fix: Audio degradation remains after pitch/time modification

[NikitaVasin]

Description

When applying the pitchShiftFilter and changing timeStretch, the original sound becomes heavily distorted, resulting in a "robovoice"-like effect.
Even after resetting the value back to 1.0, the distortion does not go away and the sound remains degraded.

Steps To Reproduce

1. Initialize SoLoud and load an audio file.
2. Activate the pitchShiftFilter.
3. Change the timeStretch value using a slider (e.g., from 1.0 to 0.5 or 2.0).

Expected Behavior

The quality should not deteriorate when changing the timeStretch.

Additional Context
• Flutter version: 3.35.2
• flutter_soloud version: 3.3.6
• Platform(s): Android/iOS

Code Sample

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await SoLoud.instance.init();
  runApp(MaterialApp(home: const HomeScreen()));
}

class HomeScreen extends StatefulWidget {
  const HomeScreen({super.key});
  @override
  State<HomeScreen> createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> {
  double _speed = 1.0;
  AudioSource? audioSource;
  SoundHandle? soundHandle;

  @override
  void initState() {
    loadSound();
    super.initState();
  }

  Future<void> loadSound() async {
    final sound = await SoLoud.instance.loadAsset('assets/sound.mp3');
    sound.filters.pitchShiftFilter.activate();
    audioSource = sound;
    setState(() {});
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: [
          Text('Speed: $_speed'),
          Slider(
            value: _speed,
            onChanged: (value) {
              _speed = value;
              setState(() {});
              if (audioSource != null && soundHandle != null) {
                audioSource!.filters.pitchShiftFilter.timeStretch(
                  soundHandle!,
                  value,
                );
              }
            },
            min: 0.2,
            max: 2,
          ),
          ElevatedButton(
            onPressed: () async {
              if (audioSource != null) {
                soundHandle = await SoLoud.instance.play(
                  audioSource!,
                  looping: true,
                );
              }
            },
            child: Text('Play'),
          ),
        ],
      ),
    );
  }
}

[alnitak]

Hi @NikitaVasin,

this is a known problem. I tried hard to solve it by trying to use a different algorithm, but I didn't succeed.

This happens more clearly with the filter attached to a sound, while using it as a global filter (SoLoud.instance.filters), it is less noticeable.

[NikitaVasin]

Thank you for your reply! Unfortunately, using a global filter doesn’t work in my case, since the application plays multiple sounds at the same time: sound effects and narrator speech, whose playback speed can also be adjusted.