal, and means including switch means connecting the output ofthe single-sideband modulator to the balanced modulator and filter means for switching off the signal to the balanced modulator when the absolute voltage amplitude drops below a predetermined threshold voltage and switching on the signal to the balanced modulatorwhen the absolute voltage exceeds said predetermined threshold voltage.

7. An audio system comprising a microphonefor generating an audio signal, a single-sideband modulator coupled to the microphonefor converting the audio signal to a single-sideband signal at a higher frequency range, centre-clipping means coupled to the output ofthe modulatorforswitching offthe single-sideband signal when the absolute voltage amplitude ofthe signal is below a predetermined threshold, means for reconverting the output of the centre-clipping means back to the original audio signal frequency, a speaker, and means connecting the output from the reconverting means.

8. An audio system as claimed in claim 7, wherein the centre-clipping means includes a full-wave rectifier, an amplitude comparator for comparing the voltage amplitude of the outputofthe rectifier with a threshold voltage, and a switch operated by the output of the amplitude comparator.

9. Afull-duplexspeaker phoneterminal substan- tally as herein before described with reference to, and as shown in, the accompanying drawings.