[mythtv] libcmyth failing to receive HD stream

Chase Douglas chasedouglas.lists at gmail.com
Sat Feb 21 23:39:54 UTC 2009

I'm working on a separate frontend project using libcmyth and I've  
been able to generate and play a live tv SD stream from an IVTV card.  
A few times I've been able to generate and play an HD stream from a  
pcHDTV-5500 card. However, usually the HD stream playback doesn't  
work. The frontend keeps requesting bytes (as low as 1 byte), and the  
backend keeps returning 0.

My guess as to why I'm seeing this only on HD streams is that the SD  
stream from the IVTV card is "always on". There's no lag time between  
starting a recording and the recording file being available apart from  
the backend processes creating the recording. However, a digital  
recording has to first tune and lock on to the signal before the  
recording actually starts. I am guessing that my frontend is missing  
some step between spawning live tv and beginning to request bytes of  
the stream. I see the following in the backend log:

2009-02-21 17:10:46.739 Preview Error: Previewer file '/home/mythtv/ 
recordings/1840_20090221171043.mpg' is not valid.
2009-02-21 17:10:46.740 Preview Error: Run() file not local: '/home/ 
2009-02-21 17:10:46.746 Preview Error: Preview process not ok.
1840_20090221171043.mpg.png) exists: 0 readable: 0 size: 0
2009-02-21 17:10:51.531 RingBuf(/home/mythtv/recordings/ 
1840_20090221171043.mpg): Invalid file (fd -1) when opening '/home/ 
2009-02-21 17:10:51.567 MainServer::HandleAnnounce Playback
2009-02-21 17:10:51.568 adding: mythtv as a client (events: 0)
2009-02-21 17:10:51.580 RingBuf(/home/mythtv/recordings/ 
1840_20090221171043.mpg) Error: Invalid file descriptor in 'safe_read()'
--- last line repeating indefinitely ---

I've run a tcpdump of a MythTV frontend to see what messages it sends  
back and forth. I see it send on a control port:

ANN Playback
*QUERY_RECORDER IS_RECORDING (backend returns 1)
*QUERY_CHECKFILE (backend returns 1 and filename)
*QUERY_CHECKFILE (backend returns 1 and filename)

Then a new control port and the video streaming port are spawned and I  

ANN Playback
ANN FileTransfer (on the streaming port)

And from then on things progress as one would expect, with the new  
control port requesting blocks and the new streaming port receiving  
the blocks. My frontend using libcmyth leaves out any of the messages  
I've starred (*) above. Which, if any, of these messages are required?  
Does this explain why I'm having issues with the digital stream, or am  
I missing something? Is there something on the event connection, which  
I'm not really listening to, that is salient?

Chase Douglas

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