A technical support community for Apple Logic Pro users.

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lagerfeldt
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Plug-in Delay Compensation broken with external side-chain

Thu Jan 11, 2018 5:26 am

Plug-in delay compensation is not applied to the side-chain routing which means that signals drift out of sync and vary depending on how many plug-ins have been applied to the track(s).

Turning off plug-in delay compensation can make some tracks sync again while the rest of the mix will drift radically instead.

This might not be a major problem with some situations in a mix, but it renders stem limiting with the new FabFilter Pro-L 2 using external side-chain input completely useless. Makes me consider switching to Cubase, where this is working correctly, since it's a pretty nifty feature when you're doing stem mastering bounces for live, TV, etc.

Logic Pro X 10.3.3
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loopsinner
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Re: PDC broken with external side-chain

Tue Jan 23, 2018 9:01 am

Can't seem to replicate the bugs :(



It works as expected on my side, unless i did something wrong here?

How do i replicate the issue?
• Logic Pro X 10.6.2
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lagerfeldt
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Re: PDC broken with external side-chain

Tue Jan 23, 2018 9:19 am

Try something that adds a lot of latency like a linear phase plug-in. You'll see that the latency is compensated as expected on the channel, but not on the side-chain signal. They're out of sync.

You can also test it with something like the FabFilter Pro-L 2 and external S/C.
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loopsinner
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Re: PDC broken with external side-chain

Tue Jan 23, 2018 9:34 am

hmm... that's the highest i can go with

17,408 samples/394.7ms

Apple's Linear Phase EQ only generates 2,560 samples/58.0ms of latency

lagerfeldt wrote:
You'll see that the latency is compensated as expected on the channel, but not on the side-chain signal. They're out of sync..


I'm having a noob moment, does that mean if i side chain Taylor Swift to a drummer, the drummer signal is not properly compensated?

Like the source of the side-chain signal (the drummer in this case)?
• Logic Pro X 10.6.2
• Focusrite Scarlett 2i2 (second generation)
• Mac mini • M1 • 16 GB LPDDR4 • macOS Big Sur 11.4
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lagerfeldt
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Re: PDC broken with external side-chain

Tue Jan 23, 2018 12:26 pm

Correct.

When you're use a compressor on a track it's using the internal side-chain by default, i.e. the input signal is being fed through the side-chain to the detection circuit. Inserting a compressor on a vocal track means that the vocal is the input signal, i.e. the vocal is being fed through the internal side-chain to the detection circuit. This happens automatically and by default in a compressor.

When you substitute the internal side-chain with an external side-chain input you're using another signal (from different channel, i.e. external) to trigger the compressor. When people say "side-chain compression" they actually should be saying "external side-chain compression".

So let's imagine you still have the compressor on the vocal track, but you substitute the internal side-chain (with the vocal input) with an external side-chain, e.g. a drum track from another channel. Now the compressor on the vocal is no longer listening to the vocal itself but listens to the drum track. This means that the compressor no longer ducks the vocal in relation to the peaks of the vocal, but instead to the peaks of drums, i.e. the external side-chain input.

If you have other plug-ins on the vocal and/or drum track - which you normally do - these plug-ins will cause a delay or latency on each track, depending on how much lookahead and processing cycles each plug-in demands. Logic automatically shifts or compensates the timing of the affected tracks so they're aligned in time in the mix.

But here's the bug in Logic: the side-chain signal itself is not compensated correctly. This means that the drum track used for triggering the vocal compression is out of sync in the external side-chain but is in sync in the rest of the mix, causing anything from minor rhythmical problems with all external side-chain compression to completely breaking a feature such as sum-limiting between stems. And the amount of sync problems depends on the processing going on in the mix. Anywhere between not very audible to a second out of sync!

All other DAWs that I'm aware of compensate processing delays correctly in the external side-chains, except Logic.
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Presence
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Re: PDC broken with external side-chain

Tue Jan 23, 2018 1:47 pm

Did this just start with 10.3.3?
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Eric Cardenas
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Re: PDC broken with external side-chain

Tue Jan 23, 2018 3:36 pm

No. This exact anomaly started in Logic Pro X 10.2.1.
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Re: PDC broken with external side-chain

Tue Jan 23, 2018 4:42 pm

Thanks Eric, that goes back a ways. Was going to start testing but can only go back as far as 10.2.4 so you probably save me some grief.
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loopsinner
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Re: PDC broken with external side-chain

Tue Jan 23, 2018 11:41 pm

Thanks for the explanation lagerfeldt.

Sadly, this is beyond me, as i do not know what method to use to detect if the external side-chain signal being properly compensated, or not.

My wish is to replicate it on my end to understand it, and to help report to Logic Pro X team, but due to noobiness (LOL!) i can't seem to do that.

But I trust you and Eric, if you guys say it's there, then it's there for me.
• Logic Pro X 10.6.2
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• Mac mini • M1 • 16 GB LPDDR4 • macOS Big Sur 11.4
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lagerfeldt
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Re: PDC broken with external side-chain

Wed Jan 24, 2018 12:19 am

I'll give you a step by step guide in short while :-)
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Ploki
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Re: PDC broken with external side-chain

Wed Jan 24, 2018 9:40 am

how can they be so sloppy for such a long time
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lagerfeldt
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Re: PDC broken with external side-chain

Wed Jan 24, 2018 11:05 am

Too busy making new loops...
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Logic 10.4.1 Side Chain Delay Bug

Sun Apr 15, 2018 6:11 am

Edit Note: Merged topics about the same bug.

Signal Path A is delayed.

(I used 3 instances of the linear eq, to make it more clear)

Screen Shot 2018-04-15 at 15.06.03.png
Screen Shot 2018-04-15 at 15.06.03.png (627.08 KiB) Viewed 10382 times
 
logness
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Re: Logic 10.4.1 Side Chain Delay Bug

Tue Apr 17, 2018 3:31 am

if someone also wants to submit a bug report to apple, here are the steps to reproduce the problem

"Logic side-chain bug"

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

result:
audio 1 <-> audio 2 are NOT in sync
 
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Alan Shields
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Re: Logic 10.4.1 Side Chain Delay Bug

Tue Apr 17, 2018 7:06 am

Confirmed. Following your steps in a new empty project Audio 1 and Audio 2 are not in sync.

Alan.
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Re: Logic 10.4.1 Side Chain Delay Bug

Thu Apr 19, 2018 3:37 am

Hi Alan, thanks for confirmation!
 
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Re: Logic 10.4.1 Side Chain Delay Bug

Thu Apr 19, 2018 5:08 am

Confirmed here too. I'm moving this to the bug section.
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Eric Cardenas
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Re: PDC broken with external side-chain

Thu Apr 19, 2018 5:45 am

Topics merged.
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Re: PDC broken with external side-chain

Thu Apr 19, 2018 5:55 am

Thanks Eric. :D
David Nahmani
• My Logic Pro X 10.5 book (Apple Pro Training Series)
Contact me for Private Lessons
Logic Pro X 10.6.1
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lagerfeldt
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Re: PDC broken with external side-chain

Thu Apr 19, 2018 10:39 pm

Just for the record: Apple are aware of this report and are actively working on reproducing the problem (and hopefully fixing it).
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I came here to chew bubblegum and kick ass... and I'm all out of bubblegum
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