Quote:
Originally Posted by NewVista
Is ATSC gamut limited in the encoder or could an expanded gamut be transmitted by "errant" program material that might be, say, conforming to a proposed WCG standard?
|
The limitation is partly in the encoder, and partly in the camera. Cameras are matrixed to match rec709 primaries. If the camera RGB values are not allowed to go negative or exceed 100%, then the gamut is limited in the RGB signals. If the RGB values are allowed to go negative, (and therefore the color difference signals go outside usual values) before coding, this is one way to carry expanded gamut. This is the basic idea of the xvYCC coding in the Blu-Ray standard.
[Edit:Hypothetically this could also be done in an ATSC signal.]
http://en.wikipedia.org/wiki/XvYCC.
The idea is that non-xvYCC aware receivers simply overdrive the positive R, G, or B and clip the negative R, G, or B, which produces some distortion, similar to that with NTSC sets using non-standard phosphors in the past. That is, colors outside the Rec709 gamut are reproduced with some excess brightness in Rec709 receivers.
Another way to handle different gamuts is to use metadata and profiles to send the expanded gamut with the same range of R, G, B values (zero to 100%), which is the way it's done for color-aware computer graphics programs; but existing Rec709 receivers don't do anything with profiles and therefore would produce desaturated color. This effect can also be seen on computer programs that are not color aware, for example, by coding a jpeg file as wide gamut and then trying to view it on Internet Explorer.