Artifacts for low level signals

with 315a2539 and this bitstream ltv_STEREO_32k_d4_-stereo_24400_32.zip, we can hear at multiples places artifacts that make the fixed point synthesis to sound rougher.

It can be easily perceived between frame 2260 and 2275.

In the 2 first figures, from top to bottom:

• Left channel floating point synthesis
• Left channel fixed point synthesis
• Left channel difference
• Core decision
• coder_type decision

In the third figure:

• Left channel floating point synthesis
• Left channel fixed point synthesis
• Left channel difference
• Left channel floating point high band synthesis only
• Left channel fixed point high band synthesis only
• Left channel difference

The problem seen in the HB seems to happen almost every time we can hear the problem in the synthesis (but not all the time). So this might be related... or not