Image Processing Reference
base or they can play independently on separate time bases if necessary. The player sepa-
rates (de-multiplexes) the streams and processes them individually as necessary.
Sometimes, this transport stream is called a multiplex (on digital terrestrial TV and
digital audio broadcast radio). It may also be referred to as a transponder stream when dis-
cussing digital satellite or a channel on digital cable.
The transport stream is an important concept as far as the receiver is concerned
because a receiver will have a tuner that acquires a single transport stream at a time. In a
multi-channel broadcast environment that is delivered on multiple transponders or mul-
tiplexes, only one is acquired at a time unless you install multiple tuners. Channel hop-
ping between program streams within a transport stream is almost instantaneous but
going to a channel in another transport stream takes longer because of the retuning.
Figure 11-8 shows how the transport stream is assembled from component streams.
Tuning into a program first involves the tuner in locating and locking onto the carrier
signal for the transport stream. This may take a finite time and in analog broadcasting the
picture will start to be delivered right away. On a digital broadcast, the transport stream
must be buffered sufficiently that a segment can be reconstructed from the packets and then
de-multiplexed into the packetized program streams. These are buffered and de-multi-
plexed to obtain the packetized elementary streams . Then the stream is decoded. The decoder
must wait until the next I-frame before reconstructing a P-frame or B-frame from it.
This retuning process takes several seconds and is a major shortcoming when
designing complex multi-channel content that is built into an interactive environment.
MPEG-1 Support in MPEG-2
When standards are designed in an evolutionary way, the earlier standards tend to still be
functionally present but are wrapped with additional facilities. This is certainly the case
with MPEG-1 and MPEG-2 because one of the stream constructs defined in MPEG-2 is
identical to the MPEG-1 format. This is called a program stream and all the components
within it must conform to the same time base as was the case with those in MPEG-1.
Designing a Better Mousetrap
For a long time the world was happy with MPEG-2. It did everything we needed at the time
it was introduced, but though it was good, there is always room for improvement in video
coding techniques. Gradually, year by year, MPEG-2 picture quality got better as the
encoder technology was improved by enhancements to the coding algorithms. These
improvements enable us to squeeze just a little bit more video into those already crowded
transmission systems. Because we were just getting smarter and generating more efficient
standards-compliant bit streams, the decoders installed in consumers' TV receivers and
DVD players couldn't tell the difference. So they just happily decoded the more efficient
bit stream without any problem at all.