Specifications

5.1-Channel Production Guidelines Production Techniques
5-11
In addition, many consumers do not yet have a full 5.1-channel Dolby Digital
playback system, and instead connect their DTV set-top boxes' and DVD players'
two-channel outputs to a stereo or Dolby Surround Pro Logic system. Under these
conditions, the multichannel soundtracks will be downmixed to mono, stereo, or
Dolby Surround matrix-encoded stereo. If the multichannel soundtrack was derived
by phase decoding and upmixing a two-track master, there may be downmix quality
problems due to signal addition and cancellation as the various channels are
combined in the downmixing process. Decoding the downmixed 5.1 content with
Dolby Surround Pro Logic will make these problems even more obvious. In nearly
every case, the original stereo or Dolby Surround mix will give the best results. If
upmixing is absolutely necessary, it is best to use a decoding system without matrix
steering or even a simple Hafler matrix. Any steering logic, including that used by
Dolby Pro Logic, adds some degree of image instability that is unwelcome,
particularly for pure music applications. When using a decoder to upmix, it is
important to turn off any surround channel time delays as they can introduce
problems when the sound track is downmixed.
Sometimes additional processing is used on top of the surround decoding in the
studio, perhaps to generate an LFE channel or stereo surrounds. When this is done,
the sound may fail to meet the listener’s expectations for true 5.1-channel material;
upmixing in this way can never create a truly discrete mix and the effects of the
upmixing system will always be audible.