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 589.

• To learn how to use Delay Compensation on supported systems for automatic delay com-

pensation, see “Delay Compensation” on page 590.

• To learn about manual delay compensation,

see “Manually Compensating for Delays” on page 591.

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

lay, see “Delay Factors” on page 592.

Introduction to DSP-Induced

Delay

In all digital systems, signal processing incurs 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 592.)

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, which causes cancellation of certain frequencies.

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

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In simple terms, DSP-induced delay can cause channels 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 needn’t worry about delay compensation.

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.

All Systems Whenever Automatic Delay Compensation is not available, you can do one or more of the following:

• Apply sample-level delay to tracks with the

TimeAdjuster plug-in (see “Using the Time-

Adjuster Plug-In” on page 591).

• Manually nudge tracks earlier or later in

time as needed “Nudging Audio Tracks” on page 592.

• Use the same plug-ins on all tracks.

While learning about delay compensation, 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 Automatic Delay Compensation and the TimeAdjuster (TDM) plug-in. In addition, the variety of tools available for delay compensation help ensure compatibility if the session is to transferred to another system.

Overview of Delay Compensation

Options

Pro Tools provides tools to compensate for signal processing delays including Automatic Delay Compensation (Pro Tools|HD only), as well as several forms of manual delay compensation.

To compensate for offsets incurred by processing delays, do any of the following (as available on your system):

Pro Tools|HD (6.4 and Higher) Automatic Delay

Compensation lets Pro Tools automatically calculate and compensate for processing delay. See

“Delay Compensation” on page 590.

Delay Compensation

Delay Compensation is used to manage DSP delays that occur on audio tracks, Auxiliary Inputs, or Master Faders because of plug-in use and mixer routing. 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.

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.

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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 591).

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.

• When Delay Compensation is enabled, all other plug-ins (except TimeAdjuster) report their delay to the Delay Manager, and this delay is what is displayed in the Delay Indicator view of each track. The Pro Tools 6.4 (and higher) version of TimeAdjuster, however, does not report its Delay setting to the Delay

Manager, preventing a type of “delay compensation” loop from occurring.

• When Delay Compensation is turned off or unavailable, TimeAdjuster operates as expected and applies its Delay to the track for manual delay compensation.

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 compensate for offsets incurred by processing delays, there are different methods you can use:

• Apply sample-level delay to tracks with the

TimeAdjuster plug-in

• Physically adjust tracks in the Edit window by the appropriate number of samples (for example, by using the Nudge function).

• Use the same plug-ins on all tracks.

Using the TimeAdjuster 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.

Digidesign TDM plug-ins display their delay values in the track Channel Delay Indicators in the

Mix window. TimeAdjuster can be used to match these delay values for tracks that need to remain in phase (such as instruments recorded with multiple microphones or stereo pairs).

See also “TimeAdjuster and Delay Compensation” on page 591.

To use TimeAdjuster to compensate for a time delay:

1 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. Delay values are shown in samples.

2 Apply the TimeAdjuster plug-in to a track whose delay you want to increase (to match the the first track).

Appendix A: DSP-Induced Delays in Mixing (TDM Only) 591

3 Control-click (Windows) or Command-click

(Macintosh) the track’s Volume Indicator until the Channel Delay value is displayed for that track.

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

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

If you are using a plug-in whose delay factor you are not familiar with, you can set the delay by ear using one of two methods:

• 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.

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 doesn’t 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:

• Real-time processing (plug-in inserts). See

“Using Plug-In Inserts” on page 593.

• Mixing and routing (sends and hardware

inserts). See “Using Sends and Hardware Inserts” on page 593.

• Bouncing tracks. See “Bouncing Tracks” on page 594.

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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 in the Mix window displays the total delay, in samples, incurred on the track from the use of any

TDM plug-in on that channel.

For information on delay indication when using Delay Compensation (automatic), see

“Delay Compensation” on page 428

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 and Hardware Inserts

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|HD systems

HD Mixer Sends Delay

Stereo Mixer Pre-fader send to bus

10 samples

Surround

Mixer

Post-fader send to bus

8 samples

Pre-fader send to bus

8 samples

Post-fader send to bus

12 samples

Sends delays for Pro Tools|24 MIX-series and

Pro Tools|24 systems

System Sends Delay

Pro Tools MIXseries or

Pro Tools|24

Pre-fader send to bus

3 samples

Post-fader send to bus

6 samples

Using hardware sends also results in delays, as described 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.

On systems that support Delay Compensation

(automatic) you can define a delay value for hardware inserts in the I/O Setup dialog. See

“H/W Insert Delay Compensation” on page 84.

Appendix A: DSP-Induced Delays in Mixing (TDM Only) 593

The tables below list the delay incurred by each type of analog and digital output available in

Pro Tools|HD and Pro Tools|24 MIX systems.

HD-series I/O characteristics

Interface and

I/O Port

Surround Mixer Stereo Mixer

192 I/O or

192 Digital

I/O

AES/EBU

24 samples 22 samples

105 samples 103 samples 192 I/O

Analog

96 I/O

AES/EBU

96 I/O

Analog

21 samples

79 samples

19 samples

77 samples

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

I/O characteristics for MIX-series audio interfaces connected to MIX-series systems

Interface A/D/A delay Digital I/O

888|24 I/O

882|20 I/O

1622 I/O

81 samples

75 samples

75 samples

17 samples

17 samples

17 samples

I/O characteristics for MIX-series audio interfaces connected to HD Legacy Port

Interface and

I/O Port

Surround Mixer Stereo 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

Bouncing Tracks

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

Pro Tools System Delay

10 samples Pro Tools|HD-series

(Stereo Mixer)

Pro Tools|HD-series

(Surround Mixer)

Pro Tools|24 MIX-series

8 samples

10 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 482.

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