Side-chain PDC problem
First Let's tackle the so called "side chain PDC bug". Given the following scenario, PDC sync is not acheived.
- create track “audio 1”
- create track “audio 2”
- route track “audio 1” over “bus 1” to “aux 1”
- insert latency inducing plugin “Linear Phase EQ” into “aux 1”
- route “aux 1” over “bus 2” to “aux 2”
- route track “audio 2” over “bus 3” to “aux 3”
- insert plugin “Compressor” to “aux 3”
- change side chain of the compressor to “bus 2”
- put a identical drum-loop on both tracks
The above essentially sets up the following two parallel signal chains...and PDC does not work right, they are out of sync.
The above parallel signal chains WILL play in sync if the side-chain is removed..
So the first data point is:
Point #1 - two parallel aux signal chains, will play in PDC sync, unless side-chain is used, then apparently the reported latency from the signal feeding into the side chain bus gets forgotten for some reason
But interestingly this appears to be related to more complex AUX routing as much as anything, which may be the actual bug in some way.. PDC handles AUX channel latency by delaying all other channels, but I think perhaps Logic Pro can get confused if and when a side-chain is combined with AUX channels like above...or there could be other scenarios not having anything to do with Side-chain, where the various different AUX signal paths might confuse the PDC engine of LogicPro.
For example, if we move the latent EQ plugin from the above scenario to the first audio track and basically skip routing through AUX1 for that, the problem goes away entirely..side-chain works perfectly..:
This leads us to second important point:
Point #2 - Side-chained scenarios with latency on the channel feeding the side chain, will play in PDC sync if the channel hosting that latent plugin is not an AUX.
Another work-around I found was to use Expert Sleeper's Latency Fixer plugin on AUX-2 of this scenario, set to a value of 53ms like this:
In this solution, LatencyFixer causes LogicPro to bring everything back in sync again. Its a partially working work-around. I say partially because while this trick does line up the resulting two signal paths, the actual side chain input will be wrong...it would be receiving the side chain data late...and pumping the compressor out of sync. Harder to hear, but that is what would be happening.
(note, Latency Fixer does not itself impose any delay, it simply reports latency to LogicPro so that the PDC engine will cause a correction).
What we learn from this workaround:
Point #3 - Putting another plugin on AUX-2 later in that same chain, then brings signal-path-one back in sync with the AudioCH2 chain..
So.. my guess is that specifically when an AUX with latency is used to feed a side chain, then PDC doesn't do what its supposed to do on that particular AUX in terms of PDC. Logic somehow ignores the PDC correction.
So I think this is related to side chaining, yes, but it's also related specifically to when AUX channels are used to host latent plugins, feeding into the side chain.
The LatencyFixer work-around is really not acceptable since it would pump the compressor at the wrong timing, though this would at least avoid phasing problems between signal paths 1 and 2 of this scenario. The only work around solution for now would be to avoid putting the latent plugin on an AUX channel feeding into the side chain.
Some of these observations might apply to the automation PDC issues as well..not sure yet, I need to dig deeper.
I hope this thread may generate some useful discussion in order that we can get to the exact and accurate problem... hopefully Apple could address it. If not, at least we might be able to identify specific taboo work flows, along with work arounds.