Scaletempo2, the new default for adjusting audio playback speed, sounds noticeably worse in some situations

Important Information

Provide following Information:

• mpv version: freshly compiled latest from master (mpv 0.33.0-109-gd0c530919d)
• macOS

This is relevant because scaletempo2 was changed to the default from scaletempo in #8376

Reproduction steps

Try listening to some 5.1 audio at 0.95x speed using the now default scaletempo2 filter.
$ mpv --speed=0.95 --af=scaletempo2 some_audio
Listen for the poor quality in some situations.

Now add the old default, scaletempo, to the af filter chain and listen for the better quality.
$ mpv --speed=0.95 --af=scaletempo some_audio

Also listen to the recorded sample files below. You can use the original_source.mkv file included to reproduce the samples I recorded.

Expected behavior

The default should not make things worse.

Actual behavior

scaletempo2 is worse for minor speed changes in the 0.85x - 1.2x range. It's MUCH better for the big speed changes though, and I really appreciate it for that.

While I do appreciate scaletempo2 for big adjustments, I usually only make minor speed changes so for me so it's not a good default. I suspect that playback speed adjustments in the 0.9-1.2x range are much more common amongst users. This comment is where another users seems to have been caught up in this default change.

Sample files

• scaletempo2_0.95x_speed.wav (Bad)
• scaletempo_0.95x_speed.wav (Better)
• 1x_speed.wav (1x speed, recorded in the same way)
• original_source_audio.mkv

Start by running the provided mpv commands with the original_source_audio.mkv sample and compare scaletempo2 against scaletempo at 0.95x and across the reported 0.85x–1.2x range. Use the recorded WAV samples to confirm the quality difference; done means the minor-speed-change behavior is no worse with the default filter while retaining the reported large-adjustment benefit.