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Tomcomm, you are correct that the triad spacing does not limit the I channel resolution, but your arithmetic and nomenclature are confused.
TV resolution is not specified in line pairs, but in lines (film resolution is specified in line pairs). Furthermore, TV resolution is specified in TV lines per picture height, that is, a distance of 3/4 picture width. So, the correct number is (as you said) approx 80 lines (not pairs) per picture height per MHz, or 80x4/3=107 lines per picture width per MHz. Another way to calculate is the number of half cycles of 4.2 MHz per active scan line (~55 microseconds), or 462 lines of luma resolution. (This is a bit of an overestimate; a good rule of thumb is 440 lines per width or 330 per height.) The spacing of 664 triads per width means that the ratio of spacing to maximum luma frequency (if those sets had comb filters) would be 664/440 = 1.5. Thus, there is little chance of moire' caused by fine detail. In the actual sets with limited luma bandwidth, the ratio is a very generous 2:1. The I bandwidth is 1/2 the limited luma bandwidth, so it clearly can be carried by the screen structure without limitation.
The more important ratio in the 21 inch and later color tubes is the vertical spacing of triads compared to the line spacing, because there is always the possibility of a moire' pattern caused by the scan lines interacting with the triads.
Later smaller tubes, particularly the Portacolors, had insufficient numbers of triads to support full resolution.
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