Apple Compressor 2 - Manual (Page 106)

106 Chapter 7 Creating Dolby Digital Professional Output Files
Options for Spatial Mixing
This section offers additional insights and suggestions for using the channels in a Dolby
Digital Professional program.
Using the Center Channel
In a multichannel system, there are three ways to achieve a centrally placed sound image:
 Create a “phantom center” (mix sound to the left and right equally, as with stereo).
Commonly used, but assumes the listener is seated exactly between the speakers
(which is not possible in automobiles and not always the case in homes). The timbre
of sound is not the same as from a direct speaker because of cross-cancellation effects.
 Use the center channel alone. This creates a stable center image for listeners in any
location. (To prevent the audio from sounding too focused or narrow, its reverb can
be spread to the left and right channels.)
 Use all three front channels equally or in various proportions. This method allows for
greater control of the range of spatial depth and width. The phantom center can be
reinforced by additional signals in the center channel, which can be enhanced by
signal spread into the left/right pair. The disadvantage is that sound from all three
speakers may not blend well or may not arrive at the listener at the same time,
causing side effects such as comb filtering, shifts in tone color, or smearing. To
counteract these side effects, you can first process the additional signals to change
their spatial character, timbre, or prominence relative to the main center signal.
Using Surround Channels
Subtle surround effects can greatly enhance the listener’s sense of depth compared to
conventional stereo. Popular music often benefits from creative use of surround. But
don’t overdo it. The film industry guideline—don’t use surround effects to distract the
listeners from the story—also works well for music.
Limitations of the LFE Channel
The Low Frequency Effects (LFE) channel is a separate signal with a limited frequency
range, and is created by the mixing engineer and delivered alongside the main
channels in the mix. A “brick wall” filter at 120 Hz in the Dolby Digital Professional
encoder limits use of the LFE channel to the bottom two audible octaves. Dolby
recommends limiting the signal to 80 Hz when mixing your sound.
In most music productions (with such exceptions as the famous cannon shots in
Tchaikovsky’s “1812 Overture”), the LFE channel is not necessary. The LFE signal is also
discarded in the Dolby Digital Professional downmix process, so intense bass signals do
not stress small stereo systems. Be sure not to include vital information in the LFE
channel that would be missed in mono, stereo, or Pro Logic playback.