High frequency "modulation" in rate switching MASA to MONO output

Basic info

• Encoder (float): d18cbee3
• Decoder: 34e96575 (exists also before this)

Bug description

As part of verifying fix for issue #996 (closed), I noticed that there is a curious very high frequency modulation present in BASOP decoder that is not present in the reference float decoder. Here is a diff spectrogram of BASOP-ref generated in Matlab with

winsize = 960; figure(1); spectrogram(main-ref, hann(winsize), winsize/2, winsize, 48000); caxis([-150,-70]); colormap(gray);

The modulation can be seen at the high end (probably something like 23-24 kHz). This is obviously quite inaudible but can be a clue for some underlying issue.

Edit: additional figure highlighting the observed part in spectrogram

Ways to reproduce

Can be reproduced using the instructions in #996 (closed), that is, use the switching profile generic_start_13k2_20frames.bin

with commands

./IVAS_cod -q -masa 2 scripts/testv/stv2MASA2TC48c.met -max_band fb generic_start_13k2_20frames.bin 48 scripts/testv/stv2MASA2TC48c.wav test.bs
./IVAS_dec -q MONO 48 test.bs out.wav