To give your listeners a “surrounding” audio experience, your sound must be created with
that goal in mind. For example, if you want the sound of an explosion coming from behind the listener, you need to put the sound in the rear channels during recording and mixing.

Dolby Digital Professional Surround Sound
Dolby Digital programs can deliver 5.1-channel surround sound with five discrete fullrange
channels (left, center, right, left surround, and right surround) plus a sixth
channel for low frequency effects (LFE). Another Dolby surround option is Dolby
Surround, with four channels (left, center, right, surround).

An Overview of the Surround Sound Workflow
The following steps outline the general workflow used to go from recording the sound
to encoding it as an AC-3 surround sound file.

Note: Compressor is not a sound editing or mixing tool. You will need sound editing
and mixing tools for the first three steps outlined below.

Step 1: Record the sound
As with any audio track, the first step is finding and recording the sound that you want.
Keep your end result in mind. If you want to position particular sounds in the surround
sound environment, you should record them separately.

Step 2: Mix the sound
The second step is mixing the audio. You need a separate audio file for each channel. If
you want to create Dolby Digital Professional 5.1 sound, you need a file for each of the
six source channels—left front, right front, center front, left rear, right rear, and Low
Frequency Effects (LFE). To create files for channels, assign your sounds to channels
using a recording/mixing system that supports six-track playback.

When mixing, you can enhance the video action by dynamically moving the
sounds within the surround field. The following are some common approaches to
multichannel sound:

• Use the surround channels for effects only. Create a stereo mix, then add
“sweetening” sound effects in the surround channels.

• Create special sounds for the rear and low-frequency channels (the rumble of an
earthquake, the pounding of drums, an airplane buzzing overhead, and so on) and
add them to a standard stereo mix. (To create sound for the LFE channel, you can
extract low frequencies from the rest of your sound using band-pass filters.)

• Position instruments, effects, and voices anywhere in the sound field.
 Use the center channel for voiceover, leaving it out of the left and right channels.
(This is called stereo plus center.)

Step 3: Digitize the sound
Once your sound is mixed, save the resulting channels as sound files in one of these
formats:
• AIFF
• SoundDesigner II
• QuickTime
• WAVE

Depending on the format, a file can contain a single channel (mono), dual channels
(stereo), or multiple channels. Compressor supports Dolby Digital Professional in all
these configurations.
Sound files intended for Dolby Digital Professional encoding must conform to the
following rules:
• All source files should be the same length. (If they are not, Compressor sets the
length of the AC-3 stream to match the length of the longest file.)
• All files must have a 48 kHz sample rate (as required for DVD).
• AC-3 streams must have a multiple of 1536 samples. If the selected input files do not,
Compressor adds digital silence to the end of the files.

Note: Compressor 2 can support any kind of source files that contain surround sound
and high resolution audio up to 64 bits per sample (floating point) and sample rates up
to192 kHz.

Step 4: Make channel assignments
Assign your source audio files to specific channels of the AC-3 stream.

Assigning Files to Surround Sound Channels
To manually assign source audio files to channels of a surround sound stream:
1. Do one of the following to import the source audio files:
• Click the “Import Surround Sound Group” button in the Batch window.
• Choose File > Import Surround Group.
The channel assignment interface opens.

2. Do one of the following to assign a source audio file to a particular channel.
• Drag the source audio file from the Finder to the icon for a specific channel (for example, “L”).
• Click the icon for a specific channel (for example, “L”) and use the Open dialog to locate the source audio file intended for that channel.

The file is now assigned to the “L” (Left Front) Channel.

Step 5: Submit the sound files to Compressor
Click Submit and Compressor does the rest, giving you an AC-3 audio stream.