Clock and Timing
This section describes the use of derived and real time clocks throughout the unit.
Redundancy Modes
This section gives information relating to the various redundancy functions available.
Alarm Lists
Provides information about the alarms that can be generated by the unit.
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:
Transient pre-noise processing - to reduce "pre-noise" artifacts before sharp transients.
Enhanced channel coupling – which maintains phase relationships between channels, and improves performance of matrix decoders.
Adaptive hybrid transform processing – an improved bit allocation and quantization algorithm
SMPTE 302M: Mapping of AES3 Data into an MPEG-2 Transport Stream
Though not specifically a coding standard, this does define a method of carrying AES3 uncompressed audio streams in an MPEG-2
© Ericsson AB 2011. All rights reserved
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