User guide

Appendix d: System Delays in Mixing 31
appendix d
System Delays in Mixing
This appendix provides an overview of system
mixer delays in Pro Tools|HD Native systems,
and explains how you can compensate for these
delays for phase coherent time alignment of au-
dio in complex or critical mixing situations.
Introduction to System Delay
In all digital systems, digital signal processing
(DSP) causes signal delays of varying amounts.
These delays can vary from as short as several
microseconds to as long as several milliseconds,
depending on the type of processing or routing
being performed. With Pro Tools|HD Native sys-
tems, these delays result from host-based pro-
cessing tasks.
In addition to delay incurred by the host buffer,
each plug-in, hardware insert, and mixer assign-
ment on a track delays that track by an amount
equal to the total of all system delay factors. (For
more information on these delays, see “Delay
Factors” on page 32.)
In some cases, signal processing delays matter
only if you use a real-time plug-in on one chan-
nel of a stereo or multichannel signal but not
the others. This imparts an unequal amount of
delay to the signals on that channel, which sub-
sequently may cause undesirable cancellation of
certain frequencies (phase cancellation).
In simple terms, system delay can cause audio to
arrive at the main output (or a submix output) at
different times. To maintain time alignment,
you can compensate for these delays.
When to Compensate
You may only really need to compensate for de-
lays between tracks where phase coherency
must be maintained (as with instruments re-
corded with multiple microphones or stereo
pairs). If you are working with mono signals,
and the accumulated delays are small (just a few
samples, for example), you probably do not
need to worry about compensating for delays.
However, larger sessions with higher track and
voice counts, many plug-ins, and/or complex
mixer routing can benefit when system delays
are compensated to maintain phase coherent
time alignment.
For detailed information on using Auto-
matic Delay Compensation, see the
Pro Tools Reference Guide
.
Do not confuse signal processing-induced
delays with monitoring latency or time do-
main effects processing (such as delay, echo,
reverb, and other desirable delay effects).
Audible symptoms of phase issues include
comb-filtering and loss of high frequencies.