[mythtv] Debugging Unreliable DCT-6200 Recording

jrandall at ftrd.us jrandall at ftrd.us
Thu Sep 22 06:09:49 UTC 2005

I'm having intermittent trouble recording with my DCT-6200 cable box  
connected via firewire.  Basically it seems like about half the time  
the program/live tv doesn't start coming in.  For example, when you  
select "Watch TV" the screen just goes blank for a little while and  
then returns.  If you try a few times, it always starts working  
eventually, but it is much more of a problem for recording -- which  
fails completely half the time.

I have seen other people posting similar issues, so I know it is not  
just me.

I suspect that it is not an issue with MythTV per se.  I have also  
observed this phenomenon when using the test-mpeg2 utility that comes  
with libiec61883.  Basically, running "test-mpeg2 -r 1 > /tmp/foo.ts"  
will either start to fill up the file or will leave a 0 byte file  
"forever".  Either way, test-mpeg2 says "...Established  
Connection..." and "Starting to receive" -- it doesn't seem to know  
whether data is actually coming or not.  I also have my DCT-6200  
connected directly to a TV for debugging, and I note that it always  
glitches a little whenever you start capturing from it (picture  
freezes and audio stops for a second, then continues).  This happens  
whether the data is transmitted or not.

When the connection is not working, mythbackend says:
"Firewire: No Input in 15 seconds [P:1 N:1] (select)"
followed by:
"RingBuffer: Couldn't read data from the capture card in 15 seconds.  

Once data starts flowing, I have never observed any problems, but as  
I said, it has this problem about 50% of the time.  When using test- 
mpeg2 to capture, it isn't really a problem, since I note right away  
that no data is flowing and retry until it works.  But, mythtv fails  
much harder.  It takes many seconds to timeout on live tv before  
letting you retry, and has the problem on channel changing as well.   
And recording shows only works about half the time, because it is  
either go or no-go... there seems to be no retry possible.

How should I go about debugging/fixing this?

We could probably add a way to detect that no data is flowing over  
firewire after the receive connection has been established and then  
retry the connection, but should this problem be fixed here or at a  
lower level (such as in libiec61883?).  Unfortunately, the  
linux1394.org server still seems to be down, so I can't check the  
buglist there to see if this is a known problem or not.

It seems probable that the real problem here is with the firmware in  
my cable box, but since it is not up to me to update that, I'd rather  
see a workaround in software.

Could someone who is familiar with the firewire STB support in mythtv  
let me know where the best place would be to put the fix?  I can  
start hacking around, but I'd rather do it "right" the first time.



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