[mythtv] bttv recordings run fast

Bruce Markey bjm at lvcm.com
Sat Mar 6 04:28:34 EST 2004

bobnvic at everestkc.net wrote:
> I figured out the issue.  I had the HQ and MV quality setting set
> for the mpeg4 capture and the CPU was pegged while recording.
> After turning off the HQ and MV quality settings, the CPU usage
> dropped to about 30% and no longer exhibited the issues while
> actually looking better than it had previously.  It appears that
> the CPU usage may have increased slightly in the current cvs to
> push it over the edge.  However the bttv color adjustments make
> the picture look far better.  

The tendency is to think that if the picture looks 'bad' then
higher resolution or compression parameters will make it better.
However, none of these can correct poor color reproduction
which is the biggest reason for the picture not looking good.

The 'adjust' filter corrects an outright mistake in the bttv luma
range but there is just plain a problem where color saturation is
washed out at higher luma values. I've been playing with the adjust
parameters to abuse it in overcoming the problems. Try this for yucks:

update channel set contrast=21000,brightness=32768,colour=32768,hue=32768;
update channel set videofilters='adjust=16:214:1.0:22:233:1.0';

The trick is to record at low contrast to avoid the problems at
high luma then stretch the contrast back out with 'adjust'. Also,
narrowing the chroma values stretches the saturation range so
that light colors are more dense. 

For me, this makes the colors in all ranges almost identical to
to the cable signal fed directly to the TV. This makes live video
look much more like video with less of that 'old film' look. Once
the colors are right the picture should still look good even at
lower resolutions.

> I have no idea why the high CPU usage would result in the issues
> that I saw in playback, but I'm quite happy again with my setup.

It's not just 'high' CPU usage, it's CPU shortfall. If the CPU
can't keep up, it eventually has to drop frames (see Lucy in the
chocolate factory). The player expects to have 29.97 frames per
second and will slew to bring the audio and video into sync.
However, if there are a lot of dropped frames so there there are
only about 20 per second while the player is trying to play 30,
there will be a constant tug'o'war going on while it tries to
line up the audio/video timing.

--  bjm


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