GB2197166A

GB2197166A - Controlling gain in speech-controlled telephones

Info

Publication number: GB2197166A
Application number: GB08626676A
Authority: GB
Inventors: Sven Langsted
Original assignee: Storno AS
Current assignee: Motorola Solutions Danmark AS
Priority date: 1986-11-07
Filing date: 1986-11-07
Publication date: 1988-05-11
Also published as: GB8626676D0

Abstract

A speech-controlled telephone comprises a transmit path having a microphone coupled to a communication channel output via a transmit amplifier (2) and a transmit attenuator (4) and a receive path having a loudspeaker coupled to a communication channel input via a receive amplifier (6) and a receive attenuator (7). The speech signal levels in each path are detected by detectors (8, 9) and the gains in each path are controlled via a differential amplifier (10) so that attenuation is decreased in a path with speech signals and increased in the other path. The total gain is kept below a maximum level, which maximum level is reduced when a comparator (12) detects high-level signals in each path indicating a howl-round condition. <IMAGE>

Description

SPECIFICATION Speech controlled telephones This invention relates generally to speech-controlled telephones and more particularly to mobile or portable radiotelephones provided with so-called hands-free operation.

Such telephones generally have separate transmit and receive paths each connected to a communication channel and each having an amplifier and an attenuator between the communication channel and a microphone or loudspeaker respectively. The loudspeaker is desirably located near to the microphone, preferably in the same cabinet. When the loudspeaker reproduces sound at speech-level, the associated microphone may pick up a crosstalk signal which may provide a stronger signal than any direct speech. A strong acoustic coupling is therefore likely, providing a large risk of acoustic feedback or howl round condition as is well known. It is common practice, therefore to control the amplifiers and attenuators using the speech signals so that the channel in use has a high gain and the other channel is cut off providing a half-duplex mode of operation.

In this case, at any particular time, the telephone is either in a transmit mode, idle mode or receive mode, as is well understood. The transmit and receive attenuators are controlled by a single circuit and are complementary in function, i.e. in response to a control signal supplied from the control circuit, one is operated at maximum gain while the other is operated at minimum gain and vice versa. The two attenuators are never both on or both off.

Thus, for instance, if the user is speaking, the telephone is placed in the transmit mode wherein the transmit attenuator is operated at maximum gain while the receive attenuator is operated at maximum attenuation to inhibit a receive signal from adversely affecting the transmission of the speech signal from the user. Similarly, in response to a received audio signal the receive attenuator is automtically switched to maximum gain while the transmit attenuator is operated at maximum attenuation. In the idle mode, whenever audio signals are neither being transmitted nor received, the gains of both the transmit and the received channels are reduced to some mid value.

It will be appreciated that in the above-described half-duplex mode of operation it is possible for the conversation to be somewhat disjointed as each party can only talk when the other party has finished and if one party tries to break in on the other then some words may be lost. Furthermore, when one party is talking, the receive channel is fully attenuated giving a "dead" feel as if the party at the other end had been cut off.

It is therefore an object of the present invention to provide a speech-controlled telephone with a full duplex mode of operation.

Accordingly, the invention provides a speech-controlled telephone comprising a transmit path having a microphone coupled to a communication channel output via a transmit amplifier and a transmit attentuator, a receive path having a loudspeaker coupled to a communication channel input via a receive amplifier and a receive attenuator, means for detecting the speech signal levels in each of the paths and for controlling the gain in each of the paths in a complementary fashion by decreasing the attenuation in a path with speech signals and increasing the attenuation in the other path such that the total gain is kept below a maximum level. In a preferred embodiment of the invention there is further included means for comparing the signal levels in the two paths and for varying said maximum level of the total gain according to said signal levels.Preferably said maximum level of the total gain is decreased if high levels of signals are detected in both the received and the transit paths.

It will be appreciated that the communication channel may be either a land telephone line or a radio link.

The invention will now be more fully described by way of example with reference to the drawing which is a block diagram of the transmit and receive paths of a speech-controlled telephone according to the invention.

Thus, the telephone comprises a transmit path from a microphone, via an amplifier 2, a filter 3 and a transmit attenuator 4 to a transmit channel, and a receive path from a receive channel via a filter 5, a receive amplifier 6 and a receive attenuator 7 to a loudspeaker. The receive path provides the necessary gain between the receive channel and the loudspeaker and, similarly, the transmit path provides the necessary gain between the microphone and the transmit channel.

In order to control the gains in the two paths, the level of signals in each of the paths is detected. There is provided therefore, a transmit detector 8 which detects the level of speech signals in the transmit path and a receive detector 9 which detects the level of speech signals in the receive path. The outputs of both detectors 8 and 9 are passed to a differential amplifier 10 which controls the attentuation levels of the transmit and receive attenutors 4 and 7, the controlling signal to transmit attenuator 4 being inverted by inverter 11 so that the attenuation levels in attentuators 4 and 7 are complementary. In this way the attenuation in the path with speech signals is reduced while the attenuation in the opposite path is automatically increased so that the loop gain is always kept below 0 dB.

In order to provide full duplex two-way communication and to avoid unpleasant clipping of the received signals, the maximum attentuation which may be inserted in either of the two paths is kept below an adaptively controlled maximum level. In the idle state, attenuation is inserted in both paths to provide a loop gain of about -20 to -30 dB.

Under normal speech conditions the maximum attenuation in each path will be about 10 dB.

This will permit full-duplex communication but will not be sufficient to prevent a howl-round condition under unfavourable acoustic conditions. The presence of howl-round is indicated by the simultaneous appearance of high-level signals in both the receive and transmit paths.

This is detected by passing the output of the two detectors 8 and 9 to a comparator 12 which compares the signal levels in both paths and if a howl-round condition is sensed, then the loop gain is further reduced by allowing an increase in the signal attenuation in each path by up to a maximum of 60 dB. A fast attack/slow-release circuit 1 3 is inserted in the controlling path from the comparator 12 to the attenuators 4 and 7. When howl-round occurs, the attenuation in the branch without speech signal is instantaneously increased to 60 dB whereafter it returns towards its normal value with a time constant of 5 seconds. If howl-round reoccurs, the attention is again increased to 60 dB, an this procedure continues until potential stability has been restablished.

It will be appreciated therefore that the use of a first circuit for controlling the attenuation balance so that the signal gains in the two directions are automatically adjusted in a complementary fashion keeping the total loop gain below a maximum level and to use a second circuit to control the maximum value of loop gain by sensing the onset of a howl-round condition and reducing the permitted maximum level of loop gain when such howl-round is detected, allows full-duplex operation of a speech-controlled telephone providing better speech quality and improved stability when compared to known speech-controlled telephones.

Claims

1. A speech-controlled telephone comprising a transmit path having a microphone coupled to a communication channel output via a transmit amplifier and a transmit attenuator, a receive path having a loudspeaker coupled to a communication channel input via a receive amplifier and a receive attenuator, means for detecting the speech signal levels in each of the paths and for controlling the gain in each of the paths in a complementary fashion by decreasing the attenuation in a path with speech signals and increasing the attenuation in the other path such that the total gain is kept below a maximum level.

2. A speech-controlled telephone according to claim 1 further including means for comparing the signal levels in the two paths and for varying said maximum level of the total gain according to said signal levels.

3. A speech-controlled telephone according to claim 2 wherein said maximum level of the total gain is decreased if high levels of signals are detected in both the received and the transmit paths.

4. A speech-controlled telephone according to any preceding claim wherein said communication channel is a land line.

5. A speech-controlled telephone according to any one of claims 1 to 3 wherein said communication channel is a radio link.

6. A speech-controlled telephone substantially as hereinbefore described by way of example with reference to the drawing.