[tubesrule]

Hi Terry,

Originally Posted by Penthode

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.

I believe shifting the CPA subcarrier to match the current line rate is the right thing to do. While it won't use the original subcarrier frequency, it will use the original subcarrier to line rate relationship and will provide for the original viewing experience.

Originally Posted by Penthode

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.

I had included in my prior response a section on how current NTSC and PAL formats are constructed but later removed it. This gets back to my previous comment about what is to be accomplished. (see below)

Originally Posted by Penthode

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.

I think your last comment is pretty close to how I feel. While we can certainly use some of the "tricks" applied to current NTSC and PAL to make the image look better, and while this would be a worthwhile exercise to see what could have been, my feelings are the initial design goal would be to recreate CPA exactly as it would have been during the trials.

From my point of view none of the specifics matter other than what the final target is. Since it's just VHDL coding and requires no physical hardware modifications to the converter, we can play with the specifics ad infinitum. (actually the FLASH memory is only rated for 100K write cycles )

Some impact would be imparted on the set as specific crystals would need to be cut for different trials. This is not expensive or difficult, but needs to be considered. Since the crystal is socketed it could be quickly replaced if you want original performance or "enhanced" performance.

If you could work through the specifics of what the best enhanced format would be, I can get it coded and then we would have both available for testing.

Darryl