5.2.3 Check for Mid LSF stability
26.4473GPPCodec for Enhanced Voice Services (EVS)Error concealment of lost packetsRelease 17TS
The interpolation of the mid-LSF can create unstable LSFs under packet erasure conditions. Let the 
th sub-frame LSFs are given by
. We denote the last sub-frame LSF of 
th frame as 
where 
. Let us denote the 
th LSF dimension of the 
th sub-frame of frame n as 
where
.
The end-LSF quantizer quantizes
. Then mid-LSF quantizer interpolates the mid-LSFs as follows.
(21)
where i-th dimension of the weighting vector 
is given by 
. The vector elements 
are not constrained. In particular if 
interpolation generates a mid-LSF 
between 
and
. However if 
or 
interpolation might generate a mid-LSF 
outside
. This could potentially create LSF clustering that result in an unstable LSF synthesis filter. To remedy this situation, a potential instability is detected as described below.
In the frame that follows the packet loss, the decoder checks whether the computed mid-LSFs are ordered correctly i.e. 
. If violation of this rule is detected the LSFs are considered as potentially unstable. If potential LSF instability is detected, decoder uses a fixed weighting value 
(typically 0.6) for mid LSF interpolation as follows.
(22)
The mid LSF interpolation based on equation (22) is continued until frame 
such that the frame 
is the first frame after frame 
that uses safety net quantization for quantizing its end LSF.