Specifications

Standards and Practices for Authoring Dolby
®
Digital and Dolby E Bitstreams
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The LFE channel in contrast, is specifically produced with low-frequency information
exclusive to this channel. Content producers have specifically created this LFE
channel to include extra sound-effects information for emphasis in effects, such as
explosions, crashes, gunshots, and such.
In a consumer playback environment, often low-frequency information from the main
channels is redirected to the subwoofer speaker and added to the existing LFE
channel, if any. If there is no LFE channel present in the Dolby Digital stream, the
subwoofer speaker contains the low frequencies redirected from the main speakers.
The capability to redirect low-frequency information is called Bass Management, and
is present in all Dolby Digital decoders. The LFE channel can also be redirected to
the main speaker channels through the use of Bass Management, for those playback
environments lacking a subwoofer speaker.
When producing a music-only multichannel program, the entire musical contents of
the program should be placed in the main channels, just as they are in stereo music
recordings. The LFE channel should only be used when the bass levels are so high as
to require a substantial decrease in overall program volume to accommodate them,
such as might occur with the cannon shots in Tchaikovsky’s “1812 Overture.”
Just as with any full-bandwidth audio signal delivered to consumers via DVD, CD, or
DTV, it is the job of the playback system to get the most out of the signal. Consumer
Dolby Digital systems that use a combination of smaller speakers and subwoofers
incorporate bass management to ensure that the speakers most able to reproduce the
bass are used to the best advantage. There is no need to tailor the frequency content of
the program in anticipation of the many different playback systems it may encounter
in the home.
Remember: just because the “point-one” channel is there does not mean it must be
used. It is perfectly acceptable to create a five-channel Dolby Digital stream
(encoding in 3/2 mode, as opposed to 3/2L mode) without an LFE channel present.
Why do I need the Dolby Recorder?
The Dolby Recorder program is a Microsoft Windows-based application that allows a
Dolby Digital data stream to be recorded onto a computer hard drive. It creates a file
with the .ac3 file extension that can then be ported over to a DVD authoring system
and married to the encoded video content.
The DP569 Encoder outputs a Dolby Digital data stream carried within the envelope of
an AES/EBU digital audio pair, although the actual area taken up by the Dolby Digital
data is only a fraction of the AES/EBU space. The rest of the unused area is filled out
with zeros. The AES/EBU envelope allows the Dolby Digital data to be routed and
stored using much of the same equipment that passes AES/EBU digital audio.
However, for a DVD authoring system to use this data stream, the excess zeros must be
stripped off. In simplest terms, this is what the Dolby Recorder application does.