Chroma Sub-sampling Demystified

Affordable video sources convert RGB video from the camera chips to Y’CbCr components and sub-sample - reduce the resolution of - the two chroma components, Cb and Cr relative to the luma to reduce the amount of data needed to record the video. Although in normal use, this this reduction in resolution is not always visible, upon colour correction, it can be made more prominent, and needs to be dealt with.

There are two main ways in which chroma sampling artifacts can be dealt with - smoothing and sharpening/reconstruction.

Some codecs, like Apple’s DV codec for DV NTSC 4:1:1 video, don’t smooth the chroma, leaving it blocky looking. This is accurate for work in the DV codec environment, but is not good for viewing, special effects, compositing or colour correction. Once the chroma is smoothed, it looks better, but it has no more detail than that with which it began.

Chroma sharpening/reconstruction techniques analyze the full resolution luma to help re-create what the chroma must have looked like before it’s resolution was reduced.

4:1:1 chroma sampling Diagram (before smoothing)

4:1:1 chroma sampling after smoothing - no more information, but more pleasant to look at.



Smoothing plugins are supplied for 4:1:1 (use for NTSC DV), 4:2:0 (HDV, PAL DV, MPEG sources) and 4:2:2 (practically everything else). Even though HDCAM is 3:1:1, upon capture over SDI, the deck will smooth the chroma up to 4:2:2. Depending upon the exact 4:2:2 codec used, it may not be necessary to use 4:2:2 smoothing as the codec may do that itself. However, you can still use any of the smoothing options if you feel that extra smoothing is needed though.