Information Technology Reference
In-Depth Information
If the received error rate increases for any reason, once the correcting power is used up, the system will degrade
rapidly as uncorrected errors enter the MPEG decoder. In practice decoders will be programmed to recognize the
condition and blank or mute to avoid outputting garbage. As a result digital receivers tend either to work well or not
at all.
It is important to realize that the signal strength in a digital system does not translate directly to picture quality. A
poor signal will increase the number of bit errors. Provided that this is within the capability of the error-correction
system, there is no visible loss of quality. In contrast, a very powerful signal may be unusable because of similarly
powerful reflections due to multipath propagation.
Whilst in one sense an MPEG transport stream is only data, it differs from generic data in that it must be presented
to the viewer at a particular rate. Generic data are usually asynchronous, whereas baseband video and audio are
synchronous. However, after compression and multiplexing audio and video are no longer precisely synchronous
and so the term isochronous is used. This means a signal which was at one time synchronous and will be
displayed synchronously, but which uses buffering at transmitter and receiver to accommodate moderate timing
errors in the transmission.
Clearly another mechanism is needed so that the time axis of the original signal can be re-created on reception.
The time stamp and program clock reference system of MPEG does this.
Figure 7.3 shows that the concepts involved in digital television broadcasting exist at various levels which have an
independence not found in analog technology. In a given configuration a transmitter can radiate a given payload
data bit rate. This represents the useful bit rate and does not include the necessary overheads needed by error
correction, multiplexing or synchronizing. It is fundamental that the transmission system does not care what this
payload bit rate is used for. The entire capacity may be used up by one high-definition channel, or a large number
of heavily compressed channels may be carried. The details of this data usage are the domain of the transport
stream . The multiplexing of transport streams is defined by the MPEG standards, but these do not define any error
correction or transmission technique.
Figure 7.3: Source coder doesn't know delivery mechanism and delivery doesn't need to know what the data
mean.
At the lowest level in Figure 7.3 is the source coding scheme, in this case MPEG-2 compression results in one or
more elementary streams, each of which carries a video or audio channel. Figure 7.4 shows that elementary
streams are multiplexed into a transport stream. The viewer then selects the desired elementary stream from the
transport stream. Metadata in the transport stream ensure that when a video elementary stream is chosen, the
appropriate audio elementary stream will automatically be selected.
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