Annex J to ITU-T Rec. G.728 defines a 40 kbit/s extension optimized for voiceband data signals of the existing Annex G/G.728 16 kbit/s fixed point specification. The main difference between the codec described hereby and the codec described in Annex G/G.728 is the application of a Trellis-Coded Quantization (TCQ) approach to codebook search. The TCQ approach replaces the analysis-by-synthesis approach to codebook search of ITU-T Rec. G.728 only in voiceband data (VBD) mode.

The backward adaptation of the predictor achieved in VBD mode is almost identical to the backward adaptation achieved in speech mode (ITU-T Rec. G.728). Additionally, the same adaptation cycle is used for both speech mode (ITU-T Rec. G.728) and VBD mode. In speech mode, the 40 kbit/s reverts to the LD-CELP of ITU-T Rec. G.728.

This annex includes an electronic attachment containing test vectors for implementation verification of Annex J/G.728.

Amendment 1 provides corrections to inconsistencies identified in the description of the gain compensation module of Annex J/G.728. These changes do not affect the existing test vectors.