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Old 06-29-2020, 11:13 AM
RTGraham RTGraham is offline
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Default Timing Issues with Tempo-Sync'd Plugins that Synchronize to Measure Boundaries

Hi Pro Mixing forum -

I've posted this in the Pro Tools 2020 forum, but someone suggested that I chime in here as well.
I've been (finally) chasing down a long-standing issue where plugins that process audio in a rhythmic fashion, and are meant to present their results musically "on the grid," wind up being out-of-time as a result of delay compensation in the session.
One simple example is the AIR Filter Gate that comes bundled with PT in the Creative Collection.

There's a more thorough description, along with a short list of steps to duplicate the problem, at my original thread here:
http://duc.avid.com/showthread.php?t...errerid=166880

If anyone has any insights as to whether there's already a fix for this (an option in Preferences, or a configuration setting, etc.), I'm all ears.
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Old 06-30-2020, 12:13 PM
RTGraham RTGraham is offline
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Default Re: Timing Issues with Tempo-Sync'd Plugins that Synchronize to Measure Boundaries

Just pasting the (revised) beginning of my original post here, so that anyone who doesn't want to click through to the other forum can easily see the summary and steps to repeat the main issue.
(There's additional detail at the link to the original post.)

PROBLEM:
Audio plugins that provide tempo-synced processing, and also require synchronization to measure boundaries, exhibit timing problems when a delay-compensated plugin is present before them in the signal path.

EXAMPLES:
Pro Tools comes bundled with the A.I.R. Creative Collection. Within that collection are the Filter Gate plugin, which "slices" or gates the audio (while also offering the option to filter it) on a rhythmic basis, with a variety of parameters for controlling that behavior. When operating properly, it can provide rhythmic effects that stay in time with the music - i.e. "on the grid"; and also the Pumper plugin, which mimics the effect of a typical EDM sidechain operation whereby the kick ducks the other instruments - which also, when operating properly, provides a rhythmic effect whose relationship to "the grid" is predictable.
Third-party plugins like xfer's LFOTool and SoundToys' Tremolator similarly depend on knowing where the downbeat occurs.

VERY QUICK SERIES OF STEPS TO DEMONSTRATE THE PROBLEM:
(1) Create a new session.
(2) Create two mono audio tracks. Name the first one "CLICK" and the second one "Signal Pulse."
(3) Insert the Click II plugin on the CLICK track. The default settings are fine, and most likely will be Click 1 at a quarter note with the "Classic Click Acc" sound and Click 2 also at a quarter note with the "Classic Click" sound, with "Follow Meter" enabled. Make sure the click is enabled in the session settings.
(4) Insert a Signal Generator plugin on the THIRD insert (Insert C) of the Signal Pulse track. Its settings can really be anything, but it makes it easy to hear what's going on if you set it to a square wave, 120 Hz, -15.0 dB.
(5) Insert a Filter Gate plugin on the FIFTH insert (Insert E) of the Signal Pulse track. Set Pattern to Straight, Attack to 0%, Hold to 15%, Release to 5%, Filter Mode to Off, Rate to 1/4 note, Swing to 50%, and Mix to 100%. Leave Env and LFO both at 0%.
You will now hear the square wave pulsing like a click, even though the transport is not running.
(6) Engage the transport (press play). The Filter Gate plugin will "jump" to match the timing of the engaged transport, and you will hear the click and the square wave pulse in perfect synchronization.
(7) Stop the transport and insert a Maxim plugin in the FOURTH insert (Insert D) of the Signal Pulse track. Set its Mix to 0% (fully dry). We are forcing delay compensation because Maxim is a plugin that induces latency.
(8) Start playback again. You will hear that the click and the square pulse are no longer perfectly synchronized. They should be, however, as the Filter Gate plugin is designed to know where in the measure it is. This is the problematic behavior.
(9) Insert another Maxim plugin in the FIRST insert (Insert A) of the Signal Pulse track, also with its Mix set to 0%. You can leave the transport running while you do this, or stop and restart it. The problem will become more pronounced now that there is longer delay compensation.

THINGS THAT DO NOT FIX THIS:
• Buffer size
• Any of the "Delay Compensation for External Devices" options in Preferences
• Routing the track to an Aux before it reaches a hardware output
• Using, or not using, Low Latency Monitoring
• Using, or not using, a Master Fader

The one thing that DOES fix this is turning off Delay Compensation.
But in a mix session, that completely throws off the music's timing and synchronization and precision, and introduces phase issues.

Is this a Delay Compensation bug? Or is this a flaw in the way Pro Tools reports its timecode to timing-dependent plugins when delay compensation is present in a session?
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