Automatic gain control

Details Automatic gain control Automatic gain control

1998-11-02

2002-03-26

Dorvil, Richemond (Department: 2741)

Data processing: speech signal processing, linguistics, language

Speech signal processing

For storage or transmission

C704S201000, C704S231000, C455S136000, C455S138000, C375S251000, C367S065000, C235S462260, C342S092000

Reexamination Certificate

active

06363343

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a gain control circuit in particular, but not exclusively, for a telephone.
DESCRIPTION OF THE PRIOR ART
When a user of a telephone speaks into the microphone, the microphone converts the sound into either an analogue or digital electric signal which is then amplified and processed before the signal is transmitted onto the telecommunications network, which may be a wired or wireless system.
Typically the amplification of the voice signal is fixed and set at a level suitable for volumes used in normal conversation. Unfortunately, if a user, say the calling party, wants to lower the volume of their voice, for example when privacy is required from people nearby or the user wants to minimise the disturbance to other people, the called party may have difficulty in hearing the calling party. One solution to this problem has been to incorporate into the phone a ‘whisper’ mode which is selected by the user when required. When in this mode the gain of the voice signal is increased by a fixed amount. This solution enables the called party to hear quiet conversation as if the caller was talking at normal volume. This solution works well when the caller is talking quietly, however if the caller starts to talk at normal volume while the ‘whisper’ mode is still active the called party may be deafened and/or acoustic coupling between the microphone and speaker may take place.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, there is provided a gain control circuit for a telephone, the circuit comprising a signal strength calculator for calculating the strength of a signal; and a gain controller adapted to modify the strength of the signal in accordance with a predetermined criterion based on the strength of the signal calculated by the signal strength calculator.
In the present invention the gain applied to the signal, typically a speech signal, increases as the strength of the signal decreases. This automatically compensates for the strength of the input signal. Therefore, if a user wishes to lower the volume of their voice the called party will still hear the speakers voice without any appreciable change in volume.
Preferably the circuit further comprising a speech activity detector for determining the presence and/or absence of speech, wherein the gain controller sets the gain applied to the signal to a predetermined level based on whether presence and/or absence of speech in the signal is detected.
Preferably the speech activity detector is coupled to the gain controller.
More preferably the speech activity detector is coupled to the signal power calculator.
Typically the gain controller sets the gain applied to the signal to the predetermined level based on whether the level of speech in the signal falls below a predetermined threshold as calculated by the speech activity detector.
Typically the predetermined level is the last gain value set by the gain controller before the speech level fell below the predetermined threshold.
Preferably the predetermined criterion is that the gain applied to the signal varies substantially linearly in relation to a change in the signal strength.
According to a second aspect of the present invention, we provide a method of controlling the gain of a telephone audio signal wherein the strength of the audio signal is varied in accordance with a predetermined criterion based on the strength of the audio signal.
Preferably the strength of the audio signal is set to a predetermined level when the level of speech falls below a predetermined threshold.


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