1598 Chapter 45
That film sound has been the starting point for 5.1
digital surround in the home has enabled the accumula-
tion of invaluable experience in mixing, recording, and
distributing multichannel digital audio. What’s more,
the widespread adoption of the Dolby Digital 5.1 format
for consumer applications has resulted in the most direct
link from program producer to home listener ever,
giving the former unprecedented control over what the
latter actually hears.
This is because the Dolby Digital bitstream carries
not only the soundtrack as originally mixed, but also
metadata, or data about the data, that can be used to
control the home listener’s Dolby Digital decoder. For
example, while the same unrestricted multichannel
audio content is delivered to every system, the consumer
decoder can be instructed by metadata precisely how to
downmix a 5.1-channel soundtrack for stereo or Pro
Logic surround playback, or even mono playback. Meta-
data also allows the original mixers to pass onto the
home decoder instructions that will, when the listener
wishes, create a compressed version of the soundtrack
on the fly for late-night viewing, when unrestricted
dynamic range could bother family or neighbors.45.7 More Home Theater Channels from Existing
Content
The success of the 5.1 format, followed by the introduc-
tion of EX in 1999, led to the consumer electronics
industry offering home theater listeners more channels
by means of advanced matrix decoders and additional
amplifiers in equipment such as audio/video receivers
(AVRs). The first step was to offer decoding of the extra
surround channel on DVDs of EX films for 6.1 play-
back, with the extra surround channel reproduced by a
third surround speaker placed behind the listening area.
A further variation soon followed, using two back sur-
round speakers in a mono configuration reproducing the
EX channel.
This 7.1 format, Fig. 45-8, as it is now known by the
CE industry, has taken on greater popularity with the
introduction of matrix technologies such as Dolby Pro
Logic IIx and Harman-Kardon’s Logic 7. These derive
stereo back surround channels from regular 5.1, and
even stereo, program sources, and today nearly all home
theater AVRs in the $300 and up range are equipped for
7.1 playback.*
Deriving surround from stereo content became a
practical and successful proposition with the introduc-
tion in 2000 of Dolby Pro Logic II, a 5.1 matrix-based
decoding technology. Using a concept originally devel-
oped by audio pioneer Jim Fosgate, the decoder in
essence seeks out surround cues occurring naturally in
stereo content, such as ambience in music recordings.
Pro Logic II can also be used to encode specific Left
and right surround information up front onto stereo
soundtracks, to achieve specific directional surround
effects on playback with Pro Logic II decoding. This
approach to delivering encoded surround content via
stereo formats is replacing the older Dolby Surround
technology for applications such as stereo broadcasting,
and is used by some video game and console manufac-
turers as a practical and effective alternative to higher
performance, but processing-power-hungry, digital 5.1
interactive audio.Figure 45-7. The speaker setup for digital 5.1-channel
surround sound in the home is about the same as that for
earlier four-channel surround. In a 5.1 setup, however, the
left and right surround speakers are fed separate left and
right surround information, where in a four-channel system
they reproduce the same mono-surround information.
* For home listeners not ready to commit to seven
speakers in their living rooms, some 7.1 AVRs
make it possible to use the extra two amplifier
channels for stereo playback in another room,
with the main system set up for standard 5.1 play-
back.