Subjective quality of several 9.6-32 kb/s speech coders

Ratings obtained from subjective tests indicate that for single encodings of speech, near equivalence to 64 kb/s PCM is obtained at 32 kb/s with ADPCM/AP and SBC, and at 24 kb/s with APC and ATC/VD. Tandem encodings of 32 kb/s ADPCM/AP are significantly better than 16 kb/s APC and SEC. Eight synchronous encodings of 32 kb/s ADPCM/AP are rated about the same as eight asynchronous encodings of 64 kb/s PCM. The subjective test results are used to develop preliminary coder performance functions in terms of Q (subjective ratio of speech power and speech-dependent noise power) in terms consistent with a previously reported quantization distortion opinion model. These relations, in conjunction with a proposed CCITT planning rule, indicate that a 32 kb/s ADPCM/AP coder can be allowed in each of the national extensions of an international connection. Reported results apply only to coder quantization effects on speech quality; other signals (e.g., voiceband data, signaling tones) and impact of coder delay on echo performance are not covered.