fix: Change Speed seem like effect pitch too

[docaohuynh]

Description

When I change speed audio seem like pitch change too

Steps To Reproduce

Change speed like this

currentSound = await SoLoud.instance
        .loadAsset('assets/audio/tiengviet.mp3');
  }

handle = await SoLoud.instance.play(currentSound!);
SoLoud.instance.setRelativePlaySpeed(handle!, 2);

Expected Behavior

Only change speed, keeping the current pitch

Additional Context

Tested on IOS simulator

I using flutter_soloud 3.4.5

flutter doctor
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 3.35.7, on macOS 26.1 25B78 darwin-arm64, locale en-VN)
[✓] Android toolchain - develop for Android devices (Android SDK version 36.0.0)
[✓] Xcode - develop for iOS and macOS (Xcode 26.1.1)
[✓] Chrome - develop for the web
[✓] Android Studio (version 2025.2)
[✓] VS Code (version 1.106.3)
[✓] Connected device (4 available)
[✓] Network resources

• No issues found!

[alnitak]

Hi @docaohuynh, yes, this is a normal behavior. It's like you play a 33 rpm vinyl at 45.

If you want to change the play speed and pitch, there is an example that shows this, but somehow using the filter for a single sound instead of a global filter could cause some distortion problems (hope to fix).

[docaohuynh]

HI @alnitak

this is a normal behavior

I try to use other packages like just_audio change speed do not affect to pitch. Is change pitch actually normal?

[docaohuynh]

Even if I change pitch

final shift = 1 / speech;
SoLoud.instance.filters.pitchShiftFilter.shift.value = shift;

The sound is a little bit distorted. And then set everything to default the sound still not normal

SoLoud.instance.setRelativePlaySpeed(_currentHandle!, 1.0)
SoLoud.instance.filters.pitchShiftFilter.shift.value = 1.0;

[alnitak]

And then set everything to default the sound still not normal

I should find another algorithm for the pitch shift filter or fix it. It is still experimental.

[bemain]

I should find another algorithm for the pitch shift filter

If you decide to, you should probably take a look at StaffPad's algorithm (now used by Audacity). I believe it was recently open-sourced, and is supposed to be really good.

[alnitak]

Thanks @bemain, I'll try to look at that for sure!

@docaohuynh, meanwhile you could try the rewritten pitch shift filter, even if it is not really related to this issue. Please follow this to do that.

[docaohuynh]

Hi @alnitak I change to dev but nothing change for me, The sound still little bit distorted when change speed

flutter_soloud:
    git:
      url: https://github.com/alnitak/flutter_soloud.git
      ref: dev

[docaohuynh]

And try different method, still distorted

Future<void> setSpeed(double speed) async {
    _speed = speed.clamp(0.5, 3.0);
    if (_currentHandle == null || _currentAudioSource == null) return;
    // soloud.SoLoud.instance.setRelativePlaySpeed(_currentHandle!, _speed);
    // final pitch = 1.0 / _speed;
    // soloud.SoLoud.instance.filters.pitchShiftFilter.shift.value = pitch;
    // logIelts('Set speed: $speed, pitch: $pitch');
    _currentAudioSource!.filters.pitchShiftFilter.timeStretch(
      _currentHandle!,
      speed,
    );
  }

[alnitak]

@docaohuynh I think I can't do better than this. This filter could have better results for music or for voices. If you like, you can try to change the 2 numbers in this line. Both must be a power of 2. You can try with the first to maybe 1024 or 4096, and the second to 8 or 16.

If you find that it should be useful, I could add a parameter to the filter to manage that value(s).

[docaohuynh]

@alnitak I have changed and still not effect. But when I change speed to 1.0, I deactivate pitchShiftFilter the sound normal again, I think when I apply pitchShiftFilter it makes the sound distorted

Future<void> setSpeed(double speed) async {
    _speed = speed.clamp(0.5, 3.0);
    if (_currentHandle == null || _currentAudioSource == null) return;
    soloud.SoLoud.instance.setRelativePlaySpeed(_currentHandle!, _speed);
    if (_speed == 1.0) {
      if (soloud.SoLoud.instance.filters.pitchShiftFilter.isActive) {
        soloud.SoLoud.instance.filters.pitchShiftFilter.deactivate();
      }
    } else {
      if (!soloud.SoLoud.instance.filters.pitchShiftFilter.isActive) {
        soloud.SoLoud.instance.filters.pitchShiftFilter.activate();
      }
      final pitch = 1.0 / _speed;
      soloud.SoLoud.instance.filters.pitchShiftFilter.shift.value = pitch;
    }
  }

[eddyleelin]

I should find another algorithm for the pitch shift filter

If you decide to, you should probably take a look at StaffPad's algorithm (now used by Audacity). I believe it was recently open-sourced, and is supposed to be really good.

It seems like the StaffPad time-stretch / pitch-shift algorithm in the Audacity repo would inherit Audacity's GPL (v2+/v3+), which seems incompatible with flutter_soloud's MIT license (I am clueless about legal stuff though)? Anyway, I would definitely like to have a good, working algorithm 😸

@alnitak FYI, I also tried to use the dev version you made using the SMB pitch-shift algorithm, but my audio (speech) still sounds heavily distorted and has lots of sound artifacts.

I just tried forking the dev branch (and patching the latest PRs from main) and then implemented the SoundTouch algorithm for time-stretch. At least for speech, it sounded much better! I'll do some more exploring tomorrow and see if I can understand the algorithm and the code better (again, Codex 5.2 helping me here out a lot).

As a note, SoundTouch is licensed with LGPL; still not sure what the consequences of that means?

[alnitak]

Hi @eddyleelin, I've seen StaffPad's GPL license, and I agree with you that it is not a choice for flutter_soloud.

I tried your fork using the filter/pitchshift.dart example! Thanks for your effort!! It has some glitches when I change values with the sliders, but the result seems very good.

I'm not sure about the LGPL license either. What I remember about it, is that the developer could use the code within a commercial and closed app, but there are some other limitations I can't remember. I know that a project could own more than one license kind, but I don't know how much (no)good is for flutter_soloud. Maybe @filiph has some thoughts?

Anyway, I would stick with Apache or MIT licenses.

I was asked to look at Signalsmith, which appears to be good and is made under the MIT license.

[filiph]

I am Definitely Not a Lawyer™.

One thing that has been drilled into my brain is that changing a license for an OSS project with more than 1-2 collaborators is massive pain. I'm not sure if what we're discussing here constitutes "changing a license".

Another thing I have been taught (though I don't claim I actually understand) is to avoid copyleft licenses like LGPL. I do like the idea of copyleft, emotionally and philosophically. But AFAIK they bring legal uncertainty. Will my game have to be open source if I use flutter_soloud? If not, does that hold for every major market? Can someone sue me in 5 years from now? Meanwhile, MIT is so simple, short and permissive that the uncertainty is a lot lower.

So my vote would be not to incorporate LGPL code here UNLESS someone who's actually a copyright lawyer tells us it's okay.

[alnitak]

@filip, as far as I know, the LGPL license is added to the MIT license of flutter_soloud and anyone can use the sources of the LGPL license in a closed and commercial product. It just needs to be told somewhere in the app. This is all I know, but anyway, I'd like to stick with a MIT license too.

@novas1r1 suggested me to look at SignalSmith which has a MIT license. I tried to use it, and it sounds very good compared to smbPitchShift. It is now in the dev branch and can be tested using examples/lib/filters/pitchshift.dart.

Despite the excellent job made by @eddyleelin trying with SoundTouch, if this Signalsmith works well and better, I'd leave it as the official one.

So, please let me know!