Distribution vs. Contribution Encoding: Part 2

Continuing the discussion of source vs. contribution encoders we began last time, we indicated that, while encoding essentially involves discarding some of the pixels in an image, source encoders typically retain twice the number of pixels for each frame of video as a distribution encoder. In industry jargon, source encoders are said to operate in 4:2:2 mode while distribution encoders operate in 4:2:0 mode.

The other difference we mentioned between source and distribution encoders, besides the number of pixels retained in an encoded image, is the number of bits used to denote colors in a pixel, known as “8-bit” color, “10-bit” color, and so on.

As the name implies, 8-bit color uses 8 bits to denote each of the three primary colors used to compose pixels: 8 bits for red, 8 bits for green, and 8 bits for blue – 24-bits in total for each pixel, or enough bits to distinguish 256 distinct shades of each primary color. Since any shade of one color can be combined with any shade of each of the other two colors, the total number of possible colors with 8 bit encoding is 256 x 256 x 256 = 16,777,216 possible colors. Since, even under optimal viewing conditions, the human eye can distinguish fewer than 10 million colors, 8-bit color is more than adequate to reproduce every color we can possibly see. For this reason, 8-bit color (24-bit pixels) is also known as “true color”.

But that is on the viewing or distribution side. On the source or contribution side, just as it is often desirable to retain twice the number of pixels needed to accurately reconstruct an image, it is often desirable to have significantly higher color resolution than needed for viewing. By allowing 10 bits to denote each primary color, the number of primary shades quadruples, from 256 to 1024, providing a total pool of 1024 x 1024 x 1024 = 1,073,741,824 colors.

While having a billion colors available does nothing to enhance the viewing experience – the human eye is simply incapable of resolving the tiny differences between the hundred nearest shades in a billion color pallet – it can and does help on the source side, when video is edited. That is because many editing steps are “lossy”, which is to say, the edited images contain less information than the original images. By starting with a billion color pallet, however, the editing losses are generally imperceptible. Reducing a billion color pallet to millions of color does not degrade the viewing experience, since the hundreds of fine distinctions lost are imperceptible to the eye. Whereas reducing a 16 million color pallet to thousands of colors does result in a clearly perceptible loss of color fidelity.

The same argument holds for the desirability of starting the editing process with twice as many pixels as actually needed for viewing. Losing half the pixels from a 4:2:2 image does not result in any significant loss of image quality. Whereas losing half the pixels from a 4:2:0 image does noticeably degrade the image.

At Telairity we support both distribution and contribution encoders and all our designed and manufactured in the US. Visit our website to learn more.

Distribution vs. Contribution Encoding: Part 1

As television technology continues to improve, broadcasters want to ensure that viewers are getting a crisp, clear image. This is where special contribution encoders come into play, as distinct from the normal sort of distribution encoder, used to render images for end-user viewing.

The main difference between the two is how color, the essential attribute of a digital picture element or “pixel” is treated. There are two separate issues here. One issue is the number of bits used to denote the color of a pixel, generally referred to as “8-bit” color, “10-bit” color, and so on.

The other issue is the number of pixels that are retained in an encoded picture. The terminology used here is a bit more obscure. 4:4:4 encoding means that 4 out of every 4 (i.e., all) pixels are retained in the encoded image.  As a rule, encoders do not support this mode, since the essential function of an encoder is to reduce the number of pixels needed to reproduce an image. 4:2:2 means that 2 out of every 4 pixels are retained in the encoded image. As a rule, this mode is only supported by source encoders. Distribution encoders instead support 4:2:0 mode, in which only 1 pixel is retained out of every 4.

Perhaps surprisingly, retaining just a quarter of the pixels in an image is sufficient to allow the image to very accurately reconstructed during the decoding process; indeed, few people can detect any difference between a 4:4:4 image, a 4:2:2 image, and a 4:2:0 image. As a consequence, for purposes of viewing, the extra pixels retained in 4:4:4 and 4:2:0 can be regarded as simple overhead, better dispensed with in a distribution encoder, since the fewer the bits that need to be transmitted or stored, the lower the cost of digital video.

Source encoders, however, are not designed merely for viewing pictures, and so the rules for them are different than the rules for distribution encoders. In our next blog, we will consider this issue in more depth, along with the issue of “8-bit” vs. “10-bit” color.