Technical information

Dispersion The degree to which loudspeakers spread their sound production in all
directions, rather than directly to their front. All loudspeakers widely disperse
their bass. But as the wavelength of a sound nears the size of a driver's cone
diameter, the sound tends to become more directional.
Distortion An inaccuracy in the reproduction of a signal. In the case of audio, it is
normally regarded as being composed of harmonic distortion and intermodulation
distortion. But used more broadly, it can also encompass frequency response
variations and noise. In the case of lossy compression technologies, some
distortion consists of spurious noise (not harmonically related) surrounding the
signal. When 'distortion' is quoted as a specification without qualification, it
normally refers only to harmonic distortion.
Dither Very low level noise, usually 'white’ in character, added to a digital audio
signal to reduce harmonic distortion. It typically is just a random variation in the
least significant bit of the digital signal. In some systems, such as Sony's SBM,
the noise is shaped to yield a lower noise floor in the more easily audible midrange
and low treble, by pushing much of the noise into the near-ultrasonic.
Dolby Digital Dolby Digital is an encoding scheme invented by Dolby Laboratories
as a way of compressing digital audio so that it uses a lot less data space. It is
also known as AC-3. Originally developed as an audio compression system for US
digital television, it achieved prominence by allowing multiple channel sound tracks
to fit onto standard 35mm cinema film prints (in between the sprocket holes on
the film!), it has become the de facto standard for DVD. The compression system
uses perceptual encoding, similar to DTS, MPEG audio (including MP3) and Sony's
SDDS and ATRAC. It can carry up to 5.1 channels of sound, but does not
necessarily have that many. Dolby Digital 2.0 (that is, stereo) can be encoded
with Dolby Pro Logic surround sound. The Dolby Digital bitstream can also carry
codes (metadata) to control playback parameters in the Dolby Digital decoder.
Dolby Digital apparently supports bit rates of up to 640kb/s, but on 5.1 (or
higher) channel DVDs the bit rates actually used at 384kb/s and 448kb/s.
(Note, the 'k' here stands for 1,000, not 1,024). Dolby Digital bitstreams also
include metadata for controlling the operation of the decoder.
Dolby Digital EX 6.1 A new surround sound standard which provides the usual
5.1 channels plus an additional channel: the centre rear channel. Unlike DTS ES
6.1 Discrete the additional channel is not carried discretely but is encoded into
the two rear channels in a similar way to the front centre channel is encoded into
a Dolby Pro Logic sound track. Movies prepared for Dolby EX 6.1 presentation in
cinemas should have the same encoding on DVD. However EX 6.1 is compatible
with 5.1 channel systems in the same way that Dolby Pro Logic is compatible with
stereo systems. More correctly, this should be termed LucasFilm THX 6.1 since