Image Processing Reference
Figure 15-1 Transponder bit-rate distribution.
feeds an AutoCue unit. Sometimes speech recognition is used to track the autocue script
and synchronize it to the audio track that is being encoded and added to the outgoing
stream. This gets complex right away, without even trying to do anything very advanced.
Broadcasting services have the same requirements for synchronized data feeds.
Some advantage can be gained by the fact that many programs are made well before
broadcast. Although the output is encoded live, it is often being played out from a prere-
corded source and is not very often “live to air” from an incoming feed. Many of the ben-
efits of offline preparation are still available to the program creator. Some data will be
synchronous to the real-world time and delivered once only, at the time of broadcast.
Other data feeds, perhaps having to do with the delivery of a text service or interactive
application, are delivered at the same time as part of the transport stream but are played out
independently. These would normally be driven from a carousel, which is a cyclically rotat-
ing buffer of data that is played out repeatedly. This is necessary because receivers typically
do not have much cache memory available and it is quite likely that the viewer has come to
the program some time after the most recent transmission of the data. Therefore, significant
numbers of viewers may not yet have received the data and it must be transmitted periodi-
cally to ensure that they all receive it. This is of course a very wasteful technique in terms of
bandwidth but is necessary in order to keep the costs of set-top boxes to a minimum.
The transport mechanisms convey the streaming data over a very long distance before it
reaches your player. Internet-streamed video will have very likely crossed the Atlantic at
least once and if you are a European user, it may have crossed the Atlantic several times if
the routing is complex. It all depends on your ISP and how you are connected to the wider
network. Certainly the content will have travelled several thousand miles and the routes