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Old 06-23-2011, 08:35 PM
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Penthode Penthode is offline
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Location: Kitchener/Waterloo Ontario Canada
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This prototype I find incredibly interesting in light of the early development of the NTSC standard. Certainly, it should be possible to generate a 1952 CPA signal to run the set. I would certainly hope that the set not be modified from the CPA design if it indeed is still a CPA receiver.

Is the subcarrier crystal 3.89MHz?

I came across another interesting paper published in the March '53 Proceedings of the IRE. It is entitled "Generation of NTSC Color Signals" by Joseph Fisher and pertains to the earlier CPA specification.

It appears that for the NTSC '52 tests, field rate R-Y switching was employed. Vestigial sideband equal bandwidth R-Y and B-Y chroma channels were used. Color subcarrier was 3.89MHz and it appears only standard 1/2 line offset interleaving was used.

CPA at field rate as opposed to line rate probably was a better choice in 1952. The reason for CPA was to cancel phase errors in transmission and to facilitate vestigial sideband chroma channels without quadrature crosstalk. Because there was no way to electronically delay and combine alternate field or line information economically in 1952, the errors would be cancelled visually. This is very much akin to simple PAL in Europe. The field rate equivalent to "Hannover Bars" would result from transmission phase errors.

The real failure of the 1952 CPA tests therefore must have been due to the flickering akin to Hannover Bars and the 610 kHz chroma subcarrier to audio carrier beat (this was before the line and field rate was shifted to 15,734.26Hz and 59.94Hz respectively to ensure that the sound carrier was a multiple of the scan rates). In addition, the alternating R-Y axis would prevent proper 1/2 interleaving of the chroma within the luma.

Note that PAL later adopted the 1/4 line offset to help mitigate the interleaving problem. PAL also introduced a picture rate subcarrier frequency offset (25Hz) to further conceal the interference. (In hindsight, a 29.97Hz offset could have been added to help conceal the NTSC dot crawl. But the standard was already established and the simpler luma to modulated chroma arrangement in NTSC probably makes this unnecessary).

Last edited by Penthode; 06-23-2011 at 08:46 PM.
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