Videography with ILCs

Chroma Sub-sampling

If you look at the specifications of video recording in ILCs, you will find that the manufacturers are prominently describing that recording is done using 4:2:2 sampling. Though these numbers look a bit complicated it is an important point, specifying what is called as ‘Chroma Sub-sampling’. In brief this is what it means. A video stream has two components. First is the ‘Luma’ (symbol for this is Y’) and this defines the brightness part. The second is the ‘Chroma’ and conveys the colour information. Separating this way into two components allows more optimal use of resources.

Since the eye is more sensitive to variations in brightness than in colour, more resources are allocated to the former (Y’). This process or more precisely ‘encoding’, where less resolution is given to chroma information compared to brightness, is called ‘Chroma Subsampling’. Pictures 1, 2 and 3 show what this means. Picture 1 shows the Luma part with full resolution, and Picture 2 shows the Chroma component sampled at less resolution. Picture 3 shows how we perceive when both are combined.

In practice, the Chroma is in turn split into two components Cb and Cr. If Y’, Cb and Cr are all sampled equally, and the scheme is called 4:4:4. This gives the best quality but is very demanding on resources. On the other hand if the two chroma components are sampled only at half the frequency of the luma, then there is a substantial reduction in resources needed but with no apparent loss of quality. This encoding is called 4:2:2 and is used by most high-end video cameras. If the chroma sampling is further halved, we get the 4:1:1 sub-sampling but here quality suffers and is no longer considered suitable for serious work. It is only good for low-end consumer applications. Thus the 4:2:2 subsampling now being offered on some ILCs offers extremely high-quality video on par with professional equipment as far as this factor is concerned.