CELP coding at 6.55 kbit/s for digital mobile radio communications

The work carried out in designing a speech codec suitable for the half-rate channel of the pan-European digital mobile radio (DMR) system is described. A set of CELP (code-excited linear prediction) schemes was selected with bit-rates around 7 kb/s, on the basis of performance without channel errors and complexity. Then the selection of a single scheme was made, taking into account performance with channel errors. The effect of transmission errors was evaluated by measuring the spectral distortion and the segmental signal-to-noise ratio (SNR) for each corrupted bit of the encoded parameters. These data, combined with informal listening tests, resulted in the definition of classes of importance for the bits. The bit error rate (BER) thresholds at which the distortion, due to errors, is perceived were determined for each class. This information was used to optimize the design of a channel coder for the selected speech coder