chris_vk3aml wrote:So far as I am aware DC restoration is a post-mechanical TV concept.
I find that hard to believe, help me out here Stephen, Baird MUST have a patent on it?;-)
Actually, whilst DC restoration may not have been around then the need for it would have been - it is there as long as there is an AC coupled interface.
chris_vk3aml wrote:So far as I am aware DC restoration is a post-mechanical TV concept.
I am not aware of any 30-line receiving circuit published in the 1930s that even hinted at the usage of DC restoration. Many black and white TV sets of the 1960s and early 1970s had no DC restoration either. These simply could not do a 'fade to black', they always faded to grey! DC restoration only became absolutely essential when the mixing of three colour video components were necessary.
It depends on your definition of DC restoration (and I am including clamping in that definition), but all AC coupled video systems must have it to some extent be it in the form of DC Biasing of amplifiers or AC light sources, otherwise, on average, half of all frames would be black! It may not have been obvious from my previous post but the method I have been using is really just a small improvement over simple DC biasing, and can also be thought of as a differential unity gain block. This serves to get a mapping to a pixel for all samples but does nothing about the variation in contrast as picture content changes.
(I refer to contrast here but it could be just as easily brightness depending on how it is being handled)
chris_vk3aml wrote:As Gary says, "purely looxury"! In the 1970s, those of us working in NBTV really did live in the proverbial video "paper bag in a septic tank"!
That includes me by the way, in fact you can include the 1960s... ( I suppose that tags me as a nerd, since I seem to be one of the few people able to remember it... the 60s I mean)
chris_vk3aml wrote:I'm not sure, but I think DC restoration was another A D Blumlein/Marconi-EMI concept, like flyback EHT, but it certainly was not universally applied in the monochrome days. Even today, I have a cheap little portable 5" monochrome set which my wife and I take on holidays up country, and it very obviously has no DC restoration.
Yes I was aware of that fact but my understanding was that it's exclusion was a cost cutting exercise rather than it not being necessary. It seems to me that the pictures on these sets should have looked at little like when that horrible macrovision copyright protection scheme when it is invoked.
chris_vk3aml wrote:One problem with DC restoration lies in reproducing NBTV recordings with 'ringing', overshoot or excessive treble boost, where the restoration may actually lock to the dark tips of video overshoot - a very odd effect when DC restore is applied.
Absolutely, and this is the very thing I first came across when processing video through my soundcard, I found that the video can actually sag below the level of the sync tips! In this case the algorithm needs to be smart enough to realise it is not in a sync-pulse.
chris_vk3aml wrote:Like gamma correction, DC restoration is one of those things that is a 'nicety' but which one can really live without. I find 32 line pictures without aperture correction, or with streaky rasters due to using a round scanning spot without spot wobble (in CRT case) or via using Nipkow disc with undersized circular holes far more annoying. However it WOULD be nice if we had it.
Hmmmm, I can't agree entirely when I see people's noses poking out of a sea of grey...
chris_vk3aml wrote:The way I obtained DC restoration was simply to place a diode across my CRT grid to the CRT cathode, this would charge the video coupling capacitor feeding the CRT very approximately so that the modulation applied was always generally upwards from a black datum, but it was very approximate.
Indeed and that is pretty much exactly what I am doing in software, it's just choosing the time constant (your capacitor and whatever resistance is there) that's the difficult bit (when there are no syncs) - I feel that it should vary somehow with picture content.
chris_vk3aml wrote:Dear me, Gary, you must think that our mechanical scanners circa 1972 were built by Blumlein himself with the facilities of a major research laboratory!
No, just the ingenuity of a JLB, and the facility equivalent of a small attic over an artificial flower shop...
chris_vk3aml wrote:Only joking, but I think you'll find that pictures derived from mechanical scanners will be ALL OVER THE PLACE in that regard, even today.
I am under no illusions there, in fact that is exactly why I am going to all the trouble. I know that it is a major problem for my software, but doesn't seem to be such a problem for mechanical (and probably analogue electronic) monitors so I trying to determine what the difference could be, but maybe simply adding a DC bias is sufficient for our purposes.
chris_vk3aml wrote:How do you INSERT the dc component into a constant sync pulse stream with a mechanical scanner, let alone RETRIEVE IT ???? !!!!
Well that may indeed be difficult, but I was asking about how to do it with sync-less streams - now THAT'S a knife!
chris_vk3aml wrote:Oh dear, dear, dear....
indeed, indeed, indeed
Thanks for the input, it helps to look at things from a different perspective sometimes even if it leads to the conclusion that it can't be done or is just unnecessary
Cheers,
Gary
Last edited by gary on Wed Apr 30, 2008 1:37 pm, edited 1 time in total.