[mythtv-users] artifacts in pvr-150 recordings, only recently

Raymond Wagner raymond at wagnerrp.com
Fri Mar 6 16:39:04 UTC 2009

jarpublic at gmail.com wrote:
> If the signal is too weak/strong is their some way I can check this
> without going out and buying amps/attenuators and seeing what happens?
If you're recording the analog feed, weak/strong signal would appear as 
fuzzyness, static, scrolling lines, or a handful of other things that 
are recognizable as analog errors.  What you see in that picture are 
macroblocks.  MPEG video is divided into macroblocks, and when the 
bitrate drops too low, you can make out the borders between them.  It 
doesn't look like lack of data; that shows up as pausing, or large 
numbers of black or very wrongly colored macroblocks.  All I can assume 
is that somewhere along the line, video bitrate drops very very low.
> I guess I can watch some TV tonight straight from coax to my TV and
> see if anything shows up there. Then I could tell if it is Comcast or
> not.
That's a good idea.  Watch a show, and record it with mythtv; see if the 
recording has artifacts not seen in the live version.  Record a known 
good source, like a dvd player (would Macrovision make this not work?), 
or video output from a computer.  You should have direct access to the 
framegrabber, I believe on '/dev/video24'.  Try capturing/transcoding 
some video off that with ffmpeg or vlc, and see if you have the same 

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