Method of and device for echo cancellation

DETAILED DESCRIPTION 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.