| 摘要 |
A method of testing the degree of degradation imposed on signals transmitted over a communications link from a transmitter to a receiver, comprises the steps of transmitting a predetermined test signal using spare capacity detected by a voice activity detector when no speech is to be transmitted over the communications link, comparing the test signal received by the receiver with the reference signal, and generating therefrom a measure of the degradation of signals caused by the communications link. Typically the test signal is longer than the speech gaps, so a part of the test signal in each such gap, the receiver assembling a complete version of the degraded test signal for comparison with the reference signal. Alternatively a test sequence shorter than the reference signal is repeatedly transmitted over the communications link, and the receiver compares each of the resulting series of degraded test sequences with an undegraded version of the test sequence to identify errors in the degraded test sequences, and reproduces said errors in a copy of the reference signal to generate a degraded version of the reference signal, for comparison with an undegraded version of the reference signal. In a third arrangement the transmitter generates an error protected version of part of the information-containing part of its normal transmissions, for subsequent non-real time transmission as the test signal, and the receiver stores the corresponding part of the normal transmission, as degraded, for use as the reference signal for subsequent comparison with the received error-protected test signal.
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