[mythtv-users] Slow Performance - Improving Picture Quality

Bruce Markey bjm at lvcm.com
Sat Mar 22 06:07:12 UTC 2003

Ryan A. Carris wrote:
> Also, if it helps, my understanding of MP4 settings:
> Resolution - by far the biggest factor in determining the amount of 
> processing power.  Increasing this increases processor requirement by a 
> factor of 4.
> Bitrate - lowering this will reduce quality by dropping data, therefore 
> no affect on processor power, but will affect disk space.
> Max/Min Quality - lowering this will improve quality by using more 
> compression and more processor power.  I haven't seen swing of more then 
> 10% changing from the default quality settings and max settings.

So if I drop the bit rate to zero and set the quality to all
1s I'd get perfect recordings that take up no disk space? ;-)

To get fewer artifacts, you need more information which
means a higher bit rate. The Quality factors are artificial
limits to mark when to do something desperate. They don't
add more information to the stream. In fact, you can cause
encoding to go screwy if they are too far out of line.

If the bit rate is low, the encoder saves space by making
blocks instead of tracking each individual pixel. Therefore,
you can usually get more unique information on screen by
raising the bit rate rather than raising the resolution. And,
as you point out, raising the bit rate has less impact on the
CPU (but will use up more disk space).

Broadcast NTSC or PAL is crappy. Resolutions beyond a width
of 480 just give a more perfect reproduction of the crappy
picture. I believe this is why the commercial products use
widths from 352 thru 544. Higher widths are a waste of disk
space for no real perceivable quality improvement.

So, my favorite setup right now is 544x480, 4400 scaled bit
rate. This only use ~60% CPU time on a 1.3GHz Duron. A bit
rate of 5500 can look a little better for lots of motion but
not enough better to justify wasting more disk space. I record
lots of basketball games and it still looks okay despite the
motion. The T... commercial product attached to the same TV
has much more noticeable artifacts during basketball games.

--  bjm

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