Apple Compressor (4.0) - Manual (Page 80)

Chapter 5 Custom settings and output formats 80
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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.
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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.
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Dialog Normalization: Specifies the average volume of the dialogue, using decibels of full
scale (dBFS), in your sound files relative to full modulation. The playback device uses this
information to maintain similar volume among different AC-3 streams. The goal is to make all
AC-3 encoded audio files have the same listening level, regardless of the source file.
It’s especially important to properly use the Dialog Normalization feature if you have different
audio files (with different volume levels) going on to a DVD. If you know the average level for
each file, enter that number in the Dialog Normalization field for each file. For example, if the
dialogue in your Final Cut Pro project averages around –12 dB on the audio meter, enter “–12”
in the Dialog Normalization field.
Note: This normalization is between the audio streams on the DVD, not within an individual
audio stream itself. For that type of normalization, use an audio editing tool. You can enter
values from –1 dBFS to –31 dBFS. The difference between the value you enter and 31 dBFS
(which represents the normal dialogue listening level) is the amount that the source audio will
be attenuated.
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If you enter –31 dBFS: The attenuation is 0 dB (31 dBFS–31 dBFS), and the source audio levels
are not affected at all.
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If you enter –27 dBFS: The attenuation is 4 dB (31 dBFS–27 dBFS).
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If you enter –12 dBFS: The attenuation is 19 dB (31 dBFS–12 dBFS).
The louder the source file, the smaller the value you enter, and the more the audio in the
encoded file is attenuated during playback.
If you don’t know the sound level of the source file or for some other reason want to ensure
the Dolby Digital Professional encoder does not affect your sound levels, enter “–31” for the
Dialog Normalization and choose None from the Compression Preset pop-up menu (located in
the Preprocessing tab).
Important: It’s critical that you set Dialog Normalization correctly if you intend to use any of
the Compression Preset settings. The Compression Preset settings assume that the audio, after
the dialogue has been normalized, is at the normal listening level of 31 dBFS. Levels that are
consistently louder than that result in distorted sound and erratic levels.