in case of PLC, applying SNS up to L_spec might not be enough: In case the transition frame from DTX after an inactive period is lost, L_spec is assumed to represent a regular TCX frame,
however, this frame is nevertheless acting as an transition frame as also visible in L_frameTCX; thus, the safer approach to prevent high frequency artifacts is to apply the SNS up to L_frameTCX;
in case this is not necessary, x[] is filled with zeros, and the multiplication is not causing any additional harm