Method of and Device for Echo Cancellation

1. Field of the Invention

The invention relates to a method of echo cancellation for a full duplex loudspeaking telephone, in which the loudspeaker signal picked up by the microphone is cancelled by means of an adaptive filter. It also relates to a device for echo cancellation with an adaptive filter.

2. Description of the Related Art

Such a method and a corresponding device are known, for example from EP 0,301,627 B1. Said document describes an adaptive filter which operates substantially free from delay. The coefficients of the adaptive filter can be adjusted only in those time intervals in which there is no speech at this location, so that the microphone then only picks up the echo signal, which is cancelled to an optimum extent by means of the adaptive filter.

For a correct operation of the adaptive filter it is essential that the levels of the signals at the input of the adaptive filter and the combining circuit, i.e. the summing circuit, are in a given ratio to one another. This is generally achieved by arranging an amplifier before the loudspeaker and an amplifier after the microphone, the gain of the two amplifiers being adjusted in such a manner that the ratio between the levels of the two signals has a given value. However, these levels depend on the geometrical position of the loudspeaker relative to the microphone and on the environment, and the adjustment must be repeated upon every change of place.

It is an object of the invention to provide an echo cancelation method and device for a full duplex loudspeaking telephone, which provides substantially automatic adjustment of the ratio between the levels of the signal at the input of the adaptive filter and the signal generated by the microphone for the purpose of combining.

Owing to the adjustable attenuation of the microphone signal prior to combining it is achieved that the desired signal level ratio is adjusted independently of the loudspeaker amplifier and the microphone amplifier and of the position of the loudspeaker relative to the microphone, the signal produced at the transmitting line by the microphone being independent of this adjustment because the attenuation of the microphone signal before the transmitting line is cancelled.

The invention is also very advantageous in the case that the gain of the amplifier preceding the loudspeaker is changed, for example, for a better intelligibility of the other station of the connection, because the additional attenuation can then automatically be changed accordingly in an opposite sense. Since this change in attenuation is also cancelled the signal at the transmitting line yet remains constant as far as the microphone signal is concerned. In principle, this also applies to a change of the gain of the microphone amplifier.

An embodiment of the sole invention will be described in more detail, by way of example, with reference to the sole drawing. The drawing shows a full duplex loudspeaking telephone having an input line 10 connected to an amplifier 12 whose output is connected to a loudspeaker 14.

The acoustic signal emitted by the loudspeaker reaches a microphone 30 via the acoustic echo path 18, which microphone also picks up acoustic speech signals and noises produced at this location. The signal generated by the microphone 30 is applied to the transmitting line 20 via an amplifier 28, a first attenuator circuit 26, a combining circuit 24 and an amplifier circuit 22.

The output signal of the combining circuit 24 appearing on the line 23 is also applied to an adaptive filter 34, which receives the signal appearing on the input line 10 at an input 33 via an attenuator circuit 36 whose function will be described later. At its output 35 the adaptive filter 34 generates a filtered signal which, in the case that the microphone 30 only picks up the acoustic signal via the echo path 18, is opposite to the signal on the line 25, so that the output signal of the combining circuit 24 appearing on the line 23 is basically zero. As a result of this, the microphone signal component emanating from the echo path is always cancelled exactly even if the microphone 30 picks up additional speech signals at this location.

For a correct operation of the adaptive filter the level of the signal on the line 25 should be 6 dB below the level of the signal on the input line 10. In the present device this is achieved as follows.

The attenuator circuit 26 and the amplifier circuit 22 each have a control input connected to a control circuit 40 via a line 41. The attenuation of the attenuator circuit 26 and the gain of the amplifier circuit 22 are controlled via the line 41, in such a manner that the gain is inverse to the attenuation, so that the effect of the attenuator circuit 26 and that of the amplifier circuit 22 cancel one another. The control circuit 40 receives the signal at the output of the attenuator circuit 26 via the line 25 and the signal on the input line 10 and forms the ratio between the levels of these two signals. At a given instant, for example in response to a signal from a user, the control circuit 40 actuates a signal generator 44, for example a noise generator, via a line 45, which generator produces a signal on the input line 10. This signal is applied to the control circuit 40 and, via the line 25, the control circuit receives the signal produced via the loudspeaker 14, the echo path 18 and the microphone 30 and in response thereto changes the attenuation of the attenuator circuit 26 via the line 41 until the levels of the signals at the lines 10 and 25 have the predetermined ratio of 6 dB. This is then independent of the setting of the gain of the amplifier 12 for the loudspeaker 14 and of the gain of the amplifier 28 for the microphone 30. It is then assumed that during this adjustment process no other noise or speech signals reach the microphone 30.

The amplifier 12 for the loudspeaker 14 generally has a variable gain in order to allow the reproduction level to be adapted to local requirements. If after the adjustment the gain of the amplifier 12 is changed this change is also transferred to the control circuit 40 via the line 15, which control circuit changes the attenuation of the attenuator circuit 26 in an opposite sense. If the gain of the amplifier 12 is, for example, increased by 3 dB the attenuation of the attenuator circuit 26 is also increased by 3 dB, so that consequently the ratio between the levels of the signals on the lines 10 and 25 remains constant again. Since the attenuation of the attenuator circuit 26 is always compensated by a corresponding gain of the amplifier circuit 22, the ratio between the levels of the signals at the transmitting line 20 and the line 27 connected to the output of the microphone amplifier 28 will remain constant.

With the described adjustment process it is possible that the attenuator circuit 26 is adjusted exactly to 0 dB in order to obtain the above level ratios. If the gain of the amplifier 12 is then reduced the attenuation of the attenuator circuit 26 would also have to be reduced, i.e. it would have to operate as an amplifier. If this is to be avoided an attenuator circuit 36 may be arranged before the input 33 of the adaptive filter 34 to provide a suitable attenuation of the signal at the input line 10 to the adaptive filter 34 rather than to reduce the attenuation of the attenuator circuit 26 beyond 0 dB. This attenuation of the attenuator circuit 36 is adjusted by the control circuit 40 via the line 43 in such a manner that the difference between the attenuations of the two attenuator circuits 36 and 26 has a value determined by the setting during the adjustment process, if necessary corrected by a change in the gain of the amplifier 12 via the line 15, in which case it is advantageous if for each setting either the attenuator circuit 36 or the attenuator circuit 26 or both have zero attenuation. For echo cancellation a change of the gain of the amplifier 12 is equivalent to a change of the gain of the microphone amplifier 28, so that if this gain is also variable a signal is applied to the control circuit 40 via the line indicated by the dashed arrow 47 and also produces a change of the attenuations of the respective attenuator circuits 26 and 36.

It is to be noted that several parts of the present device, particularly the control circuit 40, the attenuator circuits 26 and 36, the amplifier circuit 22 and the combining circuit 24, and if desired also the variable amplifiers 12 and 18, can be realised by means of a single signal processor.