Multiplex communications – Wide area network – Packet switching
Patent
1993-06-23
1995-03-07
Hsu, Alpus H.
Multiplex communications
Wide area network
Packet switching
379409, H04B 320, H04M 908
Patent
active
053964888
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a method for non-linear signal processing in an echo canceller, in which method the level of the signals passing through the echo canceller from the first end of a transmission connection to the second end is attenuated to a predetermined blocking level, when the level of the first end signals entering the echo canceller is lower than a given threshold level, and the level of the signals passing through the echo canceller from the first end to the second end is raised back to the normal unattenuated level, when the level of the first end signals entering the echo canceller again exceeds the given threshold level or when a double talk situation is detected.
BACKGROUND OF THE INVENTION
End-to-end connections of a data transmission system, such as a telephone network, often show long transit time delays, in consequence of which echo is observed, for instance in the case of normal speech, when a signal is reflected from the far end of a connection back to the talker. An echo canceller is an analog or digital device for processing a signal, such as a speech signal, which device is used for reducing echo by subtracting estimated echo from the echo (signal) occurring in the connection.
An echo canceller may comprise a non-linear processor, which only allows a passage of signals exceeding a given threshold level and blocks a passage of low-level signals, as is illustrated by a characteristic curve in FIG. 2. Non-linear processing is used for lowering the level of a residual echo caused by an incomplete echo cancellation. This results in a better overall attenuation (total attenuation consisting of echo path attenuation, echo cancellation attenuation and non-linear processing).
Incomplete echo cancellation is possible if considerable non-linear distortion occurs on the echo path (many PCM coders on the echo path, for instance). Combined quantized distortion may prevent the echo canceller from producing an echo signal with sufficiently low level at the output port of the echo canceller. Therefore, it is often advantageous to use a non-linear processor, though not absolutely necessary.
In an ideal case, a non-linear processor does not cause distortion in the speech signal coming from the near end, i.e. from the direction of the echo path. In practical applications, however, distortion may occur, in which situation it is to be recommended to shunt the non-linear processor during double talk (both ends are talking) and near end talk conditions.
The threshold level used in a non-linear processor for signal blocking decisions may be either fixed or adaptive. A fixed threshold level may be determined on the basis of the achieved echo cancellation, speech levels and line conditions. As a general rule, the threshold level shall be set about a few dB higher than the peak levels of the residual echo occurring when the echo cancellation is in operation. A good compromise can be achieved by employing a threshold level adaptive to the line conditions and speech levels. By using a high threshold level an undesired exceeding of the threshold level caused by high-level residual echo is prevented, and by using a low threshold level a distortion of speech during interruptions in speech is reduced.
SUMMARY OF THE INVENTION
The object of the invention is to provide a method for signal processing by means of a non-linear processor in such a manner that the resulting interference will be minimized.
This is achieved by a method characterized in that the level of the signals passing through the echo canceller is raised ramp-like from the blocking level to the given threshold level during a predetermined time.
In the preferred embodiment of the invention, the non-linear processing is terminated by raising the signal level ramp-like, preferably linearly, from the blocking level to the threshold level instead of affecting the level change as a rapid step. By means of such a "smoothening", the little audible stir caused by a quick change in the signal level can be avoided in the telephone
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Hsu Alpus H.
Nokia Telecommunications Oy
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