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MOTOROLA INC. Technical Developments Volume 13 July 1991
A SELF-TALKING DETECTOR FOR ECHO CANCELLING MODEMS
by Yong-Hwan Lee
INTRODUCTION
Self-talk is an abnormal operation that may occur in 2-wire echo cancelling modems. It is caused by a sudden line disconnection. When the modem is oper- ating under a mild channel condition, the receiver may not recognize a sudden line drop and may continue its operation based on the near end echo signal, i.e., the transmitted data will be received. Unfortunately, it usually takes quite a long. time (usually several min- utes) for the modem to recognize the line drop. There- fore, it is highly desirable for the modem to have a fast self-talking detector so as to minimize the time inter- val in this mode.
A sudden line drop can be easily and rapidly detected in 4-wired modems by observing the incoming signal energy. Such methods, however, cannot be efficiently applied for echo-cancelling modems, because the residual echo signal is strong compared to the received signal in a self-talking situation. When the line is suddenly disconnected, the impedance characteristic of the hybrid circuit HC shown in Fig. 1 is also suddenly changed, which results in a different echo signal. Thus, the echo canceller cannot properly remove the echo signal until it adjusts itself to this kind of abrupt change in the channel, which usually takes quite a long time. Therefore, methods based on the received signal energy are likely to be unreliable in detecting a self- talk and will in any case take a long time.
Self-talk can be detected using the fact that the received signal becomes identical to the transmitted signal. Since the transmitter(Tx) and the receiver(Rx) may use different scrambling and descrambling schemes, we can use the output of the TX scrambler and the input of the Rx descrambler, denoted by d, and d,, respectively. In a self-talking mode, the data sequence of d, will be equal to that of d, after some amount of time delay. Note that, even though the
same data is transmitted by both the local and remote modems in the normal mode, the data d, and d, cannot be identical because of the different scrambling poly- nomials. Therefore, if the data stream d, is identical to that of d, (with some time delay), it can be declared that the line is dropped.
For efficient realization, we devise a sequential self-talk detector composed of two tasks, the detection and confirmation processbrs. A single test cannot sat- isfy the required detection performance such as the true and false detection time. Thus, an additional con- firmation process is required for reliable performa'nce. Since the modem is basically operating based on the baud rate L, the detector is designed based on a single symbol(baud) data. ThiS makes the detector imple- mentation independent of, the data rate. However, it is not necessary for the test to be performed at every baud interval. The detector measures the Hamming distance H between d, tind d...
