Y'CBCR 8bu/v 4:4:4 formats:
In the variant Y'CBCR 8bu 4:4:4 format, the chroma components are also 601-compliant, but the luma component has an unbiassed range of [0..219], leaving more headroom but no footroom.
Headroom and footroom are useful for filter slop during editing. However, they are by definition not representable as distinct colors on television monitors or computer displays, and must be limited to the nominal range before they can be viewed. Moreover, leaving headroom and footroom reduces the number of distinct displayable colors.
Note that SheerVideo also supports the corresponding Y'CBCRA 8bv 4:4:4:4 and Y'CBCR 8bu 4:4:4:4 formats, which contain an 8-bit alpha (A) component at each pixel in addition to the luma and chroma components. The alpha component, used during blue-screening and other compositing techniques, specifies how visible or how opaque the pixel is. In the 601-compliant Y'CBCRA 8bv 4:4:4:4 format, the alpha component has the same standard video range as the luma component: [16..235]->[0..1]. The Y'CBCRA 8bu 4:4:4:4 format has an 8-bit full-range alpha component, [0..255]->[0..1].
QuickTime 6 doesn't include an encoder for any of these formats, so to convert the RGB 8b footage to Y'CBCR 8bu/v 4:4:4, we wrote our own codec to translate between the RGB and Y'CBCR color spaces using the equations published in BT.601-4.
At three 8-bit components per pixel, the uncompressed size of each of these Y'CBCRA 8bv 4:4:4 frames is 3 x 768 x 512 = 1,179,648 bytes. Note that on the Macintosh, an uncompressed Y'CBCR 8bu/v 4:4:4 image is actually always represented with 4-byte pixels, as if it were Y'CBCRA 8bu/v 4:4:4:4, even though the 4th byte is unused. Thus to compare SheerVideo Y'CBCR 8bv 4:4:4 images to actual uncompressed Y'CBCR 8bu/v 4:4:4 images on the Mac, multiply the uncompressed sizes and the compression powers listed here by 4/3.
Inside QuickTime, the Y'CBCR 8bv 4:4:4 and Y'CBCRA 8bv 4:4:4:4 formats are known as 'v408' or k444YpCbCr8CodecType, with the components in the order CbY'CrA. Y'CBCR 8bu 4:4:4 and Y'CBCRA 8bu 4:4:4:4 are known as 'r408' or k4444YpCbCrA8RCodecType, with the components in the order AY'CbCr. Apple's JPEG encoder supports Y'CBCR 8bu 4:4:4 input. Apple's NTSC DV-25 codec supports both Y'CBCR 8bv 4:4:4 and Y'CBCR 8bu 4:4:4 for input as well as output.
By losing a couple of bits or more of image information per pixel through round-off errors, the conversion from RGB 8b to Y'CBCR 8b inevitably seriously degrades the image quality. But the video world tolerates this degradation because that's how color television works. And color television works that way because, when it was being invented in 1950, the National Television Standards Committee (NTSC), in defiance of the Federal Communications Commission (FCC), had the marketing foresight to make it compatible with monochrome television, which only uses the luma channel.