This Coder is designed to be robust for indicated frame erasures. An error concealment strategy for frame erasures has been included in the decoder. However, this strategy must be triggered by an external indication that the bitstream for the current frame has been erased. Because the coder was designed for burst errors, there is no error correction mechanism provided for random bit errors. If a frame erasure has occurred, the decoder switches from regular decoding to frame erasure concealment mode. The frame interpolation procedure is performed independently for the LSP coefficients an the residual signal. The G.723.1 interpolates a lost frame by simulating the voice characteristics of the previous frame and slowly damping the signal.

The residual interpolation is performed in two different ways, depending on the last previous good frame. The frame is checked with a voiced / unvoiced classifier (cross-correlation maximisation function with a given decision threshold). If the current frame was marked as erased, and the previous frame was classified as unvoiced, the current frame excitation is generated using a uniform random number generator, scaled by the previous computed gain value. In the voiced case, the current frame is regenerated with periodic excitation having a period equal to the value provided by the classifier. If the frame erasure state continues for the next two frames, the regenerated vector is attenuated by an additional amount for each frame. After 3 interpolated frames (90ms!!), the output is muted completely.