Missing sync pulse and the club standard --- Why?
Posted: Tue Jun 19, 2007 7:03 am
I have been giving thought to various ideas of implementing adaptive, rock-solid sync control of the Nipkow wheel. I keep finding myself wondering how the club standard evolved and why the frame indicator is expressed in the manner in which it is.
My understanding is that the start of the frame (and of line 1) is indicated by a missing pulse. In my view, this makes frame sync more difficult and less deterministic than it could be. The sync schemes I've seen thus far involve using one-shots to project where the missing pulse would have been. Line one is thus sync'd to where we *think* it should be, not necessarily where it really is.
I think a better standard would have been to arrange for a missing pulse at the *last* line of the frame, not the first. The double-wide gap in pulses could be detected using one-shots, and used to "arm" circuitry intended to detect the real start of the frame. In this case, the start of the frame is indicated with a real event (the hard, rising edge of the sync pulse created by the wheels optical encoder), not the guessed-at timing of an event that never occurs.
I suppose the existing standard could be executed using the latter approach by simply moving the disk's optical detector to an angular position representing one line earlier. This would cause the image to be displayed as if we were sync'ing on the missing pulse, when in fact we were sync'ing on the first real pulse after the gap.
Any thoughts on this?
Pete
AC7ZL
My understanding is that the start of the frame (and of line 1) is indicated by a missing pulse. In my view, this makes frame sync more difficult and less deterministic than it could be. The sync schemes I've seen thus far involve using one-shots to project where the missing pulse would have been. Line one is thus sync'd to where we *think* it should be, not necessarily where it really is.
I think a better standard would have been to arrange for a missing pulse at the *last* line of the frame, not the first. The double-wide gap in pulses could be detected using one-shots, and used to "arm" circuitry intended to detect the real start of the frame. In this case, the start of the frame is indicated with a real event (the hard, rising edge of the sync pulse created by the wheels optical encoder), not the guessed-at timing of an event that never occurs.
I suppose the existing standard could be executed using the latter approach by simply moving the disk's optical detector to an angular position representing one line earlier. This would cause the image to be displayed as if we were sync'ing on the missing pulse, when in fact we were sync'ing on the first real pulse after the gap.
Any thoughts on this?
Pete
AC7ZL