A 21-tone signal-to-total-distortion testing standard

It is noted that ADPCM (adaptive differential pulse code modulation) forces 9600 b/s Fax V.29 modems to silently drop back to 4800 b/s. Fax customers have the right to connections without ADPCM. Unfortunately, none of the existing IEEE Std 743-1984 measurements can identify that an ADPCM is present on a connection. A novel 21-tone signal-to-total-distortion measurement is being added to IEEE Std 743 to remedy this situation. Measurement of the signal-to-noise ratio of one period (62.5 ms) of the 21-tone test signal permits rapid determination of whether one or more ADPCMs is present on a connection. The averaging of eight periods (0.5 s) of the 21-tone test signal permits determination of the number of ADPCMs on a connection, if it is known that only four-bit ADPCM is possible. If a connection has only one ADPCM with a variable number of bits, as in the case for submarine cable and satellite connections, then the number of bits can be identified