Performance of 32 kb/s ADPCM in frame erasures

We report the performance of the CCITT standard G.721, adaptive differential pulse code modulation, when channel impairment causes erasure of frames of information. The structure of the G.721 decoder is analyzed to explain the behavior observed during frame erasures and when correct transmission is restored. Various techniques have been tried to the decoder when frames are lost and when correct transmission is restored. Though the decoder, at times, takes long to gain synchronism with the encoder, the amplitude of the output signal approaches the error-free output much more rapidly. This fast recovery of amplitude helps in reducing the impact on the perceptual quality of speech. From objective results and informal listening tests, it was found that simple silence substitution performs as well as other frame substitution techniques. Resetting the encoder and decoder simultaneously at the end of erasure is seen to improve the performance. Since silence substitution is the easiest to implement, this technique seems the most appropriate one for application to wireless communications. This work is targeted towards understanding the implications to local exchange networks of wireless system alternatives that could provide access to those networks