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1080i playback - What does it take?
PostPosted: Tue Apr 18, 2006 11:05 pm Reply with quote
Bigchris
 
Joined: 18 Apr 2006
Posts: 15




I'm running a 3.0Ghz P4 HT and a FX5200 w/XvMC with Mythtv .19 kernel 2.6.13-15.8 or Win2k. I can record 1080i to a mpg2 file just fine, but when I play the file back in zine or mplayer I see momentary jagged edges on anything that moves in the picture. A flashbulb going off creates momentary black and white bars across the picture.
Playing the same file in Win2k I see no jaggies or bars but it'll skip a few frames every so often.

This feels like a hardware speed problem but my cpu utilization during playback is only a tick above 20%. Are 7200rpm hds too slow? PCI bus congestion? What does it take?
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PostPosted: Wed Apr 19, 2006 8:58 am Reply with quote
Scott Larson
 
Joined: 15 Oct 2003
Posts: 713
Location: Portland, OR




If you have deinterlacing turned on then those blocks are in the content, not the fault of your system. This happens when there's too much data to fit into the data rate the station is sending. Since these frames also require more computing power to display, the application you're using in Windows isn't fast enough to display them. Try having it display them in slow motion. I bet you'll see the exact same artifacts you see with Myth.

Flashbulbs and strobes are the most common content that causes this. Since these cause everything in the image to change instantly, there are no redundant blocks from the previous image so the encoder has no choice but to send a low bitrate blocky approximation of it.
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PostPosted: Wed Apr 19, 2006 10:54 am Reply with quote
Bigchris
 
Joined: 18 Apr 2006
Posts: 15




Scott,
Thanks for the reply and you've sent me scrambling off to take a more careful look at what I'm doing with de-interlacing. If I understand you correctly, you're saying my local station is munging up the signal they're transmitting.

The station in question is one of the first in the nation to transmit HDTV and they are a Tiffany quality, cost is no consideration kind of operation. I expect technical excellence from them, and if what I'm seeing is as good as it gets, they're not going to sell any DT capable TV sets in this area. BTW, their transmitter is 20 miles away from my roof antenna and their signal strength runs about 75-78 % at my PC.

I believe that I understand your explanation, but I still think that I've got a problem...
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PostPosted: Wed Apr 19, 2006 11:42 am Reply with quote
Scott Larson
 
Joined: 15 Oct 2003
Posts: 713
Location: Portland, OR




They're not munging up their signal. They've purposely allocated a lower bitrate than what is necessary for the content. They may be using the extra bits for subchannels, they may be expecting to add subchannels in the future, they may be getting a lower bitrate feed from their network, or they may be using an old MPEG encoder that doesn't compress motion as well as newer ones. Even at the maximum bitrate you'll still see artifacts in the worst cases like strobe lights. If's an unfortunate fact of life with MPEG.

Very very few HDTV stations send HD at the maximum ATSC bit rate these days. Most have an SD subchannel or two.
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PostPosted: Wed Apr 19, 2006 2:01 pm Reply with quote
Bigchris
 
Joined: 18 Apr 2006
Posts: 15




Sorry Scott, maybe "compromising" would have been a better term. They do in fact have a couple of low-def subchannels, but what I was attempting to convey is that these folks know what they are doing and would make every effort to produce the highest quality picture possible on their main HD channel. Their main competition in town is running 720p and I'm sure these folk's intent is to provide a better viewing experience than the competition, but I am not seeing that when motion is depicted.

I had noticed that they also have a weather channel that is actually broadcast on a subchannel of a sister subsidiary station on a different frequency. That is probably an indication of their unwillingness to further compomise their HD channel's picture quality.
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PostPosted: Wed Apr 19, 2006 2:26 pm Reply with quote
Scott Larson
 
Joined: 15 Oct 2003
Posts: 713
Location: Portland, OR




One could argue that if the station has two subchannels on the same channel as their HD, they aren't making every effort to produce the highest quality HD possible, despite the positive impression they're giving you. If they were, they wouldn't be wasting HD bandwidth on subchannels. In fact even the maximum bit rate for ATSC is often not enough to prevent MPEG artifacts at HD resolution. Adding subchannels, even one subchannel, makes it worse. They probably put the weather subchannel on another station because a third subchannel would have made all of them unwatchable.

You can go to the Local HDTV Info and Reception section of the AVS Forum and find out if other people in your area are having picture quality problems. You may be surprised to find how little some stations care about HD at this point.
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PostPosted: Wed Apr 19, 2006 2:44 pm Reply with quote
Bigchris
 
Joined: 18 Apr 2006
Posts: 15




Scott,
You're tough, but you're right of course. Maybe I could have qualified what I said with something like "within the bounds imposed by running a successful commercial venture", but we're already on the same page. Smile

I'm still going to be scrubbing my deinterlace settings tonight because what I'm seeing is downright *ugly* but I'll also have a look at the link you provided. Thanks again for the help!

Chris
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PostPosted: Wed Apr 19, 2006 3:06 pm Reply with quote
Scott Larson
 
Joined: 15 Oct 2003
Posts: 713
Location: Portland, OR




If you can post screenshots, we can probably identify which ones are MPEG artifacts and which ones are something else. Wait, I have some good ones already. These came from a station broadcasting 16 Mbps with one 2 Mbps subchannel:

Blockiness and false colors with motion:





Pixelization causing a rainbow of false colors:



Note that this same station turns off the subchannel during football and basketball games and the picture quality is near perfect.
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PostPosted: Wed Apr 19, 2006 7:33 pm Reply with quote
Bigchris
 
Joined: 18 Apr 2006
Posts: 15




Scott,
Your first shot looks familiar, things get really bad when the camera pans. I'm not quite up to screenshots with Linux yet. My first attempts to snapshot in xine gave me half-height pictures and I am guessing that what I was seeing was just one field instead of the entire frame.

Your link yielded 153 pages of posts from my local area including posts from the chief engineers of some of the local stations so that looks to be a gold mine. The station that I'm strugggling with appears from the posts to be the local gold standard. I introduced myself and gave a brief account of my problem and the local Mythtv people appear hungry for company (it's an affluent area, why make what you can buy?) so I expect I'll be hearing from them.

Chris
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PostPosted: Wed Apr 19, 2006 8:59 pm Reply with quote
Scott Larson
 
Joined: 15 Oct 2003
Posts: 713
Location: Portland, OR




Your screen shots may be actually be twice the horizontal reslution instead of half height. Apparently that's how xine stores them. It's easy to resize them in gimp (that's what I do).

If that's the best station you have that I pity you. We have two stations that send full bitrate ATSC all the time and another that sends it during sports. Guess which channels look vastly better than all the others.

If something was wrong on your end, you'd see stuttering and dropped frames.
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PostPosted: Thu Apr 20, 2006 3:10 am Reply with quote
xyzzy
 
Joined: 12 Feb 2006
Posts: 225




What bigchris is talking about sounds a lot like normal interlacing artifacts. Do you only see this on the 1080i stations, and not on 720p? Try using one of mplayer's deinterlacing filters, like kerndeint, and see if it looks better.
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PostPosted: Thu Apr 20, 2006 8:31 am Reply with quote
Bigchris
 
Joined: 18 Apr 2006
Posts: 15




My bad, Scott's good! His first words were "If you have deinterlacing turned on then..." I doublechecked - it was off! That fixed the nasties!

I tried both Bob and kernel deinterlacing and both worked though both slightly degraded picture sharpness. I decided that I preferred Bob but the quality of source material at 2AM didn't lend itself to making that choice. Do you guys have a preference and why?

Scott,
It would be nice to have a full ATSC channel to view but the lack is somewhat balanced by the fact that I have 8 or 9 DT stations to choose from. The harsh reality is that each has the same amount of spectrum space to work with and their fortunes are based on the number of commercials they can air in a 24 hour period times the premium they can command based on viewer eyeballs. Good ATSC hardware and programming are expensive and my "best" station had to make a massive investment just to be in the game. I'm sure that the FCCs waffling on the transition date to DT is making them all crazy, and with all the competition out there, it's not a business I'd want to be in. The money has to come from somewhere, and so for now esthetics has to be balanced by profitability.

Long term, we may have full ATSC channels, but I'll bet they'll be based on the HBO business model, not FTA.

Chris
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PostPosted: Thu Apr 20, 2006 9:19 am Reply with quote
Scott Larson
 
Joined: 15 Oct 2003
Posts: 713
Location: Portland, OR




I do bob deinterlacing because it's free in Nvidia hardware. Also you can't do any other kind of deinterlacing in XvMC because there's no way to access the raw video after the hardware has rendered it.

No stations are making any money from subchannels, except for a few who have leased them out to USDTV and even those lucky stations have reported earning less than a thousand dollars a year from them. All stations in the US (with some exceptions) have had to invest in a separate digital station or cease broadcasting. Some stations see HDTV as the future and are planning for it to be their main signal in a few years. Others think HD is a novelty or annoyance and expect to get rich off of the subchannels. A few resent the very concept of digital broadcasting and are transmitting in 480i at the lowest legal power just to comply with FCC regulations and stay on the air.
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PostPosted: Thu Apr 20, 2006 9:44 am Reply with quote
Scott Larson
 
Joined: 15 Oct 2003
Posts: 713
Location: Portland, OR




Another issue with deinterlacing 1080i is that most forms of deinterlacing will display 30 frames per second instead of 60 fields per second. The frames are deinterlaced so the jaggies are gone but 1080i motion won't look as smooth as 720p because the display is being updated at half speed. Motion in HD video (sports, Letterman, Leno and so on) will look choppy, more like a movie than video. When the display is updated for every field, 1080i looks almost as smooth as 720p. Obviously this isn't an issue with filmed content.

I know mplayer and xine don't update the display for every field and I have patches for them (it only works with XvMC though). I think MythTV does because I'm pretty sure that's where I stole the code to patch xine and mplayer.
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PostPosted: Thu Apr 20, 2006 10:38 am Reply with quote
Bigchris
 
Joined: 18 Apr 2006
Posts: 15




I just caught a look at the noon news which is broadcast live in 16:9 HD. It looked fabulous! Last night I was looking at prerecorded reruns and I think that was why I was noticing less "crispness" in the picture.

I suspect that our choice of ATSC while most of the rest of the world is using DVB is going to hurt acceptance of DT by pushing our costs up. I'm reminded of France's SECAM which worked - but so what! If the production volume isn't there you either have to drive the price down with tax subsidies or it doesn't get wide acceptance.
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1080i playback - What does it take?
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