[mythtv-users] Really washed out picture since upgrade to 0.24.0

Andre mythtv-list at dinkum.org.uk
Sun Nov 21 15:36:09 UTC 2010


On 21 Nov 2010, at 15:18, Paul Gardiner wrote:

> On 21/11/2010 13:59, Dan Christensen wrote:
>> Can someone summarize the issues here regarding studio levels?  Maybe
>> someone can create a FAQ for myth about this?
>> 
>> - What are studio levels?
> 
> When YUV is converted to RGB, you get values from 0 to 255 for each gun.
> But not all the values are used. 16 is actually the blackest black, and
> 235 is the brightest white. Any values below 16 are called blacker than
> black, and I presume they are supposed to be clipped. Values above 235
> might be clipped, or might be shown brighter than 235 if the display
> has head room.
> 
> Most computers monitors interpret 0 as black and 255 as white, so most
> PCs expand the range 16-235 onto 0 to 255. If you drive a TV via HDMI
> like that you'll get too much contrast and you'll lose some of the
> darkest and lightest greys.
> 
>> - Where are the settings for nvidia and myth?  What are the defaults?
> 
> I don't know about the nvidia settings, but in myth, when using vdpau,
> you can add the filter vdpaustudio in the render setup to make the
> values 16-235 be mapped through unchanged.
> 
>> - What do most TVs do?
> 
> I believe most TVs expect studio levels on HDMI and full range on VGA.

Some have a setting for this, often called video or PC, other sets like the Panasonic Viera Plasma's and some Samsung LCD's only work with full range when in "game" mode, all other modes expect limited  or studio levels and will clip blacker than black and whiter than white, also called superwhite.

A good testcard will have a grey scale, this should look linear, a smooth graduation from black to white, if there is a section where white doesn't get any brighter and/or black doesn't get any blacker then you may have a display that expects studio levels, assuming brightness and contrast set correctly.

Some testcards (all BBC cards) also have five grey scale boxes, the black box will have a dark grey spot in the middle and the white box will have a light grey spot. If you have PC (full) levels driving a studio levels display those spots will not be visible.

> 
>> - How does this relate to YUV?
> 
> You don't have to worry about it for YUV.
> 
> Paul.

You beat me to it.

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