[mythtv-users] Commflag and pixilation

Brian Wood beww at beww.org
Sun Jun 10 18:22:23 UTC 2007


Lan Barnes wrote:

> 
> First the bad news. I lied to everybody (including myself). The pixilation
> is still there. So we must consider the comm flagging a different issue.
> I'm poking into it as I steal time.

Believe it or not I'm glad to hear that, as I knew when you said it that
what you were doing could not possibly have effected your "pixilation"
problem.

What you are seeing is MPEG artifacts, Macroblocking to be precise. It
is common to just about all sports broadcasts these days, and there is
absolutely nothing you can do about it.

Try watching on a "normal" digital TV set, or even watching the feed
come in at your local affiliate's studios, and you will see the same
thing. It is caused by MPEG being used for transport (backhaul) as well
as the final broadcast, and having to encode in real-time (as opposed to
a pre-recorded show where you can do multiple passes and take more than
real-time to encode.

If you look very carefully you will see that network-inserted
commercials exhibit less "pixilation", and locally-inserted ones even
less, as fewer MPEG links are involved as you insert closer to your
reception point. This can be very hard to see as commercials often don't
have the sort of fast motion that sports broadcasts do. Sometimes you
will even see the problem on the "slo-mo" replays, in "slo-mo", as the
MPEG stream was actually recorded as well.

Believe it or not, most people do not notice the problem. I've tried to
point it out to some who just can't see it. Look at some of the research
done by the Motion Picture Expert's Group (MPEG).

(I tried to find you some links to some of the IEEE and related
material, but it all seems copyrighted and not available to the general
public, but I *did* try.) Google for "MPEG perceived" for some references.

> 
> (As an aside, there is a great deal of political pressure within the
> family to not devote time to Myth. Everyone wants Myth and everyone wants
> Myth to run better -- hue is poopy and Xv isn't working, comm flagging
> ain't there for some things, I have not mastered archiving DVDs or
> transcoding ... but everything else takes precidence and my Myth time is
> openly resented. I suspect this resonates with others on the list, but
> it's another thread for another day.)

Not another thread, another issue entirely, in the realm of psychology,
not technology :-)

> 
> On the timing of comm flagging, most of my recordings can stay in the can
> for overnight, but with sports, my practice is to set games to record, let
> them get maybe a 20 minute head start, and then start watching while they
> record. The whole purpose is to let us jump the interminable commercial
> breaks. FF is nice, but when the flagging was working it was way cool.
> 
> Many people tell me I have all the horse power I need for my two capture
> cards because they have HW mpeg2 compression. I suspect the pixilation
> could be fixed by deinterlacing but that means researching TV modelines
> and bob. I've saved all the recent traffic on that, and will attack it ...
> someday.

The Pixilation can't be fixed by you, it is inherent in the present-day
transmission systems. I have worked on sports broadcasts for CBS, Fox
and others, and any discriminating viewer will see the "problem". It was
quite evident even on the last SuperBowl broadcast, and confirmed by
numerous professional engineers.

Your system might enhance the faults, but they are there in the first
place. Increasing the bitrate might minimize the problem, but not make
it go away.

Believe me, I did this for a living for many years.

BEWW


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