In 4:2:0, the horizontal sampling is doubled compared to 4:1:1.

In 4:2:0, the horizontal sampling is doubled compared to 4:1:1, but as the Cb and Cr channels are only sampled on each alternate line in this scheme, the vertical resolution is halved. The data rate is thus the same. This fits reasonably well with the PAL color encoding system since this has only half the vertical chrominance resolution of NTSC. It would also fit extremely well with the SECAM color encoding system since like that format, 4:2:0 only stores and transmits one color channel per line (the other channel being recovered from the previous line). However, little equipment has actually been produced that outputs a SECAM analog video signal. In general SECAM territories either have to use a PAL capable display or a transcoder to convert the PAL signal to SECAM for display.

Different variants of 4:2:0 chroma configurations are found in:

  • All ISO/IEC MPEG and ITU-T VCEG H.26x video coding standards including H.262/MPEG-2 Part 2 implementations (although some profiles of MPEG-4 Part 2 and H.264/MPEG-4 AVC allow higher-quality sampling schemes such as 4:4:4)
  • DVD-Video and Blu-ray Disc.
  • PAL DV and DVCAM
  • HDV
  • AVCHD and AVC-Intra 50
  • Apple Intermediate Codec
  • most common JPEG/JFIF and MJPEG implementations
  • VC-1

Cb and Cr are each subsampled at a factor of 2 both horizontally and vertically.

There are three variants of 4:2:0 schemes, having different horizontal and vertical siting.

  • In MPEG-2, Cb and Cr are co-sited horizontally. Cb and Cr are sited between pixels in the vertical direction (sited interstitially).
  • In JPEG/JFIF, H.261, and MPEG-1, Cb and Cr are sited interstitially, halfway between alternate luma samples.
  • In 4:2:0 DV, Cb, and Cr are co-sited in the horizontal direction. In the vertical direction, they are co-sited on alternating lines.

Most digital video formats corresponding to PAL use 4:2:0 chroma subsampling, with the exception of DVCPRO25, which uses 4:1:1 chroma subsampling. Both the 4:1:1 and 4:2:0 schemes halve the bandwidth compared to no chroma subsampling.

With interlaced material, 4:2:0 chroma subsampling can result in motion artifacts if it is implemented the same way as for progressive material. The luma samples are derived from separate time intervals while the chroma samples would be derived from both time intervals. It is this difference that can result in motion artifacts. The MPEG-2 standard allows for an alternate interlaced sampling scheme where 4:2:0 is applied to each field (not both fields at once). This solves the problem of motion artifacts, reduces the vertical chroma resolution by half, and can introduce comb-like artifacts in the image.

Adapted from content published on wikipedia.org
Last modified on January 22, 2020, 7:39 pm
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