[opus] Q: Bandwidth vs. bitrate

[Ulrich Windl]

Hi!

I have some MP3 audio material which is basically speech with some background noises, essentially > 120Hz and < 5kHz.
I had the idea to reduce the file size by recoding the material to Opus at 56kbps. Unfortunately the result is a file sampled at 48kHz much larger than the original.
I hope you agree that it does not make sense to create a file larger than the original (MP3). Of course the encoder sees just WAV (RAW) with possible MP3 decoding artefacts, but anyway: Is there a way to handle this?
Maybe applying a low-pass filter before sending to Opus? And why is the 48kHz selected, and why does opus have so few sampling rates (nothing between 24kHz and 48kHz)?
The frequency steps for bandwidth are: 4kHz, 4kHz, 8kHz, 24kHz (40kHz in total)
With 4,8,12,16 you'd have sample rates 8kHz, 12kHz, 20kHz, 32kHz, 48kHz. Still I'd miss 44.1kHz, though

Regards,
Ulrich