[mythtv] Box Processor Limits
dmech at neo.rr.com
Tue Jan 21 20:41:56 EST 2003
I'm really looking for a log file that says how many frames are dropped.
>From what I've gathered I need to capture stdout and errout.
If visuals are the key, then an Athlon at 1100 Mhz cannot keep up with
recording at 640X480. Live TV is not really possible here. The decoding
of the stream doesn't even use 5% CPU at this res. It's all about encoding
quality vs diskspace.
I find that 512X384 with bit rate at 3300 provides a consistent 85%
utilization of CPU. I also have audio set to 32000 with quality setting of
3. This yields nearly 1GB an hour.
This is with .7 release. Once .8 is release I'll be transitioning to my
server machine which then will have a Celeron 1.4Ghz. I'm hoping to be
able to keep that res.
Question is, how much of an impact does increasing the bit rate have on the
reduction of CPU utilization..
----- Original Message -----
From: "Bruce Markey" <bjm at lvcm.com>
To: "Development of mythtv" <mythtv-dev at snowman.net>
Sent: Tuesday, January 21, 2003 7:20 PM
Subject: Re: [mythtv] Box Processor Limits
> 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
> 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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