Plug-in Delay Compensation in Pro Tools – Part 2
This article is an excerpt from my column, “The Pro Tools Corner” at audioMIDI.com
NOTE: This was published before automatic delay compensation was standard in every Pro Tools system. Although the concepts and techniques in this article still work, you’ll want to read the section below regarding using automatic delay compensation in Pro Tools HD, it now works the same in all PT systems now.
Pro Tools Delay Compensation – Part 2
In the last installment of the Pro Tools Corner I discussed the challenges of plug-in latency in the mixer and offered up some techniques for manually combating this latency in Pro Tools. While the tips and tricks offered up last week will get you through most situations, there is an additional built-in solution for users on Pro Tools HD called automatic delay compensation that can be a real time saver. LE/M-Powered users can also gain similar functionality through a 3rd party add-on from MellowMuse called auto-time adjuster. In this week’s Pro Tools corner, I will discuss these two additional techniques for combating plug-in latency and offer up a few more tips for keeping your mixes in sync.
Pro Tools|HD Delay Compensation: (note: this technique now works in all PT versions 9+, not just HD)
Unlike native systems, which generally account for most plug-in latency within the system’s playback buffer, HD system’s using TDM plug-ins almost always suffer from small amounts of routing delay due to the inherent nature of hardware DSP processing. While the complexities of the TDM processing infrastructure are way beyond this article, one should note that plug-in latency is a serious concern when using TDM plug-ins in Pro Tools HD, so serious that Digidesign finally added a comprehensive delay compensation engine in version 6.4 of the Pro Tools HD software. While I personally try to forget about the days before automatic delay compensation in Pro Tools HD, I am consistently surprised at the number of HD users that fail to take advantage of such a critical tool in the mixing process.
Enabling Delay Compensation:
Because every TDM plug-in instance in Pro Tools causes at least a few samples of delay, automatic delay compensation is really a mixers best friend and a near hands-off approach for dealing with plug-in latency. To activate delay compensation, you must first enable it for your system in the Playback Engine (Setup > Playback Engine). The “Delay Compensation Engine” can be set to either Short (for up to 1024 samples of compensation) or Long (up to 4095 samples of compensation). If a session is currently open, Pro Tools will automatically save, close and reopen the session to enable the delay compensation engine. Once enabled in the Playback Engine, you can turn delay compensation on and off by selecting Operations > Delay Compensation.
Short or Long?
The delay compensation engine should be set according to the type of plug-ins you plan on using versus the amount of DSP resources you want to dedicate to delay compensation. Certain plug-ins cause more than 1024 samples of delay (many real-time “tuning” plug-ins, pitch shifters, etc) and therefore would not be sufficiently compensated for under the “short” setting. The down side of using longer delay compensation settings is that it cuts into the amount of available DSP your system has for other TDM plug-in processing. If you have a smaller HD system (e.g. HD|1), I recommend setting the delay compensation to “short” and manually compensating for longer delays using the techniques described in the previous article. I personally use a HD|3 and because I consistently use plug-ins that create a significant amount of delay, I leave my delay compensation engine set to “long” and find that I still have plenty of DSP left for most mixes.
Viewing delay compensation:
The Pro Tools HD mixer has a special view just for delay compensation, access this by choosing View > Mix Window > Delay Compensation. Here you will find the tracks total delay (as accumulated by either plug-ins or bus routing), a manual offset control, and the tracks total compensation. “dly” and “cmp” are calculated and adjusted automatically, so for the most part you can watch it do its magic and mix away, knowing you are taken care of. These values are displayed in samples by default, but can be switched to millisecond in the preferences > operation tab.
The longest delay in the session is denoted in orange and if a track’s total delay exceeds the engines maximum compensation amount (1024 for short and 4095 for long), it will glow red. When a track’s delay exceeds the total amount of delay compensation available you should always disable that track’s compensation by Control+Command-Clicking (Mac) or Start+Control-Clicking (PC) on the word “dly” in the delay compensation view. This will disable the delay detection on the track and at that point the track can be compensated for manually using the techniques discussed in part one of this article.
Tips for using delay compensation:
• When a track (audio or instrument) is record enabled, delay compensation for that track is automatically suspended to allow for low latency monitoring. To compensate, newly recorded tracks will be automatically shifted earlier by the amount of the total system delay after each record pass. You can force delay compensation on any record enabled track by Control+Command-Clicking (Mac) or Start+Control-Clicking (PC) on the “cmp” field, the compensation value will light up in blue to denote this.
• The master delay compensation indicator in the edit window can be used to quickly identify if delay compensation is enabled and working correctly (when green). When delay on any track has excited the total amount of compensation available, this indicator will light up red as a warning.
• You can set up delay compensation for hardware inserts in Setup > I/O set-up > Hardware Insert Delays. You will have to manually ping your hardware inserts to figure out each ones unique delay (play a transient rich sample or sine wave out into the hardware insert and record it back into another track, measure the offset to find the delay).
MellowMuse ATA: A Solution for Pro Tools LE/M-powered
Update: This is no longer necessary in Pro Tools 9 and later, read the above section.
The basic idea of any PDC system is that all tracks in the session incur the same amount of delay, thus eliminating any timing or phasing issues associated with some tracks lagging behind others. How most systems (including Pro Tools HD) accomplish this is to calculate the delay of the track with the most latency and work backwards from there. Let’s call this longest delay “X” samples. Tracks without any latency (a track without plug-ins for example) will get delayed by X samples, while tracks with a delay less than X will be delayed by X – Y, where “Y” is the amount of delay on the track in question. When all tracks, including auxiliary returns, exhibit the same amount of latency, we have a happy mixer. Like I said, because Pro Tools LE/M-Powered doesn’t do this for us automatically, we can use the ATA plug-in to calculate and compensate for us.
While the instructional videos on Mellowmuse.com are super helpful when setting this up, here is a quick breakdown of how the ATA plug-in is used in Pro Tools:
1. First insert ATA as the first plug-in on every track of your session, including the Master Fader and any Aux Tracks.
2. Inside the ATA plug-in window, set the plug-in’s “group” selector to the type of track it is located on (Audio for audio tracks, Aux 1 for Auxiliary Returns, and Master for the Master Fader)
3. Submix all audio tracks into a new stereo aux track, again placing the ATA plug-in as the first insert on this new aux. This is necessary to compensate for delay incurred on the effects returns.
4. Make sure nothing is muted, soloed or set to -infinity, and click the “P” or Ping button on the Master Fader to calculate and compensate for delay.
5. If you are going to do second or third order sends (Using sends on your aux tracks) you will need to use “Aux 2,3,4,5” groups to compensate for additional latency (e.g. feeding a delay into a reverb, or using a send on a submix). The video at mellowmuse.com really helps to understand this.
6. Be sure to ping the master fader each time you insert a new plug-in, as that additional plug-in may change the delay on that specific track.
Note: you will not see the “Dly” indicator change to reflect all tracks having the same latency. The delay is compensated for internally inside the ATA plug-in.
The cool thing about ATA is that it actually polls the delay using an audio “ping,” which provides a super accurate measure of delay (some plug-ins report their delay to the host incorrectly). I recommend setting up session templates that have the ATA plug-ins, submix and effects return routing already set up. If you follow the instructions, it works flawlessly and is way better than having to manually compensate for delay, but I’m not gonna lie, this is sort of a cumbersome workaround if you use a lot of second and third order sends in your mixes (seriously, set up templates). But if you are going to point fingers, it’s really not Universal Audio or Mellowmuse’s fault. Avid needs to come up with a PDC solution for its LE/M-powered users and join the 21st century like every other DAW has.