[mythtv-users] Slightly OT? - Video Quality

Rich West Rich.West at wesmo.com
Wed Feb 21 17:44:48 UTC 2007


Greg Kettmann wrote:
> Does (or can) MythTV Hardware provide a "professional" quality image 
> (same as expected from TiVo, etc.) and, if so, must you buy high end 
> components? (Note, current tests were done under Windows and I've 
> returned the output/display card since I was unimpressed).  I'm looking 
> for a sanity check here, before I start again. 
>
> Here are the, rather long, details.  
>
> I've been reading this forum and asking questions for 6 months or so.  I 
> decided to take some baby steps toward building a unit.  It's been a 
> disaster, but all HW and not specific to MythTV.  It's a matter of 
> finding (working) suitable hardware, _with_ an AGP slot.  Being a 
> typical geek I've a few systems floating around but so far nothing I 
> have will work.  I finally grabbed my main desktop machine and ran some 
> tests using it. 
>
> I bought a video card which supports HDTV out, specifically it has a 
> dongle for Component (YPbPr) (an inexpensive implementation of the 
> GeForce 6200-  Leadtech A6200TDH).  I've a 3-4 year old Sony rear 
> projection big screen (53" - KP53HS10) that is capable of 1080i.  I have 
> a high quality 6 foot YPbPr cable which connected to the dongle (the 
> dongle is 6" and it would be tough to get to the TV with less than say 
> 4').  I was using Windows at this point, just playing around with 
> settings and presentation.  What I found was discouraging. 
>
> The settings for 480i and 480p seemed to do the same thing on the TV.  
> It seemed to be 640x480.  It suffered from what I think is called 
> "overscan" where the "picture" was bigger than the screen and I couldn't 
> scroll around.  There was about 10-15% of the outer edge which I 
> couldn't view.  More importantly, played videos (MPG's and AVI's) were 
> very dark when viewed with PowerDVD or MS Windows Media Player.  I could 
> turn up the brighness (in PowerDVD and WMP) but then it tended to be a 
> bit washed out.  Please note that I was using "downloaded" test video 
> segments (bit torrent), but they play just fine, with good quality, on 
> my laptop. Also, normal "Windows" brightness was just fine.  It was only 
> the played video that was dark. 
>
> The next "usable" (no TV support for 720i or p) was 1080i.  There was 
> significant "jitter", kind of an interlacing thing with the image 
> jumping up and down slightly (in Windows).  No overscan.  Windows screen 
> text nearly unreadable (small and slightly blurry).  I was able to play 
> videos, again through PowerDVD and Windows Media Player and again they 
> were quite dark.  The downloaded (supposedly HDTV) video tracks were 
> weak at best and certainly no better than what I saw with the regular 
> 480x stuff. 
>
> So, first observation is that my TV, although rated for 1080i, isn't 
> really up to the task.  Not too big of a deal.  I just means I forgo 
> HDTV for awhile.  The new TV's are much better and support DVI anyway. 
>
> The video quality is an issue though.  Is the "darkness" (of played 
> videos) caused by the video card?  Normal Windows stuff was fine.  It's 
> only the played video whether in a session or full screen.  Might 
> another card provide better results?  Does anyone have any practical 
> suggestions for a card (as in, I have a card with YPbPr output, which 
> I'm using, and it works well, the card is...)
>
> How about the "overscan"?  Are there settings in MythTV to fix that?  It 
> appears I'll need to be using 480i or p but not until I can get around 
> the overscan.  I know there have been topics on that so I'm confident 
> that particular issue has been discussed on these boards. 
>
> I just wanted a sanity check on getting started and so far it's not gone 
> well.  I think I'll give up on HDTV for now and plan on just building a 
> good HD Front End later.  For best quality I think I still want YPbPr 
> output since I've never been impressed with SVideo or Composite. I'm 
> just looking for a moderately priced way to start working with MythTV, 
> but so far I can't even find suitable hardware that gives results 
> equivalent to "regular" TV. 
>
> The next step will be a capture card.  I'd figured on the PVR-150 but 
> those aren't conveniently available at the moment and people seem to be 
> getting the replacement card, which has no Linux Drivers available.  
> There is plenty of discussion on capture cards here. 

Sorry for this being so long, but you had a lot of things to comment on
in your message. :)
I have a similar setup: older rear projection TV with component input.

I'll say right off that the Tivo doesn't provide a "professional"
image.. it just has a good MPEG2 decoder. :)  The overscan is still
there.. you just never really noticed it before.

Direct from nVidia's site
(http://www.nvidia.com/object/analog_component_out.html):
"For component out, NVIDIA determined through market research that 15%
overscan is common with many TVs (both SDTV and HDTV)."
and, since one would want to correct things by attempting to adjust the
overscan by doing the reverse (underscan), that page also has a comment
on that:
"Underscan correction always reduces the quality of the video being
watched since it changes the size of the native video resolution by
compressing the size of the pixels."

Unfortunately, because of the nature of displaying via standard
television inputs, you are going to get overscan.  How much, really
depends upon the set.  For example, my 27" CRT has barely any overscan
(although there is some), but my rear projection (56") has a huge amount
of overscan.

Now, connecting to any modern TV with VGA or DVI is the absolute best
method for handling this, since, with that cabling, the TV appears, to
the computer, as if it were a computer monitor, allowing for better
adjustment and picture.  My LCD TV, because it is connected via VGA,
acts exactly like a monitor, which, by design, does not have any
overscan. :)

However, since it sounds like your situation is similar to mine, you
don't have that option.  Component is really your best bet for the best
picture quality (check out
http://www.mythtv.org/wiki/index.php/ComponentOut) since you can pump >
480i over it.  S-Video and composite are limited to 480i signals.

With that in mind, there are issues to consider with upscaling any of
the images.  For example, regular TV is broadcast at 480i, and, if you
try upscaling it at all, you will have artifacts plain and simple. 
Always keep in mind the quality of your source: 480i is the worst
picture quality, and "upscaling it" won't make the picture better
(really) and, with some effort, it will just make it more tolerable. 
Standard DVD's are 480p.  Upscaling standard DVD's isn't as noticeably
bad as upscaling SDTV since the picture is already progressive.

Other things to consider are:
o If you are just using analog cable, you're getting the worst possible
signal from your service provider which will directly impact your
viewing pleasure when you insert a PVR (depending on your service
provider, of course).
o If you have digital cable, then it is best to use the higher-quality
video connection between the cable/sat company's set top box and your
capture card.  Coax, it seems, will give you a worse image than
composite, or even better, s-video.
o If you are using a digital set top box, keep in mind that your set top
box is receiving the video in a compressed format (eg: MPEG2),
decoding/decompressing it, and sending it out the appropriate video
connection.  With your PVR, you are now capturing and re-encoding that
image (probably in MPEG2 format again) to be displayed later.  All of
this compression and re-compression does have a negative impact on the
resulting image (although, this can be kept to a minimum).
o Some cable companies, with digital cable, provide a 1394 (firewire)
port which spews out raw MPEG2, which eliminate the need for a capture
card on the PVR.
o Choosing the right theme can help hide the visibility of the overscan
in the menu system.

Since you stated you'll deal with HD later, then you best bet might be
to just display everything at 480p, and, using MythTV's de-interlacing
method (using the "bob" algorithm), you will be happy with the results:
TV will look ok scaled up to 480p and DVD's will look as they should. :)

I personally haven't noticed anything being any darker than usual. 
However, I never tried things on the Windows side, so I can't say for
certain.  Additionally, using my nVidia 6200 with mythtv de-interlacing
everything, SDTV looks good and DVD's look great. :)

I've migrated off of our DirecTivo to MythTV, and I haven't looked back.
:)  And the WAF (wife approval factor) has remained high. :)

-Rich


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