[tig] A Bit about Bits

Dave Corbitt david.corbitt
Thu Jan 29 01:36:55 GMT 2004


>>I just think the TIG is great and the first few bucks I make when I 
>>finish setting up our main TK suite at Cinelab will go to the tig.
>
>I know that video is represented in 4 bits for Luma and 2 bits each 
>for color difference I kind of assumed that those samples were the 8 
>bits we were all talking about, I think I am mistaken,  Is it that 
>each R,G and B are represented as 8 bits per channel (or 10) and the 
>4:2:2 is a separate representation with RGB being translated into 
>YUV?
>
>Bit Confused.
>
>-Rob-

Hi Rob,

Don't confuse bits with samples. Each pixel is made of a numerical 
value to describe it's amplitude. The accuracy of that number is 
expressed as a binary and the more bits per sample, the more 
accurately that amplitude can be expressed. In the world of your old 
MKIII machines, with a Digiscan 4, 4:2:2 was an 8 bit signal. That 
would be 8 bits for each sample of video, and each pixel is sampled 
in Luminance at full bandwidth and half bandwidth or half as many 
clock samples for r-y/ b-y or u / v . Sequentially this would create 
a chain of samples like this:  Y-u-Y-v-Y-u-Y-v........ etc. Later 
machines such as the URSA created 10 bit deep signals for their 4:2:2 
outputs to match more closely available recording formats and resolve 
the tonal scale a bit better. The original D-1 machines (ca. 1986) 
could only record 4:2:2 at 8 bits and that would be 8 bits for each 
"4" of luminance and for each "2" and "2" of color difference. Later 
digital tape formats recorded 10 bits (Digi Beta, etc.). More recent 
versions of URSA created dual link 4:2:2 and 0:2:2 pairs of signals 
to create full bandwidth YUV files that could be laid off to two tape 
machines running in synch or to a disc array. 4:2:2 refers to the 
relative bandwidth of the Luminance (4) and the two color difference 
signals running at half the Luminance bandwidth, thus the 2:2. The 
second signal (0:2:2) had no luminance signal but sent inbetween 
samples of color difference to fill in the missing gaps in the color 
samples thus doubling the color bandwidth to equal the luminance, a 
very important step forward for post production work with blue screen 
or other color mattes at the time.

These 4:2:2  numbers were originally chosen to represent a clock or 
sample rate of 4 times the color subcarrier frequency for luminance 
and 2 times for the two half bandwidth color difference signals. 
However, the actual clocking frequencies of all this is not actually 
4x and 2x Fsc, just approximately. And besides, who uses color 
subcarrier nowadays anyway in a digital and data world? Sometimes HD 
was written as 22:11:11 to keep the same shorthand as the old 4:2:2 
shorthand and graphically show the larger amount of info contained in 
HD over SD.

I hope this helps clear up a few things.
-- 
Dave Corbitt
Madison, NJ 07940





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