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.
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