Ericsson SPR1100, Руководство пользователя

Страница: 199 / 256

Home > SPR1100 Handbook > Appendices > A Brief Introduction to Audio Coding Standards
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.
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.
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 subbands
and these subbands 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.
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 subbands, which correspond to the critical bands of the human ear. The
number of bits is fixed for each subband but there are additional bits that can be allocated
to any subband where encoding quality has suffered. Dolby recommends stereo signals may
be coded at 192 kbit/s, and 5.1 at 448 kbit/s, 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.
Dolby Digital Plus
Dolby Digital Plus offers enhanced performance over Dolby Digital. Some of the algorithm
improvements are: