Programme Delivery Control
Programme Delivery Control (PDC) is a
system that controls suitably equipped video recorders by using hidden codes in
the Teletext service. With PDC, if a TV programme you
want to tape is delayed or re-scheduled, the video recording will automatically
be re-scheduled too, so you don't miss any of it.
Each TV programme has a Programme Identification Label or PIL. This label contains the date, channel and start time. Just before the TV programme begins, the TV Company transmits a PDC code containing the PIL, which causes the VCR to start recording. This signal is transmitted, once per second, throughout the duration of the programme. So instead of using its built-in clock the VCR uses the PDC signal to decide when to start and stop recording. This way you don't have to worry if everything is running late - the programme still gets taped.
Because a TV programme can start early as well as late, the VCR 'looks out' for the PIL some time before its programmed start time. If it is programmed to record from more than one station, the video will channel hop, as necessary.
How to program a PDC video
The majority of VCRs
PDC VCRs can be programmed in the same way as ordinary machines: either manually or (if available) with VideoPlus+. In the UK the BBC recommends you program the VCR by using the published start time or VideoPlus+ number from a weekly listings magazine. It's usually these times that are contained in broadcasters' PILs. (The end time isn't so important for programming, since it is not used in the PIL)
Some PDC videos can also be programmed by selecting the TV show directly from the Teletext listings page with a cursor. These machines tend to be more expensive because they contain a Teletext decoder. This method does have the advantage that it's quick and easy and you don't have to have a magazine to hand. It's also far more accurate.
When programming a PDC video manually the video determines the PIL it requires from the date, channel and start time that you program it with. Similarly with VideoPlus+, the PlusCode is decoded to determine the PIL that the VCR will use. This can cause problems. For example if a programme was originally scheduled to be broadcast at 21:00 when the broadcaster had determined its PIL, then 21:00 will be the start time used. If however, the show was then set to be moved to 21:30 before the weekly TV magazine was published, its pages will show 21:30 as the start time. So, no matter if the show actually starts at 21:30 and you program your PDC VCR with this actual start time, your VCR will not tape the programme because the PIL broadcast contains the time 21:00. To get around this problem, the TV listings magazine 'Radio Times' always indicates the start time (and corresponding VideoPlus+ code) used by the broadcaster to determine a PIL wherever it differs from the latest known actual start time.
With a StarText/PDC VCR you don't have to worry about knowing a VideoPlus+ code or whether the listings contain the same start times as used by the broadcasters when they determined the PILs. What happens is when you program using the Teletext page, the PIL for the programme you're taping is actually looked up. The PIL for each programme is hidden from view, below the bottom line of the Teletext page, where it can be accessed by the video recorder during programming. This way, the Teletext page can show the latest known start time while the VCR can determine the correct PIL containing the original start time. The drawback of this system is that where Teletext services only carry listings for today's and tomorrow's TV, it is not possible to program a VCR for say, a week ahead.
Of course, if your VCR is equipped with PDC, but the TV station is not broadcasting a PDC service, then whatever programming method you use, the video recorder will only start and stop recording at a pre-programmed time, with the consequence of possibly missing off parts of programmes that run late.
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