Function / Arb waveform generators - Why no PAL/NTSC colourbars etc as standard? - Page 1
Forum section: electronics / testgear

author: Macbeth
author_rank: Super Contributor
timestamp: January 25, 2014, 11:15:33 pm
content: I've been playing with my Siglent SDG1025 and its arb waveform generation and it's inbuilt patterns (staircase, etc). One thing that has struck me is the darn thing just repeats arbs at whatever frequency you like. But more "complex" waveforms such as the classic colourbars for video should be easily generated (I would have thought as an inbuilt waveform). I mean even the most cheap to expensive 'scopes still have Video trigger on line or field capability.Is there a way of making a colour bar / cross hatch pattern using these gens? It seems trivial - instead of just repeating one arb waveform, have a simple script that can do a field pulse, 625 lines of staircase, and of course a colourburst for example?Has anyone managed to do this with arb gens?

author: TerraHertz
author_rank: Super Contributor
timestamp: January 26, 2014, 04:53:40 am
content: I'd guess the reason they don't, is that there's a very complex relationship between the color subcarrier and the horizontal sync frequency. Takes quite a lot of logic to get it right, per the standard. If it isn't right there's no point doing it.You can't even just store the waveform for one field and play that back, as it takes 4 fields in NTSC, and 8 fields in PAL, before the waveform repeats exactly.Seehttp://www.deetc.isel.ipl.pt/analisedesinai/sm/downloads/doc/ch08.pdfThis is why studio reference Sync Pulse Generators (SPGs) are so complicated.

author: Bored@Work
author_rank: Super Contributor
timestamp: January 26, 2014, 07:51:01 am
content: People have generated PAL-like1, or NTSC-like1composite video signals and patterns with cheap microcontrollers and primitive DACs. E.g. this one uses a primitive 5 bit DAC to generate a patternhttp://www.serasidis.gr/circuits/colour_bar_gen/colour_bar_gen.htm.The nice thing about analog monitors is they are forgiving. I.e. they are made to tolerate some deviations from the ideal signal form, because they are in fact never feed with an ideal signal during normal operation.If you want to do it with an arb generator you probably need one with large memory to hold the signal for a complete frame. If I would have to do it (no, I have no intention to do it), I would probably write some software to once generate all sample values for one frame, and load the output of that software into the arb generator.1"like" because they have skipped a few things here and there. Good enough for fun and giggles. Probably not good enough to serious measurement.

author: TerraHertz
author_rank: Super Contributor
timestamp: January 26, 2014, 08:15:45 am
content: Yes it's not hard to generate a signal that a typical monitor will accept as color bars. But what do you want - a test signal that is actually valid NTSC or PAL bars, complying to the standards? Or just color bars to exercise any old monitor?Someone designing commercial test equipment would be nuts to generate non-standard video test signals.Incidentally the NTSC 4-field and PAL 8-field sequence really do matter, since all tape video editing systems depend critically on the subcarrier to H-sync phasing doing the right thing.

author: Bored@Work
author_rank: Super Contributor
timestamp: January 26, 2014, 08:32:14 am
content: Quote from: TerraHertz on January 26, 2014, 08:15:45 amChinese vendors add all sorts of strange samples to their arb generators. I think it is more the amount of build-in memory they need to have to store the samples for whole frames that prevents them from doing it. And that for now they can't just copy the samples from somewhere but would have to spend some time during development to calculate them.

author: PA4TIM
author_rank: Super Contributor
timestamp: January 26, 2014, 09:27:41 am
content: I do not know much about analog television (they stopped transitting that a few years ago, over here). I have no clue if my R&S arb generator from the 90's can not do this, it is not mentioned in the manual (and when that R&S was made there still were lots of analog analog TV's and needs for a test signal. Also this generator was the topmodel of R&S ) So why would a modern one need to have that ? Is ) Are this test signals UHF/VHF or do you have to modulate a RF generator with an LF signal ? (just curious)I know brands like Philips made special TV test generators that do what you want. The have a lot of knobs. I could have 3 by now for free if I needed one. I see them now and then for (allmost) nothing because only a few hobby users still need one to restore an old TV. (but it can be different in other countries)

author: KJDS
author_rank: Super Contributor
timestamp: January 26, 2014, 10:33:51 am
content: Quote from: PA4TIM on January 26, 2014, 09:27:41 amI regularly see big piles of them at auction and they are rarely sold, even at very low prices.

author: nctnico
author_rank: Super Contributor
timestamp: January 26, 2014, 11:04:24 am
content: Quote from: Macbeth on January 25, 2014, 11:15:33 pmThe AWGs with more memory support using segments of data. You can build long and complex waveforms from small chunks of memory.Either way PAL and NTSC are pretty much obsolete. You can pickup a video generator for peanuts on Ebay.

author: Macbeth
author_rank: Super Contributor
timestamp: January 26, 2014, 05:03:13 pm
content: I don't mean I actually need to generate a proper studio grade PAL colourbar frame. I was just wondering if instead of having a very large sample, with some small changes within it, it is possible to use a mixture of the stored arb waveforms. Colourbars are just one example.It would seem trivial to implement as a feature. ie Run ARB1 x 100, Run ARB2 x 5, repeat. That kind of thing, rather than having a very large sample of 100xS1 + 5xS2 as a single arb signal.