Improved quality CELP base-band coding of speech at low-bit rates

An improved CELP (code-excited linear prediction) baseband (BB) coder was tested using informal subjective listening tests. The overall bit rate of the coder was 4.7 kb/s. The subjective quality of speech scored just over three on a five-point mean opinion scale. Improvements are described that increase the score by about 0.35 over its original form. This means that in terms of quality the CELP BB coder at around 4.8 kb/s can replace the analog ACSSB operating even at 51 dB/Hz because in its original form it had subjective quality equivalent to ACSSB. To further investigate the performance of the coder the authors introduced random channel errors into the bit stream between the encoder and the decoder. At bit error rates of 10^-3 and less, the coder did not need any forward error protection. When the bit error rate was around 10^-2 an unexpected distortion occurred; the output energy level of the coder was very low whenever there were errors in the pitch filter or the optimum vector gain parameters. This was due to the new adaptation scheme used for the optimum vector gain. The authors solved this problem by adapting the optimum gain with the maximum block energy of the pitch filter memory or by simply using the energy of the whole of the pitch filter memory