Pro Tools Notes

Latency

Published in SOS February 2003
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Technique : Computers

The snare track has an extra plug-in inserted, causing slightly more delay. To prevent phase problems with spill in other tracks, the Time Adjuster plug-in adds delay to the other drum channels. Pro Tools displays the delay in each track (as highlighted).

Using DSP-based audio systems is the answer to problems of audio latency, right? Experienced mixers using digital systems — including Pro Tools TDM — will tell you there's more to the story...

These days, anyone getting into the music technology field quickly learns about latency in audio paths passing through a computer. We also learn that this is a problem for host-based systems, with the delay being a necessary 'breathing space' to allow our CPU a chance to run all the maths behind signal mixing and processing. What's more, latency is all about the problem of delays in the monitoring chain. Except it's not: even in 'real-time' DSP systems there's potential for delays to cause problems with feel and foldback monitoring, and in particular phase misalignment in mixing. And we haven't even mentioned DirectConnect, HTDM and MIDI latencies yet!

First let me point out that latency has always been an issue in multitrack audio recording. In a studio without any computers or digital gear, you've still got acoustic latency to deal with: the time it takes for sound to propagate through the air (and other materials in fact) to the mic. With close miking this delay is small, but if you have two mics open and they both pick up the same sound, then even tiny differences in distances from the source will cause phase cancellation (a hollow, flangy effect) when the signals are mixed together. Engineers learn to account for this problem when setting up for a multi-mic recording session through careful mic placement, isolating sounds as best as possible, and even introducing artificial delays to keep all signals in phase. If, like me, your recording tends to stick to soft synths and maybe a single mic, you can still hear this problem on TV quite often — a good bet is a studio discussion programme, where two boom mics may both be open at some points.

Getting back on topic, in an analogue studio latency is no longer an issue once the audio reaches the mic and becomes an electrical signal. Audio travelling through analogue circuitry, such as a mixer channel strip followed by an insert or send/return, might take a few microseconds. This is too short a delay to cause phase problems within the human audible frequency range. In a digital system, however, you have to be on guard for further signal delays. Firstly, in any digital system it takes time to go through analogue-to-digital convertors and vice versa. If you're using a Pro Tools TDM system with an 888/24 it takes about 80 samples to make a round trip from analogue to digital to analogue, so there's a minimum monitoring latency of a little under 2ms. Now this is a pretty short period of time, but it could have a slight noticeable effect on what a performer hears. First, at low headphone levels the interaction between the direct 'in the room' sound and the cue signal might sound phasey or even flammy. Additionally, there's the question of timing, and whether you think the through-delay could be big enough to force a musical difference when overdubbing. If in doubt, you need to think about monitoring overdubs through an analogue mixer rather than Pro Tools. Interestingly, the delays are measured in samples, so running at higher sample rates reduces latency in actual time. But to put this all in perspective, I have to say that self-monitoring delay has never come up as a problem in any TDM sessions I've been involved in, and 2ms of delay is the same as standing two feet from the source...

The real issue is in being aware of delays when routing and mixing in the TDM mix environment. Delays occur through internal buss sends, external sends and inserts, and plug-ins. Obviously, an insert or send/return via analogue connections to an external outboard processor will introduce latency of the same order as the A/D/A round-trip mentioned above. Internal sends and plug-ins are mostly just a few samples (six for a send, three for a d2 EQ). For the most part the delays are so tiny as to be insignificant, but in certain circumstances they have effects that need addressing:

Quick Tips     TDM Waves plug-ins come with extra plug-ins called [plug-in name]DLA, which are simply short delays equal to the TDM delay caused by the main effect. This saves using Time Adjuster and manually calculating the number of samples when correcting for TDM latency. When you have a mixer group with the faders at different levels, take care not to option-click any of the faders (as you might do routinely to 'zero' faders). With a group, option-clicking sets all the faders to unity and loses the relative relationships.

Scenario 1: internal send and return. A plug-in is inserted on an aux input track. Other tracks use sends (post-fader) to feed a variable amount of signal to the aux. In the main stereo mix, there is a balance of the dry signal from the audio track, and the wet signal from the aux channel. This very standard configuration requires awareness of latency when used in a digital mixer like Pro Tools. Delays will be incurred through the send and the plug-in, so it's important that the signal coming from the aux is not too similar to the dry signal, or you will get phase cancellation. Firstly this means that the plug-in dry/wet mix must be set to 100 percent wet. Secondly, this routing configuration is only suitable for some types of plug-in, such as reverbs, where the wet output is significantly different to the dry input.

Scenario 2: processing individual channels of a multi-mic recording. A drum kit or ensemble is recorded with a number of mics, and there's a large amount of spill between the mics. If any single track has a plug-in inserted, the few samples of delay will put the recorded spill between mics out of phase with each other. Additionally, if it's an insert via an external analogue unit, then the delay might cause subtle timing problems, or flamming with the spill. The best way to handle this situation is to get into the habit of matching delays across tracks that need to remain exactly aligned. The extravagant way to do this is to put the same plug-ins on all of the tracks in question. However, if it's a DSP-hungry plug-in, or you're using an external device, then you can use the Time Adjuster plug-in on the other tracks to match everything up. Pro Tools helps by keeping track of the latency of each track, measured in samples. Command-click twice on a track's volume display to switch to the delay readout. Adjust each track's TA plug-in to match the biggest delay (see screenshot). I should note that the delay indicator only shows plug-in delays, not hardware inserts. To account for latency incurred by routing out of Pro Tools, you can use the values given in the Pro Tools manual for each particular Digidesign interface. Notice that a round trip through the interfaces via digital connections is much quicker than going through D-A and A-D converters, so using effects devices with digital I/O is a way of reducing latency (unless the effects box imposes lots of latency internally!).

Scenario 3: high-delay plug-ins, and/or working to picture. Certain TDM plug-ins cause an unusually large delay. These tend to be ones that use 'look ahead', such as Maxim and DINR. This kind of plug-in is often used on the Master Fader, and as it affects the whole mix at once, won't cause timing problems in this context. However, this would not be acceptable if you were working against timecode, because your final output would be out of sync. Similarly, if you use such a plug-in on a single track then you will introduce timing errors.

An example of both these problems at once would be mixing the sound for a DVD of a live performance. If you used a very high-delay process, such as Wave Mechanics' Pitch Doctor, on the lead vocal track, you'd push the vocals late by nearly 4000 samples. This is approximately 90ms, and over two frames of video: the vocal would be both audibly and visibly out of time. In this situation, you can't delay all the other tracks, not only because it would be incredibly tedious, but because it would also put everything out of time with the video. So the best option is actually to move all the audio in the lead vocal track earlier by 4000 samples. Simply select all the audio in the track, choose Shift from the Edit menu, click Earlier and enter the number of samples.

Other times that latency can creep into the world of the TDM user are when using HTDM plug-ins or DirectConnect-based software, all of which use the host CPU instead of Digi's DSPs. From what I can tell, tracks using RTAS plug-ins are read ahead of time during playback, going via the hardware buffer that you set in the Hardware Settings page, and are not delayed with respect to other tracks. I guess this is why you can't mix and match TDM/RTAS plug-ins on one track, or use RTAS plug-ins on auxes in a TDM mix. HTDM plug-ins are processed by the host, then re-routed into your (TDM) mixer by the StreamManager extension, incurring latency. (All this happens without you knowing about it of course.) Stuff coming from a DirectConnect application also comes in via StreamManager. By default StreamManager has a buffer of 512 samples, so anything HTDM or DC will incur an 11ms delay. Many of the programs that use this format are soft synths, so this is getting towards intolerable when it comes to percussive instruments. If you have a really fast computer, you can replace the default StreamManager extension with a 128-sample buffer version. This reduces the delay by 75 percent, but the trade-off is that you'll be able to run fewer HTDM plug-ins at once. The download URL for the alternative extension is download.digidesign.com/ support/digi/mac/pt/Stream Manager_128v531r2.hqx.

For more information about the issues raised in this column, check out John Klett's 1999 article, 'Delay In Large-Format Digital Music Consoles', which you can read on-line at www.technicalaudio.com.

Current Versions     Mac OS HD: 5.3.1r2. 24/Mix: 5.1.3cs4. LE for Digi 001/Audiomedia III: 5.2.1cs1. LE for Digi 002: 5.3.2cs1. Free: 5.0.1. Windows 24/Mix/HD: 5.3.3. LE for Digi 001/002/Audiomedia III: 5.3.1. Free: 5.0.1.