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I have a rather theoretical question.

I connected my Raspberry Pi to my Composite NTSC TV and observed that the terminal video output has 640 pixels horizontal resolution with nice solid clear colors for text characters and logo. However, NTSC composite can only reliable produce less than 190 horizontal pixels without introducing color blurs and artifact colors (see Composite artifact colors) because of the 3.58 MHz color carrier frequency used to decode color from composite signal.

I am puzzled how they can do this. Does anybody know the answer?

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  • \$\begingroup\$ Sorry, missed the link: en.wikipedia.org/wiki/Composite_artifact_colors \$\endgroup\$ – Andy Dec 12 '16 at 18:02
  • \$\begingroup\$ How old is your TV? \$\endgroup\$ – Andy aka Dec 12 '16 at 18:10
  • \$\begingroup\$ I have used a pretty new HD TV. I am going to try it on older CRT TV to see the difference. \$\endgroup\$ – Andy Dec 13 '16 at 15:47
  • \$\begingroup\$ Tried with CRT - not as good as with HD TV, but still pretty decent colors with 640 pix horizontal resolution. Not even close to the screenshot from the link. May be has something to do with interlace and comb filters. \$\endgroup\$ – Andy Mar 7 '17 at 16:37
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The early computers referenced in your link used relatively low-frequency clocks to approximate the characteristics of a proper chroma subcarrier. As a result, the colors were very rough and the choices were limited.

The composite output of the RPi (BCM2835 chip) uses a much more sophisticated modulator that produces a broadcast-quality subcarrier that a TV can decode for much better color resolution and fidelity.

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  • \$\begingroup\$ This may be true, but I am highly skeptical that it is possible to detect color phase for less than one subcarrier period, let alone for less than a half period and not confuse it with luminance. \$\endgroup\$ – Andy Dec 13 '16 at 22:33
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    \$\begingroup\$ I appreciate your skepticism, but it does work. I'm trying to figure out a way to explain it to you without getting into a long-winded description of the NTSC signal structure and the advanced DSP techniques that modern TVs can apply to decoding it, including things like 3-D filters (x, y, and time). \$\endgroup\$ – Dave Tweed Dec 13 '16 at 23:36
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    \$\begingroup\$ Dave, thank you for the update. I have a pretty good knowledge of NTSC signal and professional background in DSP, so if you can give me some references about modern TV color decoding filters, I will greatly appreciate that. \$\endgroup\$ – Andy Dec 14 '16 at 14:45

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