In order to try and clear up some confusion and help people understand why "4:4:4" notation (chroma subsampling notation) is inappropriate when talking about the Red, I've written this. Thanks to Graeme for his comments and suggestions. Hope it helps.
29.Oct.2008: I've updated it with a few changes based on people's responses.
Why “4:4:4” has nothing to do with the Red One camera
First, a bit of history. Originally, there was black & white television. Then came colour. To accommodate black & white televisions, colour was added to the original luma (brightness) signal through a new chroma (colour) signal, thus keeping colour separate and black & white televisions viable. In addition to making sure that black & white televisions were still useful, this was also an intelligent division of the visual data because less bandwidth was used for the chroma signal, which the human eye is much less sensitive to compared with luma.
In the digital world, the standard for separating luma and chroma is the Y'CbCr colour space – Y’ represents luma (gamma-corrected), Cb and Cr are used for blue and red channels. The green colour information is derived mathematically. As stated above, the human eye is less sensitive to colour information than brightness information. Naturally, digital video could also use this fact to save bandwidth. In digital video, the process of reducing chroma information translated into subsampling – that is, decreasing the resolution for the chroma channels.
Analogue television used much less bandwidth for the chroma signal than the luma – approximately one quarter. So the effective ratio of bandwidth would be 4:1, with the luma signal using four times the bandwidth of the chroma. The same effect is achieved in digital video through subsampling – reducing the resolution. Due to the original, analogue numbers, subsampling uses a “4” to describe a full-resolution (luma) channel. To achieve subsampling the chroma resolution is cut to one half or one quarter. Chroma subsampling notation describes this as 4:2:2 and 4:2:0 (or 4:1:1), respectively. The first number is the luma channel and the second and third numbers are chroma channels (Cb and Cr). Normally, this describes the horizontal sampling per scan line. However “4:2:0” is a special case that indicates the chroma channels are halved both horizontally and vertically. So, a subsampled 4:2:2 or 4:2:0 image will either have lost half or three quarters of the original chroma data.
Y’CbCr is a way of storing colour visual information – that is, a colour space. RGB is another colour space. RGB, of course, stores the information in a completely different way. Instead of separating luma and chroma, all the information is derived by mixing red, blue and green channels. As RGB is not designed to separate the chroma information, it is not suited for chroma subsampling.
A CMOS image sensor most often uses a Bayer-pattern filter to produce colour information (like Red's Mysterium). The Bayer-pattern filter divides the light into red, green and blue (like RGB), so that each photosite captures one of the three colour channels. However, since a photosite only captures one channel and yet correlates to one pixel in the output, the RAW data saved is not RGB. It is Bayer-patterned RGB, which can be called "GRGB". The two G's in GRGB refer to the fact that there are twice as many green photosites as red or blue. Green is favoured because the human eye is more sensitive to green light. To get an RGB image from GRGB data, RGB data has to be re-constructed through a process called “demosaicing”. There are several approaches to this and getting the best result is both a science and an art.
Demosaicing and subsampling are performed in completely different colour spaces, GRGB and Y’CbCr, respectively. They are also two very different processes. Subsampling reduces bandwidth by reducing resolution. Information is lost in that process. Demosaicing, on the other hand, is a method of interpolation in which none of the existing data is compromised.
The Red One does not perform chroma subsampling, nor does it operate in Y'CbCr colour space. The RAW data captured by the camera and stored as Redcode RAW is Bayer-patterned RGB (GRGB) and this data is demosaiced into RGB data. It is therefore meaningless to use subsampling notation when discussing the quality of the Mysterium sensor, the quality of native Recode RAW footage or demosaiced Redcode footage. It would only be appropriate if the image data were later converted to Y’CbCr colour space, in which case, the user has complete control of the quality.