Equipment Description
Page 4-6
Instruction Manual: evolution 5000 E5500 Mobile Contribution Encoder
ST.TM.E10033.3
4.5.2
Dolby Digital (AC-3)
Dolby Digital (AC-3) has been designed to take maximum advantage of
human auditory masking. It divides the audio spectrum of each channel
into narrow frequency bands of different sizes optimised with respect to
the frequency selectivity of human hearing. This makes it possible to
sharply filter coding noise so that it is forced to stay very close in
frequency to the frequency components of the audio signal being coded.
By reducing or eliminating coding noise wherever there are no audio
signals to mask it, the sound quality of the original signal can be
subjectively preserved.
In Dolby Digital (AC-3), bits are distributed among the filter bands as
needed by the particular frequency spectrum or dynamic nature of the
program. A built-in model of auditory masking allows the encoder to alter
its frequency selectivity (as well as time resolution) to make sure that a
sufficient number of bits are used to describe the audio signal in each
band, thus ensuring noise is fully masked.
Dolby Digital (AC-3) also decides how the bits are distributed among the
various channels from a common bit pool. This technique allows channels
with greater frequency content to demand more data than sparsely
occupied channels, or strong sound in one channel to provide masking for
noise in other channels.
4.5.3
Pre-encoded AC-3
The MCE supports pre-encoded AC-3 to the standard IEC 61937,
‘Interfaces for Non-linear PCM Encoded Audio Bitstreams applying
IEC 60958’.
4.6
Overview of Data Handling
The RS-422 synchronous and RS-232 asynchronous data channels
supported by the MCE are simple bit-pipes and are not coded in any way.
This is due to the fact that the nature of the data to be transmitted is
expected to be different for individual customers, and therefore a suitable
coding scheme cannot be provided with any guarantee of performance.
4.7
MCE Functional Description
4.7.1
Diagram Locations
A functional block diagram of the MCE is shown in Figure 4.2 on Page 4-8.
The physical position of the modules mentioned in this section is shown in
Figure 4.3 on Page 4-9.
