Cursor becomes inaccurate after adjusting playback rate with custom external media

[yanmingrui96]

Is there an existing issue for this?

• I have searched the existing issues

Current Behavior

When using custom external media, the cursor is accurate at normal playback speed. However, when I try to decrease or increase the playback rate, the cursor no longer aligns correctly with the actual audio playback.

Expected Behavior

1

Steps To Reproduce

1.Visit url

2.Set the audio playback rate to 0.5x

3.Start playback

Link to jsFiddle, CodePen, Project

No response

Version and Environment

https://www.alphatab.net/docs/guides/audio-video-sync

Platform

Web

Anything else?

No response

[Danielku15]

The example is indeed a missing one part.

Generally there is no bug in alphaTab, with the IExternalMediaSynthOutput it is your task to tell alphaTab the current time and this example misses to take the playback speed of an external audio into account. The built-in backing track handler for Guitar Pro files is working as expected:

GP.Backing.Track.mp4

Incorrect in the example is: alphaTab.synth.IExternalMediaSynthOutput.updatePosition expects a time position in the alphaTab time axis and the playback rate is not accounted. Changing onTimeUpdate and seekTo to translate the time axis fixes the problem:

    seekTo(time) {
        audio.currentTime = (time * audio.playbackRate) / 1000;
    },
...
...
//
// handlers for audio tag -> alphaTab updates
const onTimeUpdate = () => {
    (api.player!.output as alphaTab.synth.IExternalMediaSynthOutput).updatePosition(
        (audio.currentTime / audio.playbackRate) * 1000
    );
}

Fixed.Rate.mp4

I'll update the documentation accordingly.