repeat \ reloop full track length accuracy

[mixxxbot]

Reported by: mevsme
Date: 2019-12-17T11:16:25Z
Status: Confirmed
Importance: Medium
Launchpad Issue: lp1856683
Tags: looping

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There are issues with loop end marker and track end detection. Thus repeating or looping of a track\loop\sample with exact number of beats\bars cause longer loops than necessary.

the discussion: https://mixxx.zulipchat.com/#narrow/stream/109171-development/topic/repeat.20accuracy

[ronso0]

The issue with loop end markers at the track end is not reproducible.

However, repeating tracks would still make the looped track longer than it actually is.
This happens because if the last audio buffer at the track end can’t be filled with existing audio samples (very likely) it is padded with silence, thus you’ll get a tiny playback gap before the playhead seeks to the track start which makes the track longer than it actually is. That means a repeated track will fall out of (beat) sync with longer tracks playing at the same speed (BPM).

For loops, ReadAheadManager::getNextSamples is handling this crossfading from loop_end to loop_start.
For track repeat a similiar solution is needed. I have a POC branch (hack) ready but I can't verify atm due to lack of appropriate tracks.
https://github.com/ronso0/mixxx/pull/new/repeat-offset-buffer-fill

@mevsme Could you share some links to example files for testing please?