BUFFER SETTINGS AND LATENCY TIMES
When recording, latency is the time it takes your computer to receive an input signal, process it, and send it to an output. With no latency, this procedure would be instantaneous. Latency values can be as low as 3.0 milliseconds (essentially unnoticeable) or as high as 50 milliseconds on up (quite noticeable). These times vary depending on the hardware buffer size (H/W Buffer Size) that you choose from the Hardware window in the Setups menu.

Anytime you convert an analog signal to digital or vice versa, the analogto- digital (A/D) or digital-to-analog (D/A) converter delays the signal by about 1.5 milliseconds. Thus, when a signal is converted on the way in to your computer (A/D) and then converted as it’s played back out of the computer (D/A), the conversion delay adds up to 3.0 milliseconds. Your computer also takes some time to process the audio (depending on the buffer size set in Pro Tools) as it comes and goes, which means the total latency of your system equals the conversion delay (3.0 milliseconds) plus double the latency amount given in the following chart (figure 4.11). For example, recording with a sample rate of 44.1kHz and a buffer size of 128 samples, the total latency is 2.9ms times 2 plus 3.0ms, equaling 8.8ms.

Fig. 4.11. Latency can be a groove killer if it’s not properly accounted for during recording.

While recording you’ll often want to operate with the lowest possible level of latency. Pro Tools LE offers Low Latency Monitoring (under the Operations menu) as an excellent option, which provides a latency time of only 3.0 milliseconds (the converter delay) because the audio you hear never actually goes into the computer for processing — there’s a mixer on your PCI card that returns the audio immediately. The following restrictions apply when using Low Latency Monitoring:
• You can only record tracks with inputs set to an audio interface (not a bus).
• You can only listen to tracks assigned to outputs 1 and/or 2.
• All plug-ins and sends assigned to record-enabled tracks (routed to outputs 1–2) are automatically bypassed, and must remain bypassed.
• Record-enabled tracks will not register on meters for master faders.

These restrictions are easy to work around. I use Low Latency Monitoring during many of my sessions and it works great. In fact, the only time I don’t use it is when I absolutely have to hear the output of a plug-in while recording.

THE BASICS OF SENDS, RETURNS, AND BUSSES
Speaking of plug-ins let’s talk briefly about sends, returns, and busses. If you’re familiar with the signal routing of a mixer, you’re probably familiar with the terms. Pro Tools uses sends, returns, and busses in the same way as a hard-wired mixer — you just have more flexible routing options.

I’ll explain with an example: Let’ say you want to put the same delay effect on several tracks using a plug-in. Instead of putting that plug-in on each of the tracks, you can put one delay plug-in on an aux track and use it for any other tracks that need delay (aux is short for auxiliary input). You do this by sending copies of all the tracks that need delay (via a bus) to the aux input with the plug-in. The bus carries the tracks to the aux input where they’re processed using the plug-in and then sent out, usually to the main stereo outputs. In this case, the aux track is called an effects return. See figure 4.12 for a visual representation of the signal flow.

Fig. 4.12. Send/return configurations allow one plug-in to effect several tracks.