Significant distortion and noise for highly tonal signals

[smatt123]

Hello,

With OPUS 1.3.1, significant noise is present when encoding/decoding highly tonal signals.

Attached is an image of a 1 kHz sine tone encoded and decoded via "./opus_demo audio 48000 1 76800 -framesize 5 -max_payload 48 input.raw output.raw"

It looks like the comb postfilter amplifies the noise at multiples of the fundamental frequency.

The noise is audible in this configuration. Are there some parameters to tune to reduce the noise?

Thanks in advance!