|Some people ask
why TiVo does not use PDC like VCR's. PDC is Programme Delivery
Control, a system whereby broadcasters send special signals
which indicate when a programme as about to start or end. It
only applies to terrestrial broadcasts so even if supported
it would not work with any digital set-top boxes.
Ted Malone from
TiVo has explained the issues surrounding using PDC with TiVo:
"I thought I would jump in and set the record straight
from TiVo's side on the subject of PDC. Rest assured that
we are (and were) completely aware of PDC throughout the design
of the UK recorder. Unlike a VCR, the TiVo recorder is constantly
running. When it's not recording a programme, it is recording
the last 30 minutes of the channel you currently have tuned.
In order to use PDC, you have to re-tune to other RF channels
to see if upcoming events are going to start on time or not.
Tuning away from the current channel to do this would cause
us the lose our 30-minute cache. You basically wouldn't be
able to use any of the TiVo features on live tv (pause, etc.)
because we would constantly be tuning away to see what the
PDC codes were doing on other channels with upcoming recordings.
You will note in
the PDC FAQ that the wording softens for show start times.
The VCR "should" start the recording early. The
reason the wording softens is that if you program your VCR
to record one programme, immediately followed by another,
you could have a problem. If the first program runs over and/or
the second program starts early, you could have a problem.
This is particularly true with TiVo since it is always recording.
For the problem
of show overruns, it's a bit different. If we were tuned to
an RF channel and recording a programme, we could monitor
the PDC code for that show to see if it were going to run
over. At this point, we run into a slight limitation of the
current TiVo architecture which is just currently being fixed
in our US version and wont be available in the UK until sometime
next year. This problem is that the entire architecture relies
on a routine called the Scheduler. It handles all events that
are scheduled to occur (recordings, etc.).
Our early architecture
was entirely EPG based, and depended on recording conflicts
that only occurred when a recording was being scheduled. Our
design philosophy was that a conflict around recording should
always be handled by the user. For instance, if you try to
record two shows at the same time, we ask you which one you
want, and offer to un-schedule the conflicting programme.
In the case of show overruns, this conflict can occur while
a recording is happening, and the viewer may not be there
to handle an impending conflict. We had no internal mechanism
to handle this conflict. Would you rather record the end of
this show, or tune away to start recording another one?"