When is 4:4:4 not really 4:4:4.

The new Sony F3 will be landing in end users hands very soon. One of the cameras upgrade options is a 4:4:4 RGB output, but is it really 4:4:4 or is it something else?

4:4:4 should mean no chroma sub-sampling, so the same amount of samples for the R, G and B channels. This would be quite easy to get with a 3 chip camera as each of the 3 chips has the same number of pixels, but what about a bayer sensor as used on the F3 and other bayer cameras too for that matter?

If the sensor is subsampling the aerial image B and R compared to G (Bayer matrix, 2x G samples for each R and B) then no matter how you interpolate those samples, the B and R are still sub sampled and data is missing. Potentially depending on the resolution of the sensor even the G may be sub sampled compared to the frame size. In my mind a true 4:4:4 system means one pixel sample for each colour at every point within the image. So for 2k that’s 2k R, 2K G and 2K B. For a Bayer sensor that would imply a sensor with twice as many horizontal and vertical pixels as the desired resolution or a 3 chip design with a pixel for each sample on each of the R,G and B sensors. It appears that the F3’s sensor has nowhere near this number of pixels, rumour has it at around 2.5k x 1.5k.

If it’s anything less than 1 pixel per colour sample, while the signal coming down the cable may have an even number of RGB data streams the data streams won’t contain even amounts of picture information for each colour, the resolution of the B and R channels will be lower than the Green, so while the signal might be 4:4:4, the system is not truly 4:4:4. Up-converting the 4:2:2 output from a camera to 4:4:4 does not make it a 4:4:4 camera. This is no different to the situation seen with some cameras with 10 bit HDSDI outputs that only contain 8 bits of data. It might be a 10 bit stream, but the data is only 8 bit. It’s like a TV station transmitting an SD TV show on an HD channel. The channel might call itself an HD channel, but the content is still SD even if it has been upscaled to fill in all the missing bits.

Now don’t get me wrong, I’m not saying that there won’t be advantages to getting the 4:4:4 output option. By reading as much information as possible from the sensor, prior to compression there should be an improvement over the 4:2:2 HDSDi output, but it won’t be the same as the 4:4:4 output from an F35 where there is a pixel for every colour sample, but then the price of the F3 isn’t the same as the F35 either!

460x150_xdcam_150dpi When is 4:4:4 not really 4:4:4.

2 thoughts on “When is 4:4:4 not really 4:4:4.”

  1. Not true. It all depends on the number of photocells in the sensor and how it relates to the output resolution (raster). The fact that area occupied by R/B is smaller than G only affects the fill factor.

    1. Yes and no. Unless there are at least 1x B sample, 1x R sample and 1x G sample for every pixel of the final output resolution then R and B with a Bayer sensor are under sampled compared to G. To not have a difference in R,G and B resolution would require a sensor with twice as many H and V pixels as the output resolution so that each output pixel is represented by all 4 pixels from the GB,RG matrix. But this is not what manufacturers are doing. They are using a sensor with for example 4k x 2k pixels and calling that a 4k resolution sensor. You can interpolate between pixels as much as you want, but if there isn’t a sample in the area you are representing then that data will be missing and what is actually there can only be estimated, thus any true detail will be lost.

      Looking at a bayer sensor you have a row of pixels that are GBGBGBGBGBGB and the next line down is RGRGRGRGRGRGRG. So the first line has NO red samples and the following line has NO blue samples, so the B and R vertical resolution is half that of G. Horizontally there is one G pixel for each B and for each R. Luma information is derived mostly from the green pixels (which is why there are more of them) but also uses some information from the R and B. In addition as the G pixels are diagonally adjacent pixel interpolation is more accurate. Because of all this the luma channel can achieve a resolution not far removed from what it would be if it was a monchrome sensor. Color though is far more problematic as the filters are not all that strong and there is a lot of leakage. This leakage alone reduces the effective chroma resolution, on top of that trying to extract accurate colors requires extensive interpolation (in part due to the cross color leakage) with adjacent same color pixels. As these are always at least a full pixel away from each other the resolution is reduced further. The end result is that chroma resolution is significantly lower than luma and accordingly G resolution is higher than R or B.

      Now IF you use a lot more pixels than your desired final image resolution then this might not be an issue. Raise the sensor resolution so that you achieve full resolution in the B and R channels are you’ll be OK, but that means double the H and V pixel count, which is not what camera manufacturers are doing.

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