Apple Compressor (4.0) User Manual
Page 73
Chapter 5
Custom settings and output formats
73
For more information about creating surround sound audio files using other output formats, see
Assign files to surround sound channels
on page 74.
Note: 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.
Following are some things to keep in mind as you configure and encode Dolby Digital
Professional AC-3 files.
Source media files
There are a few guidelines to be aware of when encoding Dolby Digital Professional output files
from your source media files.
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 duration. (If they aren’t, Compressor sets the duration of
the AC-3 stream to match the duration 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 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 to 192 kHz.
Previewing the encoder settings
The settings made in the Dolby Digital Professional Encoder pane cannot be previewed. If you
have an external surround sound device connected to the USB, FireWire, or other output from
the computer, you will be able to hear the audio assigned to the different channels, but it won’t
include any settings, such as the Dialog Normalization or Compression Preset settings.
Important:
Because the audio output is not Dolby Digital, the optical output cannot be used to
verify the audio assigned to each channel.
The only way to verify the settings is to submit the batch and listen to the results. If you’re unsure
of the settings, use the Preview window to encode a short portion of the source as a test. For
more information, see
on page 57.
You can import an AC-3 file as a source media file into a Compressor batch. Compressor includes
a built-in Dolby Digital decoder that it uses to decode the AC-3 files. This decoder properly
applies any settings you made in the Dolby Digital Professional Encoder pane, allowing you to
hear their effects without having an external Dolby Digital decoder. You can also transcode the
AC-3 files to a different format.
Note: Surround sound AC-3 files are downmixed to stereo if you don’t have an external surround
sound system connected to the computer.