Question on the mpeg like compression that occurrs during ..

Archived from groups: alt.tv.tech.hdtv (More info?)

HeadRusch wrote:
> ...we've all seen it...the fast-camera zooms or pans, the action of
> running water filmed in HD....sometimes it looks fine, othertimes its
> a pixelated mess.
>
> My question: What really causes this,a nd what can be done about it.
> I've been operating under the notion that it has to do with the way
> the original HD source was encoded...but I'd like to know for sure.
>
> Can anyone provide a decent explanation of why this occurrs. Today
> on PBS I was watching some show on Killer whales and whatnot.
> Outstanding picture...until they got into this section dealing with
> schools of shimmering Herring....and almost instantly it was like
> watching some bad 320x200 MPEG 1 being blown up 300%.....
>
> Is this the signal coming over cable not having enough bandwidth to
> carry all the information necessary to keep those fast-moving images
> sharp and without compression artifacts, or is this something
> else...??
> thx

HD is a highly compressed MPEG2 data stream so compression artifacts are
common. Interlaced formats, like the 1080i used on PBS, are not as adept
as reproducing fast motion. Whereas, a progressive signal like 720p used
on ABC and this Fall on FOX produce a more stable picture. In addition,
fast motion requires a higher bitrate because each frame changes
drastically from the previous one.

I don't know if cable systems re-encode or transcode the HD streams they
receive before transmission to your home.


--
David G.

Archived from groups: alt.tv.tech.hdtv (More info?)

"John@Smith.com" <xxxxspud@newscene.com> wrote in message
news8mh90p1pcdi5qokgqnvh21miq3p1hgdcf@4ax.com...
> On Wed, 05 May 2004 03:12:53 GMT, "HeadRusch" <headrusch1@comcast.net>
> wrote:
>
> >...we've all seen it...the fast-camera zooms or pans, the action of
running
> >water filmed in HD....sometimes it looks fine, othertimes its a pixelated
> >mess.
> >
> >My question: What really causes this,a nd what can be done about it.
I've
> >been operating under the notion that it has to do with the way the
original
> >HD source was encoded...but I'd like to know for sure.
> >
> >Can anyone provide a decent explanation of why this occurrs. Today on
PBS I
> >was watching some show on Killer whales and whatnot. Outstanding
> >picture...until they got into this section dealing with schools of
> >shimmering Herring....and almost instantly it was like watching some bad
> >320x200 MPEG 1 being blown up 300%.....
> >
> >Is this the signal coming over cable not having enough bandwidth to carry
> >all the information necessary to keep those fast-moving images sharp and
> >without compression artifacts, or is this something else...??
> >thx
>
> I was wondering too. But I see this on non-HDTV channels.
> Stuff like Power Rangers - the kid show, is the one I really notice it
> because of the fast motion and the fact Ive captured shows with my TV
> tuner card for my nephew and I see similar problems when weve watched
> it a few times on my set when its on TV too.
>

Short answer:
What you're seeing is the result of the mpeg-2 compression. For
fast-changing scenes there is not sufficient bandwidth to properly encode
the signal.

More detailed answer:
MPEG-2 encoding takes advantages of redundancies within a particular frame
as well as redundancies across frames. In scenes with very little motion,
the mpeg-2 encoder can take advantage of the fact that the picture looks
almost exactly the same from one frame to the next. This gives a great deal
of compression and keeps the bitrate very low.

However, when you get a lot of fast movement, the mpeg-2 encoder needs to
put a lot more information into the bitstream in order to reconstruct a
frame based on the previous frame. Due to bandwidth constraints the amount
of info you can put into the bitstream eventually gets capped. In this case
you're going to see some blocky distortion in the picture.

When the first frame in a group of pictures (GOP) gets compressed, it is
done in a similar manner to a jpeg photo. Future frames are variations of
that frame. When the first frame is compressed the picture is broken into
lots of 8x8 blocks which are compressed using a mathematical tool called the
DCT. When the picture gets all crummy you can plainly see all these 8x8
blocks, which is a sure sign that you're seeing compression artifacts.

I have satellite tv and my biggest complaint is that EVERYTHING is
over-compressed. The bandwidth is extra-limited for satellite since they're
broadcasting through a small chunk of the airwaves rather than over cable
where the full spectrum is available. The satellite companies tend to
over-compress tv shows to fit in the bandwidth and so I can see those darn
8x8 blocks in just about everything I watch.

Brad

Archived from groups: alt.tv.tech.hdtv (More info?)

Yes, thats it exactly. Thank you so much for the explanation.

Now, if you don't mind a followup question: Is the MPEG compression done at
the time of recording and mastering or is it done at the time of
transmission through the cable systems. IE: If the companies wanted could
they dramatically expand the bandwidth available to prevent such blocking
artifacts to appear...or is this something we'll have to deal with even when
HD-DVD comes out..I'm guessing it wont be an issue for that technology
but...you never know.
???


"Brad Griffis" <bradgriffis@hotmail.com> wrote in message
news:2f6mc.2931$eH1.1759051@newssvr28.news.prodigy.com...
>
> "John@Smith.com" <xxxxspud@newscene.com> wrote in message
> news8mh90p1pcdi5qokgqnvh21miq3p1hgdcf@4ax.com...
> > On Wed, 05 May 2004 03:12:53 GMT, "HeadRusch" <headrusch1@comcast.net>
> > wrote:
> >
> > >...we've all seen it...the fast-camera zooms or pans, the action of
> running
> > >water filmed in HD....sometimes it looks fine, othertimes its a
pixelated
> > >mess.
> > >
> > >My question: What really causes this,a nd what can be done about it.
> I've
> > >been operating under the notion that it has to do with the way the
> original
> > >HD source was encoded...but I'd like to know for sure.
> > >
> > >Can anyone provide a decent explanation of why this occurrs. Today on
> PBS I
> > >was watching some show on Killer whales and whatnot. Outstanding
> > >picture...until they got into this section dealing with schools of
> > >shimmering Herring....and almost instantly it was like watching some
bad
> > >320x200 MPEG 1 being blown up 300%.....
> > >
> > >Is this the signal coming over cable not having enough bandwidth to
carry
> > >all the information necessary to keep those fast-moving images sharp
and
> > >without compression artifacts, or is this something else...??
> > >thx
> >
> > I was wondering too. But I see this on non-HDTV channels.
> > Stuff like Power Rangers - the kid show, is the one I really notice it
> > because of the fast motion and the fact Ive captured shows with my TV
> > tuner card for my nephew and I see similar problems when weve watched
> > it a few times on my set when its on TV too.
> >
>
> Short answer:
> What you're seeing is the result of the mpeg-2 compression. For
> fast-changing scenes there is not sufficient bandwidth to properly encode
> the signal.
>
> More detailed answer:
> MPEG-2 encoding takes advantages of redundancies within a particular frame
> as well as redundancies across frames. In scenes with very little motion,
> the mpeg-2 encoder can take advantage of the fact that the picture looks
> almost exactly the same from one frame to the next. This gives a great
deal
> of compression and keeps the bitrate very low.
>
> However, when you get a lot of fast movement, the mpeg-2 encoder needs to
> put a lot more information into the bitstream in order to reconstruct a
> frame based on the previous frame. Due to bandwidth constraints the
amount
> of info you can put into the bitstream eventually gets capped. In this
case
> you're going to see some blocky distortion in the picture.
>
> When the first frame in a group of pictures (GOP) gets compressed, it is
> done in a similar manner to a jpeg photo. Future frames are variations of
> that frame. When the first frame is compressed the picture is broken into
> lots of 8x8 blocks which are compressed using a mathematical tool called
the
> DCT. When the picture gets all crummy you can plainly see all these 8x8
> blocks, which is a sure sign that you're seeing compression artifacts.
>
> I have satellite tv and my biggest complaint is that EVERYTHING is
> over-compressed. The bandwidth is extra-limited for satellite since
they're
> broadcasting through a small chunk of the airwaves rather than over cable
> where the full spectrum is available. The satellite companies tend to
> over-compress tv shows to fit in the bandwidth and so I can see those darn
> 8x8 blocks in just about everything I watch.
>
> Brad
>
>

Archived from groups: alt.tv.tech.hdtv (More info?)

I don't work for a cable company or anything so this is just my best guess
based on what I've seen. I believe that the source material is encoded with
a very good mpeg encoder early on. However, I think that the broadcasters
often re-code the mpeg in order to reduce the bitrate. There are different
reasons why broadcasters would lower the bitrate. For example, an OTA
broadcast might be doing a simulcast (both analog and digital) and so they
might need to compress the data some more. For a satellite company they are
bandwidth constrained to begin with so they would also need to re-code the
data to fit all their channels into their chunk of the spectrum.

What I've seen is that digital CABLE looks very clean, but digital SATELLITE
looks over-compressed. This makes sense in that cable has a lot more
bandwidth to work with.

When high definition DVDs come out I don't think viewing HD will be a
problem. The throughput you can get reading off a DVD is far higher than
what you can transmit over the airwaves. Therefore they will be able to
encode very high quality bit streams on the DVDs.

Brad

"HeadRusch" <headrusch1@comcast.net> wrote in message
news:To6mc.28360$_41.2054694@attbi_s02...
> Yes, thats it exactly. Thank you so much for the explanation.
>
> Now, if you don't mind a followup question: Is the MPEG compression done
at
> the time of recording and mastering or is it done at the time of
> transmission through the cable systems. IE: If the companies wanted could
> they dramatically expand the bandwidth available to prevent such blocking
> artifacts to appear...or is this something we'll have to deal with even
when
> HD-DVD comes out..I'm guessing it wont be an issue for that technology
> but...you never know.
> ???
>
>
> "Brad Griffis" <bradgriffis@hotmail.com> wrote in message
> news:2f6mc.2931$eH1.1759051@newssvr28.news.prodigy.com...
> >
> > "John@Smith.com" <xxxxspud@newscene.com> wrote in message
> > news8mh90p1pcdi5qokgqnvh21miq3p1hgdcf@4ax.com...
> > > On Wed, 05 May 2004 03:12:53 GMT, "HeadRusch" <headrusch1@comcast.net>
> > > wrote:
> > >
> > > >...we've all seen it...the fast-camera zooms or pans, the action of
> > running
> > > >water filmed in HD....sometimes it looks fine, othertimes its a
> pixelated
> > > >mess.
> > > >
> > > >My question: What really causes this,a nd what can be done about it.
> > I've
> > > >been operating under the notion that it has to do with the way the
> > original
> > > >HD source was encoded...but I'd like to know for sure.
> > > >
> > > >Can anyone provide a decent explanation of why this occurrs. Today
on
> > PBS I
> > > >was watching some show on Killer whales and whatnot. Outstanding
> > > >picture...until they got into this section dealing with schools of
> > > >shimmering Herring....and almost instantly it was like watching some
> bad
> > > >320x200 MPEG 1 being blown up 300%.....
> > > >
> > > >Is this the signal coming over cable not having enough bandwidth to
> carry
> > > >all the information necessary to keep those fast-moving images sharp
> and
> > > >without compression artifacts, or is this something else...??
> > > >thx
> > >
> > > I was wondering too. But I see this on non-HDTV channels.
> > > Stuff like Power Rangers - the kid show, is the one I really notice it
> > > because of the fast motion and the fact Ive captured shows with my TV
> > > tuner card for my nephew and I see similar problems when weve watched
> > > it a few times on my set when its on TV too.
> > >
> >
> > Short answer:
> > What you're seeing is the result of the mpeg-2 compression. For
> > fast-changing scenes there is not sufficient bandwidth to properly
encode
> > the signal.
> >
> > More detailed answer:
> > MPEG-2 encoding takes advantages of redundancies within a particular
frame
> > as well as redundancies across frames. In scenes with very little
motion,
> > the mpeg-2 encoder can take advantage of the fact that the picture looks
> > almost exactly the same from one frame to the next. This gives a great
> deal
> > of compression and keeps the bitrate very low.
> >
> > However, when you get a lot of fast movement, the mpeg-2 encoder needs
to
> > put a lot more information into the bitstream in order to reconstruct a
> > frame based on the previous frame. Due to bandwidth constraints the
> amount
> > of info you can put into the bitstream eventually gets capped. In this
> case
> > you're going to see some blocky distortion in the picture.
> >
> > When the first frame in a group of pictures (GOP) gets compressed, it is
> > done in a similar manner to a jpeg photo. Future frames are variations
of
> > that frame. When the first frame is compressed the picture is broken
into
> > lots of 8x8 blocks which are compressed using a mathematical tool called
> the
> > DCT. When the picture gets all crummy you can plainly see all these 8x8
> > blocks, which is a sure sign that you're seeing compression artifacts.
> >
> > I have satellite tv and my biggest complaint is that EVERYTHING is
> > over-compressed. The bandwidth is extra-limited for satellite since
> they're
> > broadcasting through a small chunk of the airwaves rather than over
cable
> > where the full spectrum is available. The satellite companies tend to
> > over-compress tv shows to fit in the bandwidth and so I can see those
darn
> > 8x8 blocks in just about everything I watch.
> >
> > Brad
> >
> >
>
>

Archived from groups: alt.tv.tech.hdtv (More info?)

A weak signal would most likely not cause pixelation. When you get a weak
analog signal, the noise in the airwaves seems larger in comparison (i.e.
your signal-to-noise ratio decreases). This causes the familiar degradation
with analog broadcasts. The further you are from the source, the fuzzier
your picture gets.

Digital broadcasts are much different. Rather than sending the exact analog
signal, a series of 1's and 0's are sent instead so that the receiver can
RECONSTRUCT the analog signal. With digital broadcasts you see what is
known as the "digital cliff effect." Even the signal gets weaker as you're
farther away from the broadcast, you can still perfectly decipher all the
1's and 0's. However, there comes a point where you can no longer decipher
them. As you cross that threshold you go from perfect picture to no
picture. This is what is known as the "digital cliff effect."

The pixelation is a compression artifact from the 8x8 DCTs that go on during
the mpeg-2 compression. See my earlier post for a little more on that.

Brad

"Dennis Mayer" <Polaris1@execpc.com> wrote in message
news:409974B9.C17452EB@execpc.com...
>
> Might HD set pixelation also be caused by a weak HD signal.....
>
> HD cable channels are grouped by 'carrier frequency' with in the
> cable...
>
> Any good cable signal should read +1db or better... More like
> +10db..
>
> Your local cable company Tech can verify your home HD signal level
> via a
>
> home visit.....
>
> IF gold splitters or bad cable is a problem.... the HD tuner or
>
> HDTV set (using DVI) must be of high quality to compensate for
>
> a decent HD picture..
>
>
>
>
>
> HeadRusch wrote:
> >
> > ...we've all seen it...the fast-camera zooms or pans, the action of
running
> > water filmed in HD....sometimes it looks fine, othertimes its a
pixelated
> > mess.
> >
> > My question: What really causes this,a nd what can be done about it.
I've
> > been operating under the notion that it has to do with the way the
original
> > HD source was encoded...but I'd like to know for sure.
> >
> > Can anyone provide a decent explanation of why this occurrs. Today on
PBS I
> > was watching some show on Killer whales and whatnot. Outstanding
> > picture...until they got into this section dealing with schools of
> > shimmering Herring....and almost instantly it was like watching some bad
> > 320x200 MPEG 1 being blown up 300%.....
> >
> > Is this the signal coming over cable not having enough bandwidth to
carry
> > all the information necessary to keep those fast-moving images sharp and
> > without compression artifacts, or is this something else...??
> > thx

Archived from groups: alt.tv.tech.hdtv (More info?)

What a totally bizarre response. So many holes, I don't where to begin, but
please tell me what compression scheme the Japanese and Australians are
using to avoid pixelization?

Of course there is no Chinese standard and not an HDTV signal on the air in
China, so that is total bunk...like the rest of your message...like the rest
of your messages.


"Bob Miller" <robmx@earthlink.net> wrote in message
news:wEdmc.8438$V97.2120@newsread1.news.pas.earthlink.net...

> In other words the combination of 8-VSB at 19.34 Mbps and MPEG2 cannot
> deliver fast motion 1080i video.
>
> Viola no pixelization on Japanese HDTV sets or Chinese HDTV sets or
> Australian HDTV sets or soon French HDTV sets.
>