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A Dolby Digital Primer (part 4)

Dolby Digital encodes 5.1 discrete channels which, for the home cinema enthusiast, is an ideal format. Most of us however live in a world where fewer channels are the norm, therefore some form of 'downmixing’ is necessary. There are a number of possibilities...

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Downmixing

Dolby Digital encodes 5.1 discrete channels which, for the home cinema enthusiast, is an ideal format. Most of us however live in a world where fewer channels are the norm, therefore some form of 'downmixing’ is necessary. There are a number of possibilities:

  • Full: all 5.1 channels
  • 3 Stereo: the surround channel information is distributed to the front speakers
  • Phantom: the centre channel is sent to both left and right speakers
  • Stereo: left and left surround signals to left speaker, right and right surround signals to right speaker, centre signal to left and right speakers
  • Mono: all channels except LFE to centre speaker

In addition, when downmixing to two channels, the result can be Dolby Surround compatible where a Dolby Pro Logic decoder will interpret the signals as matrixed LCRS, or straightforward stereo. All of these are handled in the decoding process, but the person doing the encoding has to be aware of the possible side effects downmixing entails. It’s analogous to checking a stereo mix in mono to make sure it still sounds OK, but here it’s a little more complicated. The encoding engineer has to bear in mind several points:

  • None of the downmix formats include the LFE (Low Frequency Effects) channel. Therefore if the LFE channel contains essential information, that information should be distributed among the front channels.
  • A mono surround channel is obtained by summing the stereo surround channels, attenuating them by 3dB and bandpass filtering between 100Hz and 7kHz. If the stereo surround channels contain identical signals, which they easily could, then the mono surround channel will come out louder than it ought to be. Also, if the stereo surrounds have significant content above 7kHz, this will be lost. Compensation may be necessary.
  • Incoherent signals in the left and right channels may be interpreted as mono surround information and will be directed to the mono surround channel. The left and right channels may have to be panned in a little to prevent this happening.
  • Identical signals in the left and right channels will be interpreted as centre channel information in a surround downmix. To avoid this, a slight delay can be applied to either channe
  • All of this implies that 5.1 mixes for domestic consumption must be checked very carefully in all of the downmix modes or some users will not be happy. Fortunately, the DP562 decoder provides for this and a potentially tedious process is at least facilitated.
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By David Mellor Thursday January 1, 2004
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