[mythtv-users] fine tuning timing of channel changing when using external channel selector

Ray Olszewski ray at comarre.com
Fri Jun 6 19:35:28 EDT 2003


Just to recap ... I have a Motorola digital-cable box (a DCT 2000) that I 
am planning to use an IR Blaster to control. I'm currently working on 
getting the LIRC parts working, and I expect the mechanics of it will sort 
out in time. But I'm facing one conceptual puzzle.

When the Motorola box changes channels, there is a lag of a couple of 
seconds between the time the (manual) remote finishes sending the 
channel-change instruction and the time the new channel displays. After 
that, the Motorola box briefly displays its own OSD with information about 
the show selected, much the way Myth itself does. I can adjust how long it 
displays this information, with 3 seconds the minimum time, but I cannot 
(or at least there is no documented way I can) disable this display 
completely. Since the overlay is part of the signal that gets sent to the 
v4l capture card, it will appear at the start of every show I record with 
Myth. And, of course, I have no way to speed up the actual process of 
changing the channel on the Motorola end.

What this means is that I need to get the Motorola box to begin the process 
of changing channels somewhere between 15 and 30 seconds before the start 
of the show I want to record. How do I accomplish this this using Myth?

With my other capture system (running vcr/avifile through crond), the 
solution is to start the process a minute early, change the channel (which 
will take a few seconds), wait a few seconds to let the OSD come on and 
off, then start recording. All easy to add into the small Perl program I 
use fto control the recording process.

But since Myth has no option to begin recording X seconds before the start 
of the show, I don't see a way to implement even approximately similar 
functionality using Myth. (Since I use ntp to sync the time and know of no 
way to insert a 1-minute time zone offset, fudging the system clock is not 
a practical workaround ... without ntp, the system clock would be too 
drifty to trust for any video start times.)

What do those of you with external channel changers (I surmise that 
satellite receivers have a similar "feature") do to work around this 
problem? For that matter, what does a TiVo do (if anyone knows) about this 
when it is controlling an external channel selector?

This is not quite the question that Calvin asked earlier today and that 
Carlos answered with the C program. That thread addresses delays on the PC 
side. I'm concerned about delays that are external to the PC, delays that 
derive from the normal operation of the Motorola DCT2000.




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