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ac7zl wrote:Any thoughts on this?
Pete
AC7ZL
Steve Anderson wrote:Why do we have sync pulses at all? Steve A.
gary wrote:How would one implement DC restoration?
ac7zl wrote: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
gary wrote:Of course you only have to advance processing the signal by one sync pulse width and you have syncs at the beginning of the line and the frame pulse at the beginning of the frame.
The reason for using a missing pulse for frame sync is explained in:
"Start or Finish" by Doug Pitt: NBTV Newsletter Vol. 29 No. 4
gary wrote:In actual fact, by club convention, a sync pulse is added at the *end* of each line except for the last one where it is omitted. So the frame sync (missing line sync) *is* actually at the end of the frame.
Steve Anderson wrote:gary wrote:Of course you only have to advance processing the signal by one sync pulse width and you have syncs at the beginning of the line and the frame pulse at the beginning of the frame.
The reason for using a missing pulse for frame sync is explained in:
"Start or Finish" by Doug Pitt: NBTV Newsletter Vol. 29 No. 4
Here we're getting into the realm of symantics, the mechanics or electronics doesn't care what we call it, it just gets on and does its job. So should we.
Steve A.
Heck, sorry about the above, it's just the project manager coming out in me.
Did I hear someone mutter "Barstard"?
Steve Anderson wrote:gary wrote:In actual fact, by club convention, a sync pulse is added at the *end* of each line except for the last one where it is omitted. So the frame sync (missing line sync) *is* actually at the end of the frame.
So is the following correct?
Steve A.
AncientBrit wrote:If we adopted crystal lock some form of "sync" detection is still necessary on the received signal stream to determine flyback.
GL
AncientBrit wrote:I record NBTV digital files in 64x32 format to disc and ensure that each frame starts at 0 pixel 0 line and is exactly of 2048 bytes duration.
So on playback no form of sync detection is necessary, the frames are taken as 2k chunks and painted straight to screen.
Neither is there any need to write header info.GL
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