Stereo BASOP Encoder: BWE noise bursts at 13.2kbps WB

Basic info

• Float reference:
  • Decoder (float):ceb8871f
• Fixed point:
  • Encoder (fixed):31b60873

Bug description

Noise bursts were observed in BWE/high-frequency region of the spectrum in stereo 13.2kbps WB (16kHz input) in multiple place of the stereo LTV signal. This might be related to #1398 (closed), but these artefacts are only observed for WB input, they are not present at 13.2kbps SWB. Some instances of these are at 2:10,0 and 2:19,0; 2:41,8; 2:42,9 seconds, see spectrograms below.

Not all of these are audible, but e.g. in the part around 2:46.0 this adds high-frequency noise over couple of frames so it does not seem to be a localized occasional issue all the time.

For the instance at 2:41.8, there is some initial analysis by @vaillancour already:

2:41.8 -> transition TCX-ACELP, same core for fix and float; -> problem in the BWE, starts when switching from TCX to ACELP ( at 48kHz the problem seems more in the float than the fix) 

Float ref:

BASOP:

Ways to reproduce

./IVAS_cod_fx -stereo 13200 16 ltv16_STEREO.wav bit
./IVAS_dec_flt_ref stereo 16 bit out.wav