Chapter 1

104 Everything You Ever Wanted to Know About DVD
acoustics in theaters, home equipment, and digital mastering processes.
The LucasFilm THX Digital Mastering program uses a patented process to
track video quality through the multiple video generations needed to make
a final format disc or tape, set up video monitors to ensure the filmmaker is
seeing a precise rendition of what is on tape before approving the master,
and other steps along the way.
THX-certified 4.0 amplifiers enhance Dolby Pro Logic with the following:
a crossover that sends bass from front channels to the subwoofer; reequalization
on front channels (to compensate for the high-frequency boost
in the theater mix designed for speakers behind the screen); timbre matching
on the rear channels; decorrelation of rear channels; a bass curve that
emphasizes low frequencies. THX-certified 5.1 amplifiers enhance Dolby
Digital and improve on 4.0 with the following: rear speakers are full range,
so the crossover sends bass from both front and rear to the subwoofer;
decorrelation is turned on automatically when rear channels have the same
audio, but not during split-surround effects, which don’t need to be decorrelated.
More info can be found at the Home THX Program Overview
(www.thx.com/consumer_products/home_overview.html).
Discs containing 525/60 (NTSC) video must use PCM or Dolby Digital on
at least one track. Discs containing 625/50 (PAL/SECAM) video must use
PCM or MPEG audio or Dolby Digital on at least one track. Additional tracks
may be in any format. A few first-generation players, such as those made
by Matsushita, can’t output MPEG-2 audio to external decoders.
The original spec required either MPEG audio or PCM on 625/50 discs.
A brief scuffle was led by Philips when early discs came out with only twochannel
MPEG and multichannel Dolby Digital, but the DVD Forum clarified
in May of 1997 that only stereo MPEG audio was mandatory for 625/50
discs. In December of 1997, the lack of MPEG-2 encoders (and decoders)
was a big enough problem that the spec was revised to allow Dolby Digital
audio tracks to be used on 625/50 discs without MPEG audio tracks.
Because of the 4 percent speedup from 24 fps film to 25 fps PAL display,
the audio must be adjusted to match before it is encoded. Unless the audio
is digitally processed to shift the pitch back to normal, it will be slightly high
(about half a semitone).
For stereo output (analog or digital), all players have a built-in two-channel
Dolby Digital decoder that downmixes from 5.1 channels (if present on
the disc) to Dolby Surround stereo. That is, five channels are phase
matrixed into two channels to be decoded to four channels by a Dolby Pro
Logic processor or five channels by a Pro Logic II processor. PAL players
also have an MPEG or MPEG-2 audio decoder. Both Dolby Digital and
MPEG-2 support two-channel Dolby Surround as the source in cases
where the disc producer can’t or doesn’t want to remix the original onto discrete
channels. This means that a DVD labeled as having Dolby Digital