5.2 Concealment operation related to spectral envelope (LPC) representation

26.4473GPPCodec for Enhanced Voice Services (EVS)Error concealment of lost packetsRelease 17TS

When the LSF parameters of the first good frame are not available, the LSF parameters of the concealed frame are extrapolated using the last LSF parameters. The general idea is to fade the last LSF parameters towards an adaptive LSF mean vector. First, an average LSF vector is calculated from the last 3 known LSF vectors as

(17)

Then, the adaptive mean LSF vector is calculated by

(18)

Then the LSF vector used for concealing the lost frame is computed

(19)

where is the mean LSF vector defined according to Table 3.

Table 3: Values of LSF mean vector

LPC Quantization == 0 AVQ	=
LPC Quantization == 1 ACELP	=

Furthermore, is defined according to Table 4.

Table 4: Values of LSF interpolation factor

Bitrates:	5.9, 6.8, 8.0, 13.2, 32 and 64 kbps	9.6, 16.4, 24.4, 48, 96 and 128 kbps
plcBackgroundNoiseUpdated == 1
plcBackgroundNoiseUpdated == 0

depends on the previous coder type and the signal class of the last good frame for the first 3 lost frames. It is determined according to Table5.

Table 5: Values of LSF interpolation factor

Last good received frame (coder_type)	Additional criteria	
UNVOICED		1
INACTIVE or AUDIO	Last_GSC_pit_band_idx > 0 and nbLostCmpt > 1	0.8
	else	0.995
UNVOICED_CLAS	Successively lost frames = 1
	Successively lost frames = 2	0.6
	Successively lost frames = 3	0.4
UNVOICED TRANSITION		0.8
VOICED_CLAS		1
ONSET		1
ARTIFICIAL ONSET		0.6
All other cases		0.4

Starting from the 4^th consecutive lost frame.

The estimated LSF vector of the concealed frame is converted to LSP representation and interpolated. The interpolation procedure corresponds to the procedure described in subclause 5.1.9.6 of [5]. The interpolation procedure calculates four or five LSP vectors, each for a given subframe of the concealed frame. The interpolation is done between the LSP vector of the last subframe of the last frame (the one before the concealed frame)and the LSP vector derived from during concealment, as described above.