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NTSC TelevisionThis page outlines the expired technology of analog NTSC off-air broadcast color TV.
Understanding NTSC is relevant because it laid a framework for broadcast digital TV.
NTSC Color TV
NTSC engineers were confronted with a problem. A color television camera outputs
five video voltages: red, green, blue, horizontal sync, vertical sync. But broadcast bandwidth could not handle all five voltages as is. In addition, they were required to maintain backward compatibility with legacy black and white televisions. Their ingenious solution reduced and combined the five video voltages with a bit of algebraic magic, a dash of modulating gymnastics, and a clever manipulation of the human eye.
The NTSC solution began by combining an understanding of our sensitivity to light with the black and white luminance signal. Our sensitivity to light is not equal across the entire bandwidth of color. We are most sensitive to green light, less sensitive to red, and least to blue. From the engineer's point of view -- the green voltage was the most significant, red much less, blue least. They also understood if all color could be derived from a prismatic dividing of white light, then the reverse could produce the (black & white) luminance signal.
Therefore NTSC engineers produced a luminance signal with a mixture of the reduced green voltage, less red, and even less blue. The following algebraic expression was their first step in a recipe toward a workable broadcast signal:
Where Y = Luminance, R = Red, G = Green, B = Blue.
Y= (0.3R + .59G + .11B)The engineers had reduced the red, green, blue voltages and computed a black and white luminance voltage. But this first step was not enough to fit the video within their broadcast bandwidth.
Squeezing RGB into A&C
Their next step 'squeezed' the three abbreviated color voltages into only two color difference signals. They converted reduced RGB into A = (R -Y) & C = (B -Y).
The encoded RGB color would ultimately be decoded as follows:
0.3Red = (A)+(Y) .59Blue = (C)+(Y) .11Green = (R+B)-(Y)
To this point the NTSC engineers had produced a black and white luminance voltage 'Y' and reduced RGB color to A&C. The resulting Y, A, & C was defined as component video.
The engineers still had a bandwidth issue. Y, A, and C was still too large for broadcast.
Plus they still needed to deal with the horizontal and vertical synchronizing voltages.
Therefore the engineers' next step reduced A and C to I and Q.
I = 87.7% of A Q = 49.3% of CThe engineers finally had a bandwidth solution via three manageable voltages: I, Q, & Y. They defined I, Q, & Y as composite video.
Ready, Set, Broadcast
The next NTSC step converted composite video to RF (radio frequency) and generated the horizontal and vertical synchronizing signals.
The engineers assigned Y, I & Q to only two radio frequencies. Y was assigned an amplitude modulated frequency. I and Q were combined (90° out of phase) and
assigned a phase modulated frequency (similar to FM).
They hid the horizontal and vertical sync signals via a mathematical manipulation of the I and Q signals. I will not include the calculation here. You'll thank me later. In addition, the video's accompanying audio was simultaneously broadcast on a separate FM radio frequency.
The NTSC signal was ready for broadcast. A television station broadcast antenna initiated cycles of electromagnetic waves that stimulated electromagnetic voltages in receiving TV antennas. The receiving television tuners and video processors decoded the modulating RF voltages as follows:
- Separate Y from I & Q
- Calculate the Horizontal and Vertical synchronizing voltages
- Recover A&C from I & Q which equals component (R-Y) & (B-Y)
- Compute RGB from (R-Y) & (B-Y)
The revived five voltages, RGBHV, were then used to recreate the original images captured by the TV camera with frames and lines of illuminated colored pixels on a television screen.
|Ed's AV Handbook.com
Batting Practice for the AV Pro and a Primer for the Novice.
Copyright 2007 Txu1-598-288 Revised 2018