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List:       mythtv-dev
Subject:    Re: [mythtv] Couple of questions and an issue...
From:       raldrich () mac ! com
Date:       2003-04-29 22:39:55
Message-ID: 7FBE1304-7A93-11D7-96CD-0030657D441C () mac ! com
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On Tuesday, April 29, 2003, at 01:35  PM, wwinright@attbi.com wrote:
>
> My questions are:
> Has anyone using the PVR card seen any issues with something that I 
> wouldn't
> nessecarily call a stutter, but maybe frames dropping, especially 
> during slow pans?
>

Since the PVR uses fixed bit rate encoding, and probably doesn't 
support motion estimation, what you're seeing is probably caused by the 
encoder running out of bandwidth for all of the changes on the screen.  
As the bandwidth requirements increase, the picture gets progressively 
more blocky, until eventually, it looks like it is being rendered by an 
Atari 800 at low resolution.

> And, has anyone seen odd lines appear on the borders of 
> characters(people) in a
> show during bright scenes, heavy action, gunfire, etc.? (I have seen 
> something
> similar on my Laptop when playing DVDs using certain video-players...)
>

This is an effect of MPEG compression (I've heard the term 'Mosquitoes' 
used to describe it).  Some advanced MPEG decoders add filters to help 
reduce effect.  I have no idea whether or not the PVR-350's decoder 
supports filters.

Generally, you'll see four kinds of compression artifacts in MPEG-2 
streams.

1 - Macropixel blockiness - MPEG-2 breaks each frame down into (I 
believe) 16x16 pixel blocks, each of which is compressed separately.  
If a large number of macropixels have changed from the previous frame, 
the number of bits available to represent each macropixel can drop too 
low, and too much data has to be thrown away.

2 - Macropixel edges - even if there is plenty of bandwidth available, 
the edges of the macropixels can become visible because the compression 
of adjacent macropixels resulted in slightly different colors.  This 
effect is most often visible in the background of scenes which have a 
gradient - sky, or darkness.

3 - 'Mosquitoes' or 'Ringing' - a sort of 'wave' is visible around 
areas of high contrast.  Each macropixel is compressed by converting 
the data from waveforms into frequencies, and then throwing away most 
of the high frequency data.  Since the waveform for a hard edge looks 
like a square wave, and since square waves require a lot of high 
frequencies to reproduce faithfully, you get 'mosquitoes' instead.

4 - Temporal crawl - This one's kinda hard to describe, but when you 
see it, you'll know what I'm talking about.  In a scene which doesn't 
change very much (a very slow pan for example), the encoder might 
decide to update one macropixel, but not the one next to it, and then 
on the next frame, update the second macropixel.  The visual effect is 
that parts of the image seem to move independently of other parts.  In 
one example, the camera was focused on a newscaster's head, which was 
just barely moving as she spoke - after compression, her nose seemed to 
be changing width as her head moved.

Sorry about the long post - I hope this helps you make sense of it 
all...

Ron

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