Quote:
Originally Posted by tubesrule
Terry,
The CPA chroma frequency was defined as exactly 495/2 * Fh. Using the original Fh of 15,750.00Hz yields exactly 3,898,125Hz. This also means that there was an exact relationship between the phase of the chroma carrier and the line frequency (i.e. the phase and position of the chroma carrier will be exactly the same on every other line). From what I have read there was no half line offset. That's something that came later. Of course anything can be done now, but I would recommend putting the set back to as close to original as possible, warts and all, to verify what they saw during the original tests. This will result in a stationary, visible pattern but that's what they would have seen.
So using the current NTSC line frequency of ~15,734.27Hz and the same 495/2 yields ~3,894,230.77Hz. This means that to maintain the same locked phase relationship between the chroma carrier and the line, we would have to use this new frequency. If we wanted to use the original 3,898,125Hz, we would loose the locked phase relationship between the chroma carrier and the line. This will change the visible dot pattern.
The issue with the SCRF converter is there is no frame memory, so rate conversion cannot be performed. The line output timing would match the line input timing, or 15,734.27Hz. With the WC converter the output is complete independent of the input and anything can be done.
So is there a design reason to have the chroma locked to the line, or does it even matter with this set? Do we want to observe the set exactly as it would have appeared during the tests? Since a crystal will need to be cut it could easily be cut for either frequency, or both could be cut and tried. To the converter it's just firmware so it doesn't matter. This will ultimately be Nik's/Steve's call assuming they take the set all the way to operation on CPA.
Darryl
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Hi Darryl,
The shift from the horizontal rate of 15,750Hz to 15750 x 1000/1001Hz was only to accommodate the 4.5MHz sound carrier. Hence shifting the subcarrier to accommodate the slightly lower scan rates should be inconsequential. I trust you agree the frame rate conversion is not necessary.
The half line offset I referred to as an odd number multiple of half the line rate and is what is used for the current NTSC system.
PAL uses a quarter line offset so that there is a quarter cycle less of subcarrier per line. This means that when the R-Y axis reverses line by line, there will not be the chance of vertical dot line up.
I have not found any information that CPA used a quarter line offset but I included in my calculation anyway.
My thoughts are this: a fixed relationship between CPA subcarrier and the scan rates is desirable but not essential. The frequency response of VHS tape is restricted so that there is no line locked relationship to fsc in VHS tape and it looks pretty good. If however wider luma bandwidth is desired (through comb filtering the source video) then maintaining a locked frequency relationship.
The quarter line offset (as used with PAL) I believe applies to CPA. But is this correct? (It requires some thought) And would the visual field averaging require the quarter line offset? Perhaps it was not used or was not necessary?
Perhaps the NTSC in 1952 was preoccupied with the edge flashing artifacts and so did not get around to thinking about CPA interleaving in greater detail? If that is the case, I would proceed with CPA fsc = 495 x 15750/2 x 1000/1001.