Opus has this feature where you can change the target bitrate freely on various parts of the song--even continuously, if you want. So I tried to create an Opus file (Falcom Sound Team J.D.K. - "Defeat Your Old Nemesis") that starts at the maximum bitrate allowed (512kbps) and continuously decreases until the minimum bitrate (8kbps).
There are some important things to note in this experiment, though:
1. Opus actually allows bitrates as low as 500 bps. However, it's very unlikely for it to reach that low bitrate due to its VBR nature and also, only up to 8kbps is the recommended minimum because anything lower than that is barely intelligible unless you already know what you're listening to.
2. Opus dynamically chooses the low-pass filter applied based on the requested bitrate. At 8kbps, it would have chosen narrowband. But I forced Opus to always encode at fullband.
3. Opus dynamically chooses whether to encode in mono or stereo based on the requested bitrate. Below around 24kbps, it would switch to mono. But I forced Opus to always encode in stereo. Nevertheless, as the bitrate goes down, the sideband data also gets eliminated so we get progressively more mono as the bitrate decreases.
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