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HDTV and Myth -- Lessons learned
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Author:  Liv2Cod [ Sun Dec 19, 2004 2:49 am ]
Post subject:  HDTV and Myth -- Lessons learned


My system consists of these pieces:
    * Pioneer Elite 58-in rear-projection TV
    * Pioneer Elite A/V receiver for decoding AC-3
MythTv box consisting of:
    * Silverstone LaScala case LC-01
    * Abit AS8 motherboard (865PE chipset)
    * Intel 530 (3GHz) microprocessor
    * pcHDTV HD-2000 card
    * nVidia 5500 graphics card (PNY)
    * Pioneer DVD-RW
    * WD 250G disk drives (qty. 3) in LVM configuration


Long story:

I'm completely facinated by HDTV. I bought my set almost four years ago which qualifies me as an "early adopter" in this area. Soon after buying my set and signing up for DirecTV I began to feel despair. Most of the content was on the major networks, and DirecTV had none of that programming. The satellite local channels were really poor -- I mean REALLY poor quality. I was viewing most of my programs with really crappy signals recorded on my Tivo.

There had to be a better way.

When I learned about MythTv I was stunned. Here was THE answer. I was amazed that they had adopted the frontend/backend architecture and designed it around an SQL database. This was how a Tivo SHOULD be designed! I read through the MythTv installation procedure. Then I read it again. I let it sit a week and read once more. Knowing my nacent skills in Linux, I figured it would be sometime mid0-2005 before I had a working box.

I followed the links from the MythTv page to KnoppMyth. Now here was the Right Idea. Package Linux with Myth and all necessary components in a single CD. Pure genius! I figured my mom could set up a Myth box with this! :lol:

Along the way I discovered my target application, HDTV, was supported by Myth but in a very early stage. I optimistically began with an Athlon 2500 processor and Abit NF7-M mainboard. I still really like that board, but it turned out to be way underpowered for high def.

Why? Because hardware-assisted playback just doesn't work.

Browse the Gossamer threads and search a bit and you will find stacks of people who can't get XvMC to work, and only a few (if any) who can. The lucky few seem to be playing back less than full HDTV resolution. (I hope I'm wrong, Xsecrets, but I'm afraid your early success with XvMC will blow away like smoke in the wind when you start playing 1080i or 720p streams.)

Here are a couple of threads I began, which attracted really excellent input from the developers of HDTV code in Myth:

HDTV in Myth -- Is it a "myth?"
HDTV in Myth -- not a "myth" CONCLUSION

I have learned an enormous amount by reading the posts of Jarod Wilson, Daniel K., Brandon Beattie, Doug Larrick, and John Poet. These are the people who write and test the code that enable HDTV recording in Myth. I recommend anyone who wants to learn, start reading their posts on the myth-user and myth-dev mailing lists.

Joe Barnhart

Author:  ceenvee703 [ Tue Dec 21, 2004 1:15 pm ]
Post subject: 

Joe: saw your post on mythtv-user concerning interlacing and deinterlacing.

Too chicken to post anything about this over there, but over here my skin's been thickened up a bit. :)

I think the bottom line for best output from the HD3000 is similar to the situation for best output with standard definition to a TV, in this sense:

* If you're outputting interlaced video (whether via a TV-out on a video card to a standard-def TV, or 1080i and VGA/DVI/scan converter to a HDTV), the fields of your output video have to match EXACTLY the fields your TV is expecting... no resizing, no timing differences, no nothing. This is why the PVR-350's video output is considered to be very good: because it doesn't resize and properly times the fields of the MPEG2 files it's playing back.

* If you're outputting progressive video, or you can't get your interlaced output to match exactly as above, your deinterlacing scheme better be good. One of the last answers to your post over there mentioned that mythtv's deinterlacing schemes are lacking with respect to Dscaler and tvtime (which are using dscaler's routines).

My guess is that hardware HDTV tuners, and HDTVs that are running in a progressive mode (or that can only do a progressive mode, like plasma or LCD), have hardware-based schemes for deinterlacing video. They do it fast and they do it well. We as HTPC people are forced to throw processing power to attempt to accomplish the same thing, with varying degrees of success.

Author:  Xsecrets [ Tue Dec 21, 2004 2:05 pm ]
Post subject: 

and of course concidering that it takes (from what I've read) at least 2.6-2.8 Ghz minimum to just decode full res hdtv in real time that doesn't leave much room for even the fastest computers to do any deinterlacing.

Author:  Liv2Cod [ Tue Dec 21, 2004 8:25 pm ]
Post subject: 

I'm getting some good input over there.

I think my issue is somewhat rare since most people use DLP or LCD displays for HD now. My old clunky CRT rear-projection set cost me a fortune and I'm not ready to pitch it yet. It's one of the few devices that takes a native 1080i signal instead of a 720p signal.

The nuances of this process are pretty subtle. Look at the choices we have:

(a) 1080i source --> myth --> 1080i output device
(b) 720p source --> myth --> 1080i output device
(c) 1080i source --> myth --> 720p output device
(d) 720p source --> myth --> 720p output device

I'm only worrying about (a) and (b) cases, whereas most people worry about (c) and (d). In my cases, the deinterlace filtering doesn't seem to apply to me since my source is either interlaced or not, but my output device is always interlaced. I have experimented with turning on "deinterlace" and choosing "bob" but it looks identical with or without.

To further complicate things, the XFree86 driver takes the 720p signal from ABC and FOX and scales it up to 1080i for me. I'm not completely sure if this happens in the nVidia driver, but I suspect so. In my case, this scaling is OK but not spectacularly good. (I'd be happy with a software implementation of the Faroudja DCDi scaler here. :wink: )

The one case where you'd expect my setup would hit one out of the park is (a) where no scaling or deinterlacing is taking place. Still, I find the output strangely lacking in resolution in the vertical dimension. Football fields are a great test pattern since they have lots of nearly-horizontal lines which look like dashed lines at times on my set. When a continuous line becomes "dashed" I start to think, "hey, I'm missing some scan lines here!"

I just want 'em back.

Joe Barnhart

Author:  Xsecrets [ Tue Dec 21, 2004 8:46 pm ]
Post subject: 

well I'll be right there with you with the Rear Projection 1080i whenever I can get a working converter. Unfortunately my set only accepts component input for HDTV.

Author:  SnapperDragon [ Thu Dec 23, 2004 6:23 pm ]
Post subject: 

Yeah, I'm gonna try it also. I have a Sony 1080i rear projection I want to shoot vga to. I got the same converter that you said you couldn't get to work, X.

Author:  Xsecrets [ Thu Dec 23, 2004 10:54 pm ]
Post subject: 

lol well I'm sure in my case it's just actually a bad converter. I have heard of too many people having success with that mode.

I guess that's what I get for buying off ebay.

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