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Audio Coding
EN/LZT 790 0023/1 R16A
C-3
C.1 A Brief Introduction to Audio Coding Standards
Where appropriate, the output Transport Stream can be made compliant with ATSC
A53(E) ATSC Digital Television Standard and DVB 101-154 v1.7.7.
C.1.1 MPEG
The Moving Pictures Experts Group (MPEG) was formed in 1988 to generate
compression techniques for audio and video. In the first version, ISO/IEC 11172-3
MPEG-1 audio, has a selection of two separate algorithms. MPEG-1 Layer I and II
were implementations of the MUSICAM algorithm and MPEG-1 Layer III (mp3) was
an implementation of the ASPEC algorithm. The algorithms have since been
improved and extended with other versions of MPEG.
C.1.2 MPEG-1 Layer I/II
This algorithm is similar to MUSICAM and only really differs in the structure of the
frame headers. Layer I is a restricted version of the full algorithm to allow a reduced
decoder to be developed. Hence, over time as the processing power of decoders
have increased by orders of magnitude, Layer I is no longer used for broadcast.
The algorithm creates 3 frames of 384 samples. Each small frame is divided into
sub-bands and these sub-bands can be coded for each frame or for all 3. There is
limited ability to allocate bits to different bands and there is no entropy coding of the
encoded samples so a relatively high bit rate is required to obtain a reasonable
quality.
C.1.3 Dolby Digital
Dolby Digital is an algorithm from Dolby that forms part of both the ATSC and DVB
standard for digital broadcasting. It is marketed under the name of Dolby Digital.
The encoder includes a psychoacoustic model to improve the quality. The signal is
divided into 32 multiple sub-bands, which correspond to the critical bands of the
human ear. The number of bits is fixed for each sub-band but there are additional
bits that can be allocated to any sub-band where encoding quality has suffered.
Dolby recommends stereo signals may be coded at 192 kbps, and 5.1 at 448 kbps
but other rates can be used if required.
The encoders have the ability to encode stereo and equivalent modes, and will also
pass through pre-compressed Dolby Digital (both stereo and multi-channel).
When in Dolby Digital Pass-through mode, glitch suppression is supported, where
the coding module monitors the encoded bitstream and if the framing structure is
incorrect, a valid silence frame or the last good frame is inserted in its place. If this
state occurs for more than a second, the encoder signals that the Dolby Digital
bitstream is corrupted.