[Tig] Colour Correction, telecine or DI.

[Richard Jackson]

    RGB levels are traditionally "full range", i.e. Black = 0/0/0 and  
White = 255/255/255 for an 8-bit/component system. By extension, a 10- 
bit/component system would have Black = 0/0/0 and White =  
1023/1023/1023. This is fine for "inside a computer frame buffer"  
use, but when SMPTE sat down to map RGB transmission into their  
Serial Digital (SDI) standards they ran into the restriction that  
levels 0-3 and 1020-1023 are reserved codes (these levels can only be  
used to designated "TRS" or sync words). This isn't a problem with  
YUV transmission, which is defined as Black = 64 and White = 940: the  
reserved codes are just super-white or super-black and (in theory)  
don't happen very often. But for RGB full-range levels, it's tough to  
live without Black and White!

    So the SMPTE spec for SDI Dual Link transmission of RGB data  
calls for similar levels as YUV: namely Black = 64/64/64 and White =  
940/940/940. This allows for signal headroom above black and below  
white (just as current YUV levels), but places the reserved codes  
well away of "normal" levels. The only problem is that systems that  
use internal levels of 0-1023 are (understandably) reluctant to  
"squish" the dynamic range down to 64-940 (and potentially re-expand  
it on the other side) just to send a video signal across an SDI link.

    This means we have (at least) two "standards": SMPTE SDI, which  
calls for Black = 64/64/64 and White = 940/940/940; and "tradition",  
which leaves Black at 0/0/0 and White at 1023/1023/1023 (even though  
SDI links will clip Black and White to 4/4/4 and 1020/1020/1020).

    - Richard Jackson
      AJA Video






On Oct 12, 2006, at 8:12 AM, Bob Friesenhahn wrote:

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>
> On Wed, 11 Oct 2006, Steve Shaw wrote:
>>
>> It seems a lot of people don't understand the video levels for  
>> Dual Link, HD and data, and what that means for workflows - and  
>> this seems to be especially true of some manufacturers.
>
> I don't fully understand the nuances of video levels for Dual Link,  
> HD and data, and what that means for workflows.  Can someone please  
> describe the nuances here?  In particular, are there secret codes  
> stored in the below-black and above-white regions?  In the pure  
> digital realm, do these regions really need to be preserved for  
> post-production?  Lastly should the united states department of  
> homeland security be concerned about those secret codes?
>
> Bob
> ======================================
> Bob Friesenhahn
> bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/ 
> bfriesen/
> GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
>
>
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