[mythtv-users] viaslice quality

Andrew Dodd atd7 at cornell.edu
Sat Feb 21 12:10:27 EST 2004

Quoting Ivor Hewitt <ivor at ivor.org>:

> Just a quick question for those using viaslice output driver...
> Are you seeing 'chunky' mpeg output?
> I'm using a dvb-t card to supply an mpeg source stream but the playback 
> through viaslice is displaying, um, chunkyness.
> Hard to describe... the effect is quite subtle,  but the effect is like 
> there are vertical bars down the screen at eight pixel intervals. almoset 
> as if the Y shading across that patch is reversed. Quite subtle, but most 
> noticeable with light greys or greens.
> The effect is particularly noticeable on a 4:3 video being played back in 
> vertical letterboxed mode... if the image pans than there mpeg corruption 
> on the extreme edges of the screen as new colours move in.
> Initially I thought this may be some problem with scaling the image, but 
> throwing loads of debugging messages and using an output resolution of 
> 720x576, shows no problems.
> So, is anyone else seeing this?
Have you tried playing the same stream through another playback device?

What you're describing sounds like classic MPEG macroblocking, and 99% of the
time, visible macroblocking is the result of a bad encoder or the encoder using
too low of a bitrate.

Worst example of that I can think of in a digital broadcast was the WWOR (UPN 9,
NYC) feed piggybacked as a subchannel on WNYW's (FOX 5, NYC) ATSC stream.  My
analog reception of WWOR was better than the digital stream, and that's SAD
because WWOR is the third worst analog channel I receive.

Last I saw, WWOR's digital feed was much improved, but it was still 480i.  :(

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