This error concealment process has been incorporated in the decoder and is functional when the frame of coder parameters has been identified as being erased. The mechanism of detecting frame erasures is not defined in the Recommendation, and will depend on the used application.

Specific steps taken for an erased frame:

repetition of the synthesis filter parameters

attenuation of adaptive and fixed-codebook gains

attenuation of the memory of the gain predictor

generation of the replacement excitation

The reconstruction strategy is based on previously received information. Therefore the missing excitation signal will be replaced with one of similar characteristics, while gradually decaying its energy. This is done by using a voicing classifier based on long term prediction gain. An erased frame inherits its class from the preceding (reconstructed) speech frame. Voicing classification is continuously updated based on this reconstructed speech signal. The generation of the replacement excitation depends on the periodicity classification. If the last received frame was classified as periodic, the current frame is considered to be periodic as well. Only the adaptive codebook is used. To avoid excessive periodicity computed the pitch delay (of the last received frame) is increased by one for each next subframe. The fixed codebook gain will be attenuated. If the last reconstructed frame was classified as non-periodic, the current frame is considered to be non-periodic as well. Therefore, only the fixed-codebook is used, by contributing randomly selected codebook and sign indices. The adaptive codebook gain will be attenuated.