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-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.

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.

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 does not 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.

Accommodating Stereo Playback

Even with the popularity of 5.1 systems, you should always address stereo reproduction. There are three basic ways to do this:

  • Prepare a new stereo mix from the original multitrack elements (using conventional stereo-mixing sessions).

  • Prepare a studio-adjusted downmix from the multichannel mix. This method takes advantage of the work that has gone into mixing the 5.1 version. It retains flexibility in the exact proportions of each channel represented in the final stereo mix.

  • Let the decoder derive a stereo downmix, based on preset formulas in the decoder. Downmix options and dynamic range control effects can be previewed and adjusted in the production studio, and a range of adjustments is possible.

Always check the mix on an inexpensive surround system to evaluate how well it sounds on modest playback systems.

Note: For more information about Dolby Digital Professional, see “Frequently Asked Questions about Dolby Digital Professional,” available at the Dolby Laboratories Inc. website at http://www.dolby.com.