Imagine that you are mixing a session, and all of your audio tracks are racing to the mix bus. When there are no plug-ins inserted on any of the audio tracks, they all reach the mix bus at the same time. It’s a multi-way tie.
Now imagine that you need a compressor plug-in on the lead vocal. We don’t realize it, but plugin processing takes a certain amount of time (usually milliseconds). So the lead vocal track now goes through the compressor plugin on its way to the mix bus. It loses the race because the compressor plugin postpones its arrival at the mix bus very slightly — so slightly that you might not even notice it.
Now lets say that in the same session you have a bass track. You decide that you’d like to duplicate the bass track so that you can process the duplicate with an amp simulator and mix it with the original. The original track has no plugins and the duplicate processed track has an amp simulation plugin on it. Who gets to the mix bus first? The original bass track. The duplicate is delayed while it is processed through the amp simulator plugin. If we add another plug in to the processed track (a compressor for example) then that track is delayed even more. When you mix it with the original bass track, you’ll hear phasing, or flanging or in severe cases you might hear flams (double-hits) on the attacks of the notes. How bad the issue is will depend upon the processing power required by the plugin, and how much power your computer provides. Some plugins require a lot of DSP and add more lateness or “latency” to the track.
Automatic Delay Compensation looks at a session and watches what plugins are on which tracks. It automatically delays ALL tracks in order to allow the ‘slowest’ track (the track with the most delay due to plugins) to catch up. If Automatic Delay Compensation is turned off, you will hear these timing issues. When turned on, Pro Tools usually does a pretty good job of making sure that all of the tracks reach the mix bus at the same time.
When we use the “hardware insert” i/o in the plugin menu, the audio track must make a trip from Pro Tools to the audio interface output (D/A conversion), into the external processor (let’s say it’s a hardware compressor), out of the external processor into the interface, then A/D conversion back into Pro Tools. That trip takes time** due to the D/A and A/D. We think that A/D and D/A conversions happen in real time but they don’t — they take a few milliseconds. Automatic Delay Compensation also calculates the timing for this type of plugin.
** note an analog processor does NOT add latency. It’s the D/A and A/D that produces latency.