Data Compression 58919.6.2 Dolby E
Dolby E is a digital audio compression technology designed for use by TV broadcast
and production professionals which allows an AES/EBU audio pair to carry up to eight
channels of digital audio. Because the coded audio frame is arranged to be synchronous
with the video frame, encoded audio can be edited without mutes or clicks. Dolby E can
be recorded on a studio-level digital VTR and switched or edited just like any other AES
digital audio signal, as long as some basic precautions are observed. Data must not
be altered by any part of the system it passes through. That’s to say, the gain must not be
changed, data must not be truncated or dithered, and neither must the sample rate be
converted. Dolby E technology is designed to work with most popular international video
standards. In its fi rst implementation, Dolby E supported 29.97 fps, 20-bit word size, and
48-kHz audio. Newer versions will support 25 fps, 24 fps, and 16-bit and 24-bit audio.
19.6.3 DTS
DTS, the rival to Dolby Digital in the cinema, uses an entirely different approach to
AC-3-coded data on fi lm stock. In DTS, the digital sound (up to 10 channels and a
subwoofer channel) are recorded on CDs, which are synchronized to the fi lm by means
of a time code. Because of the higher data rate available (CD against optical fi lm track),
DTS uses a relatively low 4:1 compression ratio.
19.6.4 MPEG AAC
MPEG-2 advanced audio coding (AAC) was fi nalized as a standard in 1997 (ISO/IEC
13818–7). AAC constitutes the coding algorithms of the new MPEG-4 standard.
19.7 MPEG-4 ................................................................................................................
MPEG-4 will defi ne a method of describing objects (both visual and audible) and how
they are “ composited ” and interact together to form “ scenes. ” The scene description
part of the MPEG-4 standard describes a format for transmitting the spatiotemporal
positioning information that describes how individual audiovisual objects are
composed within a scene. A “ real world ” audio object is defi ned as an audible semantic
entity recorded with one microphone—in case of a mono recording—or with more
microphones, at different positions, in case of a multichannel recording. Audio objects
can be grouped or mixed together, but objects cannot be split easily into subobjects.