Apple Compressor (4.0) - Manual (Page 83)

Chapter 5 Custom settings and output formats 83
Spatial mixing options
Below are additional suggestions for using the channels in a Dolby Digital Professional program.
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): This method
is 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-cancelation 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 signals 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.
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.
LFE channel limitations
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 exceptions such as the famous cannon shots in Tchaikovsky’s
1812 Overture), the LFE channel isn’t necessary. The LFE signal is also discarded in the Dolby
Digital Professional downmix process, so that intense bass signals don’t 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.
Because LFE is separate from other channels, its ability to blend with higher frequencies can be
affected by filters used to generate the LFE signal. To ensure a cohesive audio signal, keep the
entire signal together in the main channel or channels.
Avoid creating an LFE channel for material originally produced without one. Dolby Digital
Professional’s five main channels are all full range, and the LFE channel doesn’t increase the
frequency response. Dolby Digital Professional decoders offer bass management, directing
low frequencies to a subwoofer or other suitable speakers. An LFE track may interfere with
bass management.