Information Technology Reference
In-Depth Information
Figure 7.20:
The ability of the consumer to use random access to skip over commercial breaks may destroy
commercial television as it exists today.
Although the PVR can be used as an ordinary video recorder, it can do some other tricks.
Figure 7.20
shows the
most far-reaching trick. The disk drive starts recording an off-air commercial TV station. A few minutes later the
viewer starts playing back the recording. When the commercial break is transmitted, the disk drive may record it,
but the viewer can skip over it using the random access of the hard drive. With suitable software the hard drive
could skip over the commercial break automatically by simply not recording it.
When used with digital television systems, the PVR can simply record the transmitted transport stream data and
replay it into an MPEG decoder. In this way the PVR has no quality loss whatsoever. The picture quality will be the
same as off-air. Optionally PVRs may also have MPEG encoders so that they can record analog video inputs.
Real-time playback in a PVR is trivial as it is just a question of retrieving recorded data as the decoder buffer
demands it. However, the consumer is accustomed to the ability of the VCR to produce a picture over a range of
speeds as well as a freeze frame. PVRs incorporate additional processing which modifies the recorded MPEG
bitstream on playback so that a standard MPEG decoder will reproduce it with a modified timebase.
Figure 7.21
shows that a typical off-air MPEG elementary stream uses bidirectional coding with a moderately long GOP
structure. An increase in apparent playback speed can be obtained by discarding some of the pictures in the group.
This has to be done with caution as the pictures are heavily interdependent. However, if the group is terminated
after a
P
picture, all pictures up to that point are decodable. The resultant motion portrayal will not be smooth
because pictures are being omitted, but the goal is simply to aid the viewer in finding the right place in the
recording so this is of no consequence. In practice the results are good, not least because the noise bar of the
analog VCR is eliminated.
Figure 7.21:
By manipulating the elementary stream, a compliant bitstream can be produced which will play at a
different speed from the original.
