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This is worth learning as an excellent example of electrical engineering.

  1. NTSC: Never Twice the Same Color National Television Systems Committee.
  2. B&W TV:
    1. Designed in late 20s and early 30s.
    2. No frame buffer: analog signal is received and displayed in real time.
    3. The biggest contributor was Philo T Farnsworth, a Utah farmboy who got the idea for raster scan from plowing his fields. Ref: David McCullough on The American Experience.
    4. Farnsworth then was in a long fight with RCA, the 1930s analog to Microsoft.
    5. A losing competing method was John Logie Baird's mechanical TV, using a spinning disk with holes in front of the CRT.
  3. Color TV specification requirement:
    1. Add color to B&W NTSC.
    2. Upward compatible.
    3. Downward compatible.
  4. Useful properties:
    1. People see color info less precisely than B&W info.
    2. People see blue-yellow color less precisely than red-green.
    3. In typical images, a scan-line is much the same as the previous one.
  5. Engineering implications:
    1. We can lo-pass filter the color.
    2. We can especially reduce the blue-yellow resolution.
    3. The fourier transform of the intensity looks like a comb.
  6. Solutions:
    1. Rotate (R,G,B) to (Y,I,Q).
    2. Lo-pass filter I to 2/3 of normal bandwidth.
    3. Lo-pass filter Q to 1/3.
    4. Combine sort-of like this (the real equation is messier): I cos omega t + Q sin omega t
    5. Shift the combined color signal up in frequency by 1/2 the frequency of one scan-line and add to the Y.
  7. Limitations:
    1. Sudden intensity changes cause false color aliases.
    2. Sudden color changes cause Dot_crawl (false intensity aliases).
    3. Color changes that are not filtered properly are especially bad.
    4. Interference causes phase changes causes color shift.
  8. Trivia:
    1. The actual frame rate for color TV is 30/1.001 = 29.997 Hz.
    2. This reduces a beating interference problem.
    3. The first color TVs used discrete resistors, capacitors and inductors, w/o any integrated circuits or memory.
  9. Solution to color NTSC limitations: HDTV.
  10. Foreign variants to NTSC:
    1. PAL (Phase Alternating Lines) on alternate lines reverses the color-encoding phase. A systematic phase error then causes opposite hue errors on alternate lines, which partly cancel. Better sets store the previous line and average hues before display. The resulting color is better.
    2. SECAM (System Essentially Contrary to the American Method) (System Even Crappier than American Method) (Shows Every Colour All Murky) (Systeme √Člegant Contre les AMericains) (Sequential Color with Memory) transmits the red color on odd lines and the blue on even lines. Excellent ref: Wikipedia
    3. There are many variants, depending on the broadcast frequency, bandwidth allocated to video, and audio, whether the audio is above or below the video, whether the audio is modulated AM or FM, etc, etc.
    4. Sometimes standardization is not considered desirable. East Germany's TV format was deliberately incompatible with West Germany.
  11. Excellent refs:
    1. Wikipedia
    2. How Stuff Works