While playing around with Opus today I came across a CD-sourced song where the sound suddenly becomes significantly worse for a few seconds before recovering. I used a recent "free encoder pack" (2014-07-06) for foobar2000 (v1.3.3). The resulting file says it was encoded using libopus 1.1.
I first noticed the effect at 32kbps VBR. It also happens at 24kbps, but not at 16kbps or 48kbps or any higher bitrates that I tried. The effect can be observed using the attached sample, but I think the specific time when it occurs is different in the excerpt than in the full track.
Hopefully this is of use/interest for the Opus developers.
I can confirm the issue
Also confirmed. Suspicious that it's the speech-detection kicking in and switching to SILK, which only matters in the bitrate range where the codec has to choose among SILK, Hybrid and CELT modes. If you're at or below about 16 kbps, it's only SILK anyway. If you're at or above about 48 kbps it's only CELT anyway. That would also explain why it happens at a different time in the full-length version, as it's probably a marginal call. Believe there's a neural network deciding, so perhaps it can be trained to recognise this genre better in future.