Beth Cataldo, March 27, 2007

AC-3 Audio:
Compressor encodes your audio track into AC-3 audio files, a perceptual audio coding system that analyzes an audio signal and throws away the parts we can’t hear. It is a lossy compression scheme that compresses PCM audio up to a 12:1 ratio.

AC-3 is the audio standard for DVD-Video. All DVD players are required to have an AC-3 decoder. DVD players also support PCM and a few other audio formats. 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), sometimes known as “the subwoofer.”

Another surround option is Dolby Surround, with four channels (left, center, right, surround). Not all AC-3 audio is 5.1 surround sound. Dolby Digital Professional is commonly used to encode stereo files to greatly reduce their file size.

5.1 Surround Sound:
5.1 surround sound uses six discreet channels to feed speakers arranged in a matrix around a central point. A 5.1 surround field has three full-spectrum speakers in the front, two in the back and one subwoofer for reproducing low-frequency effects. It’s important to understand that Compressor cannot take stereo audio files and create 5.1 surround sound from them. If you want to create a 5.1 surround sound AC-3 audio file, you must first create the six channels of audio it requires. This must be done using other applications outside Compressor.

About the Dolby Digital Professional Encoder Pane
This section contains detailed information about the various tabs within the Dolby
Digital Professional (AC-3) Encoder pane of the Inspector window.
You make your Dolby Digital Professional settings using the tabs described below, by
either modifying an existing setting or creating a new setting in the Settings tab of the
Presets window. The Dolby Digital Professional pane opens with the default Audio tab
on top and contains the following items:
• File Extension: This field displays the Dolby Digital Professional file extension (ac3)
automatically after the Dolby Digital Professional output format is selected from the
File Format pop-up menu or the ( + ) pop-up menu in the Settings tab of the
Presets window.
• Allow Job segmenting: This checkbox allows you to turn off job segmenting. It is
relevant only if you are using Compressor with distributed processing and with two
pass or multi pass encoding. (The Compressor 2 distributed processing feature is
limited to computers that have either Final Cut Studio or DVD Studio Pro 4 installed.)
• Audio, Bitstream, and Preprocessing: These buttons open the Audio, Bitstream, and
Preprocessing tabs described next.

Audio Tab Settings
All of the most important Dolby Digital Professional settings are accessible in the
Audio Tab.
• Target System: Compressor limits available settings to those appropriate for the
target system. If you’re encoding for use with DVD Studio Pro, choose DVD Video.
• Audio Coding Mode: Specifies the audio channels of the encoded stream.

For example, “3/2 (L, C, R, Ls, Rs)” means three front channels (left, center, right), and
two rear channels (surround). “2/0 (L, R)” is essentially a standard stereo file. The audio
coding mode affects settings such as the available bandwidth and surround channel
preprocessing. If you choose Automatic, Compressor makes its best guess at the
intended Audio Coding Mode, based on available source audio files.

Note: In the Audio Coding Mode pop-up menu, “S” stands for a single rear “Surround”
channel, and in this illustration, “LFE” stands for Low Frequency Effects (also known as
“subwoofer”). In subsequent references, the Low Frequency Effects
channel is referred to as “the LFE channel” and its channel identifier code is “.S” (for
“subwoofer”). The single rear “Surround” channel is referred to as “Center surround”
(and its channel identifier code is “Cs”).

• Sample Rate: Specifies the sample rate. All files intended for DVD authoring must
have a 48 kHz sample rate as per the DVD specification.
• Data Rate: The choices depend on the coding mode and target system. The higher
the rate, the better the quality. AC-3 streams have a constant data rate. At 448 kbps,
which is the default for 5.1 encoding, one minute of AC-3 audio takes about 3.3 MB of
storage space. For stereo encoding, rates of 192 kbps and 224 kbps are typical and
will produce good results.
• Bit Stream Mode: Defines the purpose of the encoded audio material. The information
is included in the finished stream and can be read by some decoding systems.
Dialog Normalization: Specifies the average volume of the dialog in your sound files
relative to full modulation. The playback device uses this information to maintain
similar volume among different AC-3 streams. A setting of –31 dBFS ensures the
sound levels will not be altered at all.
• Enable Low Frequency Effects: Select this checkbox to include the LFE channel in the
encoded stream. (Not available for mono or stereo.)

Bitstream Tab Settings
These settings are stored in the finished stream for use by the playback device. Leave them at their default values unless you have a specific technical reason for changing them.
• Center Downmix, Surround Downmix: If your encoded audio has these channels, but the
player does not, the channels are mixed into the stereo output at the specified level.
• Dolby Surround Mode: When encoding in 2/0 (stereo) mode, specifies whether the
signal uses Dolby Surround (Pro Logic).
• Copyright Exists: Select this checkbox to specify that a copyright exists for this audio.
• Content is Original: Select this checkbox to specify that this audio is from the original
source and not a copy.
• Audio Production Information: Select this checkbox and fill in the fields below to
specify how the encoded audio content was mixed. Playback devices may use this
information to adjust output settings.
• Peak Mixing Level: Specifies peak sound pressure level (SPL) (between 80 dB and
111 dB) in the production environment when this mix was mastered.
• Room Type: Specifies information about the mixing studio.

Preprocessing Tab Settings
Preprocessing options are applied to the audio data before encoding. With the exception of the Compression Preset setting, leave these settings at their default values
unless you have a specific technical reason for changing them.
• Compression Preset: Specifies one of the dynamic range processing modes built in to the AC-3 format. The default of Film Standard Compression should only be used when you are encoding an original mix intended for cinema. In almost all cases, you
should choose None.

General
• RF Overmodulation Protection: Determines whether to use an RF preemphasis filter in the clip protection algorithm to prevent RF overmodulation in set-top-box decoders.
• Apply Digital Deemphasis: Specifies whether input audio data is preemphasized and needs to be deemphasized before encoding.

LFE Channel
• Apply Low-Pass Filter: Select this checkbox to apply a 120 Hz low-pass filter to the
Low Frequency Effects (LFE) channel. Turn off this filter if the digital signal fed to the
LFE’s input does not contain information above 120 Hz.

Full Bandwidth Channels
• Apply Low-Pass Filter: Select this checkbox to turn on a low-pass filter with a cutoff
near the available audio bandwidth that is applied to the main input channels. If the
digital signal fed to the main input channels does not contain information above the
available audio bandwidth, you can turn off this filter. Compressor will automatically
determine the available bandwidth.
• Apply DC Filter: Select this checkbox to turn on a DC high-pass filter for all input
channels as a simple way to remove DC offsets. Most mixed audio material is already
free of DC offsets.

Surround Channels
• Apply 90° Phase-Shift: Select this checkbox to generate multichannel AC-3 streams
that can be downmixed in an external 2-channel decoder to create true Dolby
Surround–compatible output.
•Apply 3 dB Attenuation: Select this checkbox to apply a 3 dB cut to the surround
channels of a multichannel film soundtrack being transferred to a consumer home
theater format. Cinema surround channels are mixed 3 dB “hot” (higher) relative to
the front channels to account for cinema amplifier gains.

Source: Apple's Compressor 2 User Manual

< previous | next>