Measurement of signal to quantising-noise ratio in ADPCM systems and its implications for echo cancellation

It is questioned whether the standard pulse code modulation (PCM) quantisation distortion test using a narrow band of noise is appropriate to adaptive systems. An investigation of such measurements has been carried out in relation to the use of echo cancellers, where the amount of echo cancellation that can be achieved is a function of the available signal/noise ratio. The recommended performance test for echo cancellers gives results consistent with those obtained from the recommended test on PCM systems, but not with those obtained from the latter test on adaptive systems such as adaptive differential PCM(ADPCM). A wideband test has been adapted from analogue intermodulation distortion measurement practice and realised using a Fourier analyser, both to produce the test signal and to measure the resulting distortion products. The method has given results consistent with the performance of echo cancellers in a software ADPCM environment. An alternative test is also proposed, based on the performance of such echo cancellers, which offers scope for the development of a portable distortion measurement instrument.