Telephonic communications – Subscriber line or transmission line interface – Network interface device
Patent
1996-09-13
1998-04-07
Hong, Harry S.
Telephonic communications
Subscriber line or transmission line interface
Network interface device
379406, 379411, 370286, 370290, 370291, H04B 320, H04B 323
Patent
active
057374104
DESCRIPTION:
BRIEF SUMMARY
This application is the National Stage Application of PCT Application Ser. No. PCT/FI94/00577 filed Dec. 21, 1994.
FIELD OF THE INVENTION
The invention relates to a method for adaptation to an echo location in an echo canceller comprising an output port, an input port, an adjustable delay unit compensating for the pure delay component in the echo path between the echo location and the output port of the echo canceller, a digital filter calculating, from a signal outgoing to the echo path, an estimate for the echo location which is shorter than the maximum allowed duration of the echo path.
BACKGROUND OF THE INVENTION
In end-to-end connections in a data transmission system, such as a telephone network, long propagation delays are often encountered. Consequently, an echo will be perceived for instance in the case of normal speech when the signal is reflected back from the far end of the connection to the speaker.
Two main factors contribute to the generation of echo: acoustic echo between the earphone and microphone of a telephone set, and electrical echo generated in the transmission systems for the transmit and receive directions of the connection.
Hybrid circuits (two-wire to four-wire transformers) located at terminal exchanges or in remote subscriber stages of a fixed network are the principal sources of electrical echo. Subscriber lines in a fixed network are normally two-wire lines for reasons of economy. Interexchange lines, on the other hand, are four-wire lines.
CCITT recommendation G.131 specifies echo tolerance graphs taking into account the ratio between the echo level and the delay. In practice, it has been found that all connections having an unidirectional delay in excess of 20 ms require an echo cancelling mechanism to reduce the level of echo. The block diagram of FIG. 1 illustrates a transmission system requiring echo cancellation. In FIG. 1, hybrid B causes the speech of speaker A to be transmitted back as echo which is cancelled by an echo canceller B. Correspondingly, hybrid A causes the speech of speaker B to be transmitted back as echo which is cancelled by an echo canceller A.
An echo canceller is a signal processing device, e.g. a device processing speech signals, employed for reducing echo by subtracting an echo estimate from the echo (signal) present in the connection. The echo canceller may be either digital or analog. Today echo cancelling devices are implemented with digital signal processing, thus enabling modelling echo paths having very long propagation delays. Since the echo path is in principle different in each call situation, it is necessary to employ in the echo canceller a method that always adapts to a new echo path at the beginning of the call. Digital signal processing offers an adaptive filter as a solution to this problem. When a signal is present or the signal level is sufficient, adaptation is initiated in which the filter coefficients are updated on the basis of the correlation of the speech signal and the returned echo signal.
FIG. 2 shows a solution of this kind for embodying the echo canceller B of FIG. 1. A speech signal RIN received from speaker A is transmitted as such to hybrid B; the signal SIN from the hybrid includes an echo of the speech of speaker A. An adaptive filter 20 calculates an estimate for this echo; the estimate is added in a summer 21 to the signal SIN received from hybrid B in reverse phase, so that the remaining signal LRET only comprises the speech signal of speaker B. Furthermore, the echo canceller B incorporates an attenuator 22 with which the residual echo is attenuated. This means that the signal LRET is attenuated when there is sufficient certainty that the signal SIN from the hybrid B only comprises the echo of the speech of speaker A and no speech of speaker B.
In echo cancellers, adaptive filters 20 must in principle have a length covering the delay in the entire echo path. It is to be noted that a considerable delay may exist between the echo canceller B and the hybrid B; such a delay is illustrated by delay blocks D.su
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Makinen Jorma
Vahatalo Antti
Hong Harry S.
Nokia Telecommunication Oy
Wolinsky Scott
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