[mythtv-users] OT: LED or Plasma (was Advice on choosing a TV)

Andre mythtv-list at dinkum.org.uk
Sat May 29 10:09:33 UTC 2010

On 29 May 2010, at 08:47, Paul Gardiner wrote:

> Andre wrote:
>> On 28 May 2010, at 10:17, Paul Gardiner wrote:
>>> Andre wrote:
>>>> On 28 May 2010, at 00:00, Paul Gardiner wrote:
>>>>> Andre wrote:
>>>>>> Alternatively and much more simply are you using studio levels (vdpaustudio) out from MythTV? If not there is a scaling of the display output 16-235 scaled to 0-255, this introduces some small artefacts that a plasma display will accentuate.
>>>>> Now I have MythTv driving the TV, I'm getting results much darker than
>>>>> any other source - all the others being much the same. I.e., dark areas
>>>>> are darker. First thought when I saw this was that it might be
>>>>> the 16-235 to 0-255 scaling, so I put vdpaustudio in the filter box.
>>>>> It made no visible difference. Any other ideas? It would be really nice
>>>>> if all my sources were the same, so I can just calibrate once.
>>>> You should be able to see the difference vdpaustudio makes, I think we established that you have a critical eye ;-)
>>> Cross posts. Turns out silly Paul didn't realise that 0.23 was out.
>> Yessss, come back when you've upgraded, can't believe you can be subscribed here and not know about it!!
> Ok sorted. vdpaustudio did it. Now all sources look the same to me now.

Ok, great, it really sounded like video/pc scaling, glad I'm not going mad(er).

>> It's going to end up as RGB or something directly related for the display anyway but the reason the TV industry uses colour difference is that a slight level mismatch on a YUV signal creates a slight colour shift but black, white & grey are still exactly black, white and grey, and it doesn't look objectionable, whereas a slight level mismatch in an RGB signal gives a colour cast to everything and looks really horrible. Add to that Nvidia have a long and messy history of very slightly screwing up colour space conversions, some broadcast people refuse to use their cards and a realtime TV graphics system I use a lot VizRT often needs tidy up rework when switching to or from an Nvidia card, yes even with the $9k specialist cards, ATI usually get it right but vdpau wasn't invented or implemented by ATI so here we are.
> I didn't think it was quite that. I thought our sensitivity to average
> values of Y, U and V were pretty much the same, but our eyes resolve
> detail in Y far more than U and V, hence the U and V can be decimated
> in MPEG and put on a lower frequency carrier in analogue.

That too, it's a major factor in video compression but that's an issue for compression/ analog encoding and broadcast distribution I was referring more to the interconnect between player and display or anything where uncompressed analog video is sent over any distance. Nvidia commonly cuts corners when doing yuv to rgb or vice versa, Miranda make significant money selling broadcast adaptors for Nvidia cards used in TV graphics, even when the cards have supposedly broadcast grade outputs, I'm regularly asked to report on why the extra expense is necessary.

I'm not sure if that's what's going on but it would be good to prove it one way or the other.

did you try 576i again?

> The thing
> I was saying about color controls in TV I'm not sure about: it was
> just that I'd realised at some point that although naively you might
> imagine that a TV taking a RGB input could map it straight through to
> the "guns", but you'd still want brightness, contrast and color
> controls and probably the easiest way to implement them is going to
> YCbCr and back.

Many cheaper CRT TVs never had colour controls on their SCART RGB inputs for this reason.

Some rare CRT TVs used to have a colour difference drive straight to the guns, the tube would de-matrix by design.

>> I see varying colour casts in shadows with vdpau, I get a similar effect when I run my PS3 in RGB mode, playing the same video, in YUV mode the cast is gone.
> That's interesting. Have you played with the itu709/itu601 setting with
> that video?

Yep, it seems to do the right thing in auto on all my current from broadcast recordings, some other more esoteric broadcaster test clips it messes up but that's not surprising, don't know about DVD or Bluray/HDDVD rips as I don't have any.

The varying colour cast is the one perhaps fixable irritation with vdpau in MythTV that's left, other media players (except mplayer)  are far behind MythTV in my experience. The unfixable irritations will require a visit to the relevant broadcaster or production house with a baseball bat or other suitable cluestick ;-)


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