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

[varenc]

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

[Hrxn]

I suggest voting with +1 and -1 on the original post to vote on changing the default.
+1 to vote for changing the default back to scaletempo (unless there is some fix)
-1 to vote for keeping the current default.

Edit: yes, can reproduce

[CounterPillow]

Suggestion: scaletempo3, which uses scaletempo for speeds between 0.80 to 1.3 and scaletempo2 for speeds outside of that range.

[TiGR]

Or maybe have it configurable separately as we have it with scale algorithms.

[varenc]

The best solution would just be to make scaletempo2 work better even at minor speed changes! Chrome's own audio scaling, which scaletempo2 is a port of, seems to work fine with minor adjustments.

@DorianRudolph, perhaps you might have an idea of why scaletempo2 performs worse than Chrome does at 0.95x speed? Is there any hope for just tweaking it to handle this use case? That would of course be the ideal solution!

My thinking is that if scaletempo is going to be restored as the default, that should happen soon to avoid further confusion for people. Also I'm basing this on the assuming that minor speed changes in the 0.85x - 1.2x range are far more common amongst MPV users, like they are for me, though I'm not sure if that's true. No matter the outcome, I'll also submit a PR for a docs update which adds a section explaining to users how to easily change the default to another audio scaler.

(@TiGR I do think the how mpv lets you choose your "audio scaling" filter is a bit idiosyncratic and hard to discover, but I think that's for a different discussion!)

[DanOscarsson]

If it works fine in Chrome it may be because some versions of their code switched to resampling between speed 0.95 - 1.06. Personally I prefer to use resampling when close to normal speed, like when playing a 25 Hz movie on a 24 Hz display. And mpv can do sync to vsync with resampling and be configured to do that so 25 Hz movies are automatically resampled to 24 Hz.
My only need for preserving pitch is when playing at a fast speed like > 1.5.
And that is what I would have expected most users need scaletempo2/scaletempo for.
But apparently that may not be true but cannot be determined without asking a lot of users.

As I have started working on some fixes to scaletempo2 (not related to speed near 1) it would be good to quickly decide which scaletempo version to use (there is one more atempo in ffmpeg) as maintaining several WSOLA implementations will just be confusing for users and additional work for maintainers. But may be needed if one cannot solve all users needs.

[realnc]

When I built mpv from git, the first thing I noticed were rather severe audio glitches when listening to audiobooks at speeds 0.9 and 1.1 (depending on whether the narrator is too fast or too slow.) It sounds like a scratched CD where the CD player is skipping.

scaletempo produces perfect results at these playback speeds. You can't even tell the sound is slowed down or sped up. It really sounds like the narrator is just reading slower or faster.

There doesn't seem to be an option to tell mpv which filter to use, so I had to put af-add=scaletempo in my config. Unfortunately, this disabled mpv's automatic filter removal when the filter is not needed. The filter is always active and shows up in the OSD all the time.

Something like an --audio-speed-filter option would be very nice to have instead of hardcoding scaletempo2 in the mpv source code.

[garoto]

[	 no-osd af add "@tempo:scaletempo" ; no-osd add speed "-0.1"
]	 no-osd af add "@tempo:scaletempo" ; no-osd add speed "+0.1"
BS	 no-osd af remove @tempo ; no-osd set speed 1.0

[realnc]

[	 no-osd af add "@tempo:scaletempo" ; no-osd add speed "-0.1"
]	 no-osd af add "@tempo:scaletempo" ; no-osd add speed "+0.1"
BS	 no-osd af remove @tempo ; no-osd set speed 1.0

I can't see what speed I'm setting.

[avih]

@realnc please file a new issue, with logs and everything else which the template requests

If you can bisect it to find the exact first commit where the issue happens - it would great info to add.

[avih]

It sounds like a scratched CD where the CD player is skipping

@realnc could you please open a new issue for this? All the reports we have so far are about subjective quality, but what you're describing is new, and could very well be an actual bug - which none of us is able to reproduce.

So please file a new issue, with logs, preferably sample files, bisect if you can, etc. It would help us identify a yet-unknown bug.

[kevin-stuart]

I will be not too helpful commenting here, but I just want to confirm this report.

I upgraded do 0.34 and wondered why voices sound robotic at speed 1.1 until I figured out that apparrently the default was changed to scaletempo2. I added af=scaletempo as option in mpv 0.34 and aparrently things went back to normal.

Unfortunately, I can't offer any samples and it may subjective, but to me it was clear as day that something had changed and voices sounded very robotic with a lot of videos (but not all!). There seem to be some exceptions, but for me, scaletempo2 is way worse.

At least please don't remove scaletempo, for me scaletempo2 is very hard to bear for many files. I can try to see if I notice some kind of regularity such as audio codecs, but for me, there is something very wrong with scaletempo2.

[richardpl]

Use atempo instead.