[mythtv-users] Odd lirc behavior when using plasma display vs. CRT display

Roger Heflin rogerheflin at gmail.com
Sat Jun 21 02:36:31 UTC 2008

Larry K wrote:
> I have a FE running myth 0.20 on FC8 that has been happily working over
> S-Video to a 27" Panasonic CRT TV.  Recently, I bought a new Panasonic 50"
> plasma, so my intent is to connect this display using DVI-HDMI, rather than
> S-Video, for obvious reasons.
> Anyway, since I have not yet configured xorg.conf for DFP output, I simply
> plugged the S-Video cable into the new plasma as an interim measure.  When I
> did this, lirc started behaving oddly.  It basically does not respond to my
> remote, and when I run irw to test it out, it very rarely (if ever) picks up
> the command.  If I simply unplug the S-Video cable from the plasma and put
> it back on the old 27" CRT, everything returns to normal.  Here, irw echos
> each and every command flawlessly.  Mythtv responds flawlessly to my remote
> control as well.
> What in the world could be causing this odd behavior?  Why would the display
> device affect lirc like this?  What can I do to correct this?  Oh.  I am
> running lirc from a serial port if that matters.
> Larry

The signal coming out of the TV it close enough (signal wise and light 
wavelength wise) to look like remote control commands and overload it.

When my LCD tv is first warming up (with anything on it) it actually overloads a 
remote control repeater that I use to forward remote control commands to another 
room with the actual device in it, and results in the repeater not being able to 
repeat any commands (even from any other repeaters).   It does not overload a 
real remote control receiver device (ie a set top box, vcr, dvd) but will 
overload the simple repeater for 10-30 seconds on startup that is forwarding 
commands, I would suspect that it is doing the same thing to lirc.

It might be that lirc's algorithm is not good enough to deal with this high 
level of noise (the repeater box is pretty stupid, it pretty much repeats 
whatever it detects as remote control commands), you might be able to get an IR 
filter for the tv (that hopefully won't screw up the picture much and be pretty 
cheap) that would reduce the interference, on mine since it is only on startup, 
I just have to wait a while for it to clear up.

You might also attempt to put the ir receiver in such a way that it limits the 
amount of signal that it can *SEE* from the tv, and/or make a tube/shield to 
limit what it gets from the tv, of course if it being screwed up by a reflected 
signal (from the walls) you may not be able to do anything.


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