Bad synthesis quality for ISM-2 @16.4 kbps

Basic info

• Float reference:
  • Encoder (float): fbeb5ef8
  • Decoder (float): fbeb5ef8
• Fixed point:
  • Encoder (fixed): 1df326bb
  • Decoder (fixed):

Bug description

The fixed point quality of ISM-2 at low bit rates is really bad, the spectral shape of the synthesis is really wrong as show in the figure below (similar for both channels and constant overall the file). Maybe LPC filter is wrong? On the next figure, from top to bottom

• Float
• Fixed
• diff
• spectrum

Ways to reproduce

Box folder: ...\Box_EXTERNAL_IVAS_BASOP_VERIFICATION\issues\issue-1194

IVAS_cod -ISM 2 G2S3.wav.0.csv G2S3.wav.1.csv 16400 48 G2S3.wav bit
IVAS_dec EXT 48 bit synfxfx.wav 

IVAS_cod_flt -ISM 2 G2S3.wav.0.csv G2S3.wav.1.csv 16400 48 G2S3.wav bit
IVAS_dec_flt EXT 48 bit synfltflt.wav