Low frequency issue for ISM-2 @ 80kbps

Basic info

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

Bug description

When encoder stereo speech at 80 kbps, the synthesis sounds muffled. When looking to individual channels, we can observe the presence of strong inter-harmonic noise in the low frequency region.

On both figures, from top to bottom:

• Float synthesis (r or l channel)
• Fixed synthesis (r or l channel)
• difference
• spectrum

Ways to reproduce

Box folder: ...\Box_EXTERNAL_IVAS_BASOP_VERIFICATION\issues\issue-1193

IVAS_cod -ISM 2 ap07a1s04.wav.0.csv ap07a1s04.wav.1.csv 80000 48 ap07a1s04.wav bit
IVAS_dec EXT 48 bit synfxfx.wav 

IVAS_cod_flt -ISM 2 ap07a1s04.wav.0.csv ap07a1s04.wav.1.csv 80000 48 ap07a1s04.wav bit
IVAS_dec_flt EXT 48 bit synfltflt.wav