Spectral parameter substitution for the frame error concealment in a speech decoder

A method for concealing the effects of frame errors in frames to be decoded by a decoder in providing synthesized speech, the frames being provided over a communication channel to the decoder, each frame providing parameters used by the decoder in synthesizing speech the method comprising the steps of:

• a) determining whether a frame is a bad frame;
• and
• b) providing a substitution for the parameters of the bad frame, a substitution in which past immittance spectral frequencies (ISFs) are shifted towards a partly adaptive mean given by:
• ISFq(i)=α*past—ISFq(i)+(1−α)*ISFmean(i), for i=0 . . . 16,
• where
• α=0.9,
• ISFq(i) is the ith component of the ISF vector for a current frame,
• past_ISFq(i) is the ith component of the ISF vector from the previous frame,
• ISFmean(i) is the ith component of the vector that is a combination of the adaptive mean and the constant predetermined mean ISF vectors, and is calculated using the formula:
• ISFmean(i)=β*ISFconst—mean(i)+(1−β)*ISFadaptive—mean(i), for i=0 . . . 16,
• where β=0.75, where
• ISF
• adaptive_mean
• ⁡
• (
• i
• )
• =
• 1
• 3
• ⁢
• ∑
• i
• =
• 0
• 2
• ⁢
• past_ISF
• q
• ⁢
• (
• i
• )
• and is updated whenever BFI=0 where BFI is a bad frame indicator, and where ISFconst—mean(i) is the ith component of a vector formed from a long-time average of ISF vectors.

An apparatus for concealing the effects of frame errors in frames to be decoded by a decoder in providing synthesized speech, the frames being provided over a communication channel to the decoder, each frame providing parameters used by the decoder in synthesizing speech the apparatus comprising:

• a) means for determining whether a frame is a bad frame;
• and
• b) means for providing a substitution for the parameters of the bad frame, a substitution in which past immittance spectral frequencies (ISFs) are shifted towards a partly adaptive mean given by:
• ISFq(i)=α*past—ISFq(i)+(1−α)*ISFmean(i), for i=0 . . . 16,
• where
• α=0.9,
• ISFq(i) is the ith component of the ISF vector for a current frame,
• past_ISFq(i) is the ith component of the ISF vector from the previous frame,
• ISFmean(i) is the ith component of the vector that is a combination of the adaptive mean and the constant predetermined mean ISF vectors, and is calculated using the formula:
• ISFmean(i)=β*ISFcosnt—mean(i)+(1−β)*ISFadaptive—mean(i), for i=0 . . . 16,
• where β=0.75, where
• ISF
• adaptive_mean
• ⁡
• (
• i
• )
• =
• 1
• 3
• ⁢
• ∑
• i
• =
• 0
• 2
• ⁢
• past_ISF
• q
• ⁢
• (
• i
• )
• and is updated whenever BFI=0 where BFI is a bad frame indicator, and where ISFconst—mean(i) is the ith component of a vector formed from a long-time average of ISF vectors.