Appendix A: DSP-Induced Delays in

Mixing (TDM Only)

This appendix provides an overview of DSP-induced delays, and explains how you can compensate for these delays to improve time and phase alignment of audio in complex or critical mixing situations.

• To learn about DSP-induced delays and when

you should compensate, see “Introduction to

DSP-Induced Delay” on page 647.

• To learn how to compensate for delays automatically with Delay Compensation (TDM

systems only), see “Delay Compensation” on page 648.

• To learn how to compensate for delays manu-

ally, see “Manually Compensating for Delays” on page 649.

• For details on the sources of DSP-induced de-

lay, see “Delay Factors” on page 651.

Introduction to DSP-Induced

Delay

In all digital systems, DSP processing causes signal delays of varying amounts. These DSP-induced 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.

Do not confuse signal processing-induced delays with time domain effects processing

(such as delay, echo, reverb, and other desirable delay effects).

Each plug-in, hardware insert, and mixer assignment on a track delays that track by an amount equal to the total of all DSP-delay factors. (For

details on these delays, see “Delay Factors” on page 651.)

In some cases, signal processing delays matter only if you use a real-time TDM plug-in on one channel of a stereo or multichannel signal but not the others. This imparts an unequal amount of delay to the signals on that channel, which subsequently may cause undesirable cancellation of certain frequencies.

Audible symptoms of phase issues include comb-filtering and loss of high frequencies.

Appendix A: DSP-Induced Delays in Mixing (TDM Only) 647

In simple terms, DSP-induced 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 DSP-induced delays.

When to Compensate

You may only really need to compensate for delays between tracks where phase coherency must be maintained (as with instruments recorded 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 DSP-induced delays are compensated to maintain proper time alignment.

In any session, if you want to maintain absolute time alignment across all tracks you should always compensate for signal processing delays.

• You can manually compensate for delays with one or more of the following methods:

• Apply sample-level delay to tracks with the

TimeAdjuster plug-in (see “Using TimeAdjuster TDM Plug-In” on page 650).

• Manually nudge tracks earlier or later in

time as needed “Nudging Audio Tracks” on page 651.

• Use the same plug-ins on all tracks.

While learning about how to compensate for delays, keep in mind that the best results can be often obtained by using a combination of one or more methods. Doing so gives you more options for managing DSP resources as needed by Delay

Compensation and the TimeAdjuster (TDM) plug-in. In addition, the variety of tools available for compensating for delays help ensure compatibility if the session is to be transferred to another system.

Automatically Compensating for Delays

If you want Pro Tools to automatically compensate for offsets incurred by processing delays, use Delay Compensation.

How to Compensate

Pro Tools systems provide tools to compensate for signal processing delays, as follows:

• You can automatically calculate and compensate for processing delays by using the

Pro Tools Delay Compensation feature. See

“Delay Compensation” on page 648.

Delay Compensation

Delay Compensation automatically manages

DSP delays that occur on audio tracks, Auxiliary

Inputs, or Master Faders because of plug-in use and mixer routing. With Delay Compensation enabled, Pro Tools maintains time-alignment between tracks that have plug-ins with differing

DSP delays, tracks with different mixing paths, tracks that are split off and recombined within the mixer, and tracks with hardware inserts.

648 Pro Tools Reference Guide

To maintain time alignment, Pro Tools adds the exact amount of delay to each track necessary to make that particular track’s delay equal to the delay of the track that has the longest delay.

Delay Compensation should be enabled during mixing and playback for optimal delay-compensated sound. In some cases when recording, Delay Compensation should be turned off.

For more information on using Delay Com-

pensation, see “Delay Compensation” on page 469.

To use Delay Compensation to compensate for

DSP-induced delays:

1 Choose Setup > Playback Engine, and choose

a Delay Compensation setting (see “TimeAdjuster and Delay Compensation” on page 649).

2 Choose Display > Mix Window Shows > Delay

Compensation View.

3 Choose Operations > Use Delay Compensation.

4 Use the track Delay Compensation displays and other settings to do any of the following, as needed:

• Define specific delay values for hardware inserts

• Bypass Delay Compensation per track

• Fine tune Delay Compensation for each track by entering a User Offset

TimeAdjuster and Delay Compensation

TimeAdjuster plug-ins behave in a special way when used with Delay Compensation, as follows:

• When Delay Compensation is enabled, Time-

Adjuster does not report to Pro Tools its Delay setting (the amount of delay to be added to the signal path). However, TimeAdjuster does reports its internal plug-in delay of 4 samples to Pro Tools, and this delay is reported in the track Channel Delay indicator (or Delay Compensation View) in the Mix window.

• When Delay Compensation is turned off or unavailable, TimeAdjuster operates as expected and applies its Delay setting to the

track. For more information, see “Manually

Compensating for Delays” on page 649.

This ability to maintain TimeAdjuster instances and their Delay settings lets you more easily transfer sessions between systems with and without Delay Compensation.

Manually Compensating for

Delays

If you want to manually compensate for offsets incurred by processing delays, do any of the following:

• Use the same plug-ins on all tracks.

• Apply sample-level delay to tracks with the

TimeAdjuster plug-in. See “Using TimeAdjuster TDM Plug-In” on page 650.

• Physically adjust audio tracks in the Edit window by the appropriate number of samples (for example, by using the Nudge func-

tion). See “Nudging Audio Tracks” on page 651.

Appendix A: DSP-Induced Delays in Mixing (TDM Only) 649

Using TimeAdjuster TDM Plug-In

You can use the TimeAdjuster TDM plug-in provided with your Pro Tools system to apply an exact number of samples of delay to the signal path of the tracks you are working with. Up to

2048 samples of delay are available.

The Channel Delay indicator (dly) in the Mix window displays the total delay, in samples, incurred on a track from the use of any plug-in on that channel. TimeAdjuster can be used to match these delay values for other tracks that need to remain in phase (as can occur with instruments recorded with multiple microphones or stereo pairs, if the tracks do not use the same plug-ins).

See also “TimeAdjuster and Delay Compensation” on page 649.

For information on delay indication when

using Delay Compensation, see “Delay

Compensation” on page 469

To use TimeAdjuster to compensate for a time delay (for example, between two tracks):

1 In the Mix window, Control-click (Windows) or Command-click (Macintosh) the first track’s

Volume/Peak/Channel Delay indicator to toggle to Channel Delay (dly).

2 Note the Delay value, shown in samples.

3 Insert the TimeAdjuster plug-in on the second track (the one whose delay you want to change to match the first track).

4 Control-click (Windows) or Command-click

(Macintosh) the track’s Volume/Peak/Channel

Delay indicator until the Channel Delay (dly) value is displayed for that track.

5 Change the delay time in TimeAdjuster by moving the Delay slider or entering a value in the Delay field, until the second track’s delay value matches that of the first track.

Testing TimeAdjuster

You can test the delay values by duplicating an audio track and reversing its phase while compensating for delay.

Using TimeAdjuster with Plug-Ins

If you are using a plug-in whose delay factor you are not familiar with, you can set the delay by ear.

To use TimeAdjuster to compensate for plug-ins with unknown time delays:

• If you are working with phase-coherent track pairs, or tracks that had been multi-miked, you can “null out” the delay. Invert the phase of the target track using the TimeAdjuster

Phase Invert button, and adjust the plug-in delay time until the signal disappears. (When they are perfectly synchronized, duplicate signals of opposite polarity cancel each other out.) When you are finished, disengage the

Phase Invert button.

– or –

• Change the delay while listening to the signal in phase, adjusting until any comb-filter effects cancel out.

650 Pro Tools Reference Guide

Nudging Audio Tracks

On audio tracks, you can use the Pro Tools

Nudge feature to nudge the affected regions in single-sample (or other) increments, to preserve phase coherency. If it is necessary to nudge a region by a large number of samples, you may want to calculate the equivalent value in milliseconds and nudge the Region in millisecond increments. If the value does not divide evenly, you can switch the Nudge value back to samples and use these smaller increments for the remainder.

The disadvantage of using this method is that it only works with audio tracks (not live inputs), and the timing relationship between tracks is permanently altered, which can affect editing.

Sample Rate and How it Affects Delay

The delay in samples caused by TDM processing is the same regardless of the sample rate because a sample is tied to the rate of the sample clock for the entire system. However, when samples are converted into a time value (milliseconds or microseconds), the sample rate (44.1 versus

48 kHz) must be taken into account.

Delay Factors

In Pro Tools TDM systems, delay is incurred when you perform the following processes:

• Bouncing tracks. See “Bouncing Tracks” on page 651.

• Real-time processing with plug-in inserts.

See “Using Plug-In Inserts” on page 651.

• Mixing and routing with sends. See “Using

Sends” on page 652.

• Mixing and routing with hardware I/O

(sends or inserts). “Using Hardware I/O” on page 652.

Bouncing Tracks

Bouncing to tracks causes additional delay, as follows:

Bus-Based Bounces When you bus a track to another track and record the result, the following delays are incurred:

Bus-based bounce delays for each Pro Tools system

HD Mixer Delay

Stereo Mixer 10 samples

Surround Mixer 8 samples

Bounce to Disk The File > Bounce To Disk command causes no delay on a TDM-equipped system since delay compensation for the bounce function is built in. This form of bouncing may be more desirable than bus-based bouncing. For

details, see “Bounce to Disk” on page 536.

Using Plug-In Inserts

Using plug-in inserts on a track causes additional delay.

For specific information on the delay amounts for DigiRack or Digidesign plugins, refer to your

DigiRack Plug-Ins Guide or

Digidesign Plug-Ins Guide .

Channel Delay Indicator

On all systems, the Channel Delay indicator

(dly) in the Mix window displays the total delay, in samples, incurred on the track from the use of any TDM plug-ins on that channel.

For information on delay indication when

using Delay Compensation, see “Delay

Compensation” on page 469

Appendix A: DSP-Induced Delays in Mixing (TDM Only) 651

To see the amount of processing delay on a track that uses plug-in inserts:

■

In the Mix window, Control-click (Windows) or Command-click (Macintosh) the track’s Volume indicator to toggle between Volume

(“vol”), Peak (“pk”) and Channel Delay (“dly”) indications.

Using Sends

When you send a track to another track and return it to a track (audio, Auxiliary Input, or Master Fader), the following delays are incurred:

Sends delays for Pro Tools TDM systems

HD Mixer

Stereo Mixer

Surround

Mixer

Sends Delay

Pre-fader send to bus

10 samples

Post-fader send to bus

8 samples

Pre-fader send to bus

8 samples

Post-fader send to bus

12 samples

Hardware sends also produce delays (see below).

Using Hardware I/O

When an audio interface is used on an insert or send, there is delay introduced by going through the D/A and A/D converter pair (for analog devices) or digital I/O connection (for digital devices) on the audio interface.

You can define a delay value for hardware in-

serts in the I/O Setup dialog. See “H/W Insert

Delay Compensation” on page 85.

The following tables list the delay incurred by each type of analog and digital output available in Pro Tools|HD-series systems.

Pro Tools|HD I/O characteristics

Interface and

I/O Port

Stereo Mixer

192 I/O or

192 Digital

I/O

AES/EBU

24 samples

105 samples 192 I/O

Analog

96 I/O

AES/EBU

96 I/O

Analog

21 samples

79 samples

Surround

Mixer

22 samples

103 samples

19 samples

77 samples

I/O characteristics for Pro Tools|24 MIX-series Legacy interfaces connected to Pro Tools|HD Legacy Port

Interface and

I/O Port

Stereo Mixer Surround

Mixer

18 samples 16 samples 888|24 I/O

AES/EBU

888|24 I/O

Analog

82 samples 80 samples

20 samples 18 samples 888|20 I/O

AES/EBU

888|20 I/O

Analog

ADAT Bridge

Optical

ADAT Bridge

S/PDIF

18 samples

18 samples

76 samples

16 samples

16 samples

74 samples

1622 I/O

Optical

1622 I/O

S/PDIF

18

76

16 samples

74 samples

On the 888|24 I/O, the Digital I/O delay incurred when using AES/EBU or S/PDIF is identical.

652 Pro Tools Reference Guide