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Research Topic: DV Colour Space

By Ian Mander, 13 June 1999.

Question: How come a DV stream is 3.5MB/sec and the compression is fixed at 5:1 when uncompressed RGB digital video is about 31.6MB/sec?

Answer: PAL (and NTSC) video does not use the RGB colour space. The colour space it does use needs less bandwidth (bytes of data per second).

RGB (also called "true-colour") has 8 bits of colour information for each of the red, green, and blue colour channels. This makes a total of 24 bits or 3 bytes. A PAL frame is 768 x 576, and being 25 frames per second makes 3 x 768 x 576 x 25 = 32400kB or 31.640526MB.

Normal PAL video uses the YUV colour space. Y is luminance while U and V are different chrominance components. U and V each use half the bandwidth that Y uses.

However, D-1 component digital video equipment employs YCbCr coding. Y is luminance. Cb and Cr are different chrominance components.

International standard CCIR-601-1 specifies 8-bit digital coding for component video with black at luma code 16 and white at luma code 235, along with chroma in 8-bit two's complement form (centered on 128 with a peak at code 224). This coding has a slightly smaller excursion for luma than for chroma; luma has 219 risers compared to 224 for Cb and Cr. The notation CbCr distinguishes this set from PbPr, where the luma and chroma excursions are identical.

To cut a long story short, fancy compression methods mean that not as much disk space is needed for the same amount of video footage when it is in its native video colour space.

RGB to YCbCr conversion:

Y = 0.299R + 0.587G + 0.114B

Cb = -0.1687R - 0.3313G + 0.5B + 128

Cr = 0.5R - 0.4187G - 0.0813B + 128

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