SyncLock: Do not seek phase when audible status changes

[ywwg]

backport of #4155 -- the other issue fixed in that PR does not affect 2.3

https://mixxx.discourse.group/t/vestax-tr-1-weird-behaviour-quantizing-drops-tempo/22715/7
Do not seek phase when audible status changes

(the syncrequest is totally unnecessary, enginebuffer already puts in a sync request when the user hits Play)

[Holzhaus]

I cannot reproduce the original issue, does someone else want to have a look? Otherwise LGTM.

[ywwg]

Steps to reproduce:

• load two tracks
• enable sync only on one of them, and also enable quantize on it
• make them different bpms
• play both
• fade the slider for the track with sync up and down

In the old code, when the fader hit the bottom, the track would jump and/or change speed to fix what it thought was a quantize error

[uklotzde]

I suppose that @ywwg has tested the fix properly. LGTM

[daschuer]

Oh, unfortunately this does not completely fix the issue, Instead of a jump, we now have the tempo dip stretched over some seconds.
This can be reproduced the best with two 440 Hz tracks and:

• Playing two tracks with sync enabled the second one silenced by the line fader
• Nudge the second track
• Bring the second track in
• Silence the first track.
• Now the nudging is reverted gradually. (Without this branch it was reverted immediately)
• This can be noted by listening the silenced track in headphone
• When you bring the track in, the phase adjustment is gone.

[daschuer]

@ywwg

when I remove

    if (isMaster(mode)) {
        m_pBpmControl->resetSyncAdjustment();

from void SyncControl::setSyncMode(SyncMode mode) the issue is gone.

Unfortunately the issue is then shifted to the sync enable case because the leader adjustment is not reset when playing as single leader. The code is in place, but somehow bypassed. Hopefully you know already how to fix it right from the main code.

[ywwg]

The sync code is ever evolving and getting simpler as time goes on :)

[daschuer]

I have filed https://bugs.launchpad.net/mixxx/+bug/1939103 to not forget the issue.

[ywwg]

got it