[mythtv] Box Processor Limits

Bruce Markey bjm at lvcm.com
Tue Jan 21 16:20:14 EST 2003


Doug Wiltanger wrote:
> I have a quick question, how can one tell that it's dropping frames?

I read I some nifty text that explained that while
recording, it can't afford to miss audio frames so if
it got behind, it would drop a video frame and repeat the
previous one (I'd quote this but I can't find it right now).
This does appear to be the way MythTV behaves. If the CPU
is nearly pegged while recording, frames will get dropped
at peak times. On playback, the motion appears jerky. The
busier the CPS is, the more frames get dropped.

A rule of thumb that I use is that you notice jerkiness
if the CPU averages less than 10% idle time.

The eye will perceive smooth motion if the frame rate is
at least 20-24 frames per second. Movies are 24fps. NTSC
television is 29.97fps (okay, 30 ;-). If you step through
a DVD you notice that about every fifth frame is repeated.
When you watch it at normal speed it appears to be smooth.

So, some frame dropping is tolerable but as it goes past
10-15% it becomes more and more annoying. If the CPU is
more than 90% busy, it drops frames, so it doesn't need
to do the work for that frame. Therefore,  the the CPU
usage is no longer an indication of how much work the CPU
would have had to do to encode all of the frames. In fact,
I tested recording at 720x480 with 2 tuners on a 1.2GHz
CPU and both show were 'recorded'. The CPU was pegged and
even though this it was only ~60% of the necessary cycles,
there was audio for both shows and a few frames per second
of video. Isaac's Athlon 1800+ would "almost" do 2 640x480
recordings but there would be noticeable frame drops.

So, the tradeoff is between higher resolution causing frame
drops and lower resolution which is less crisp. I believe
that the slight blurriness of a lower res is less annoying
than any jerkiness. For broadcast TV signal quality, you
can lower the res significantly without seeing much of any
difference.

I'd recommend using the highest resolution where the CPU
is an average of at least 10% idle while recording. Some
suggested resolutions to try for recording NTSC (sorry,
I don't have suggestion for PAL):

720x480 640x480 480x480 352x480 320x480
720x240 640x240 480x240 352x240 320x240

Does that answer your quick question? ;-)

-- bjm




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