# Question re video output (RGB vs YCbCr)



## rcbray (Mar 31, 2004)

Does anyone know what video the HD Tivo outputs via HDMI for HD? Is it RGB or YCbCr color space 709. Does it output color space 601 for SD?

I would like to use my upconverting Denon (with Avia or DVE) to check the calibration of my HD Tivo input. The Denon will output either RGB or YCbCr for HDMI. I know there is a subtle difference and would like to pick the color space actually used by the HD Tivo.


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## SpankyInChicago (May 13, 2005)

rcbray said:


> Does anyone know what video the HD Tivo outputs via HDMI for HD? Is it RGB or YCbCr color space 709. Does it output color space 601 for SD?
> 
> I would like to use my upconverting Denon (with Avia or DVE) to check the calibration of my HD Tivo input. The Denon will output either RGB or YCbCr for HDMI. I know there is a subtle difference and would like to pick the color space actually used by the HD Tivo.


Googling seems to suggest it is YCbCr.


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## litzdog911 (Oct 18, 2002)

HDMI is a purely digital video interface. "YCbCr" is another name for the "Component Video" outputs on the HR10-250 and most other HD gear. It's analog. As is the older, less common RGB interface.


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## rminsk (Jun 4, 2002)

litzdog911 said:


> HDMI is a purely digital video interface. "YCbCr" is another name for the "Component Video" outputs on the HR10-250 and most other HD gear. It's analog. As is the older, less common RGB interface.


Y'CbCr is the digital version of Y'UV. The main difference is in the scaling of the chroma signals. Y'UV dates back before RGB for broadcast. This is because Y'UV is compatable with analog B/W televisions because the Y channel is very closely related the data from a B/W camera. HDMI 1.1 is Y'CbCr while HDMI 1.2 is either RGB or Y'CbCr. HDMI 1.3 supports high bit depth for each of the channels and also adds xvYCC.


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## litzdog911 (Oct 18, 2002)

rminsk said:


> Y'CbCr is the digital version of Y'UV. The main difference is in the scaling of the chroma signals. Y'UV dates back before RGB for broadcast. This is because Y'UV is compatable with analog B/W televisions because the Y channel is very closely related the data from a B/W camera. HDMI 1.1 is Y'CbCr while HDMI 1.2 is either RGB or Y'CbCr. HDMI 1.3 supports high bit depth for each of the channels and also adds xvYCC.


I stand corrected


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## TyroneShoes (Sep 6, 2004)

It actually has little to do with analog vs. digital, except that converting RGB to Y'CbCr is commonly done before digitizing, primarily because the luminance has more pixels than chrominance when digitized. RGB is of course "red, blue, green" and typically has no true luminance signal. Y is the designation for the luminance signal by itself. Cr is the difference between R and Y, and Cb is the difference between B and Y (luminance, red, blue, and green can all be resolved by reversing the matrix encoding).

Converting RGB to Y'CbCr gives a true luminance signal which can be digitized at a higher resolution, and the difference signals for chroma can convey the full color space with less original information than RGB, while allowing chroma to be digititized at a lesser resolution. HD is 4:2:0 video, meaning that four luminance pixels in the H and V dimensions have the same spatial real estate in the raster as 2 chroma pixels in the H dimension, with those pixels subsampled to represent the identical color information on each pair of pixels in the V dimension. IOW, luma is encoded at a higher rez than chroma.

That makes Y'CrCb tailor-made for encoding digital video, while RGB would be problematic and not compress anywhere nearly as efficiently. (BTW, the " ' " or "prime" indicates gamma correction of the Y signal).

But all of these different matrix encoding designations refer to a particular method of analog encoding of video either at a point _before_ digitizing, or decoding at a point _after_ converting back to the analog domain, so one is not truly "analog" while another is truly "digital". In fact strictly speaking, they are only applicable in the analog domain while their properties can continue through digital conversions.

That said, certain designations are used typically more commonly with digital, while others are used typically more commonly with analog, yet quite often the terms can be used interchangebly, regardless of domain. For instance, YUV may refer commonly to analog video, but is also commonly used to refer to digital video paths in Sony HDTV schematics, so it is not truly restricted to one domain or the other.

Matrix encoding schemes are one thing, while whether the video is in an analog or digital form, is quite another thing. They are not interdependent, and the domain does not limit the encoding scheme, although some schemes are practical for digital while others are not.

And that is the explanation as to why the answer to the OP's question is that the digital video on HDMI is not RGB.


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