Testing a Video Signal
in [Design]
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From: Phil Allison on 18 Jan 2010 18:29 "bg" > If the frequency of the burst was off, the color hue would shift, but you > would still have color. ** Completely WRONG. The phase of the colour sub carrier is what produces hue shifts. Explains why NTSC is so prone to hue shifts in transmission and why PAL was invented to solve the problem. If the TV receiver or monitor cannot phase lock onto the sub carrier burst, then normally no colour will appear. ..... Phil
From: Mark on 18 Jan 2010 22:54 On Jan 18, 8:52 am, "Mike Warren" <miwa-not-this-...(a)or-this- csas.net.au> wrote: > Michael A. Terrell wrote: > > >Mike Warren wrote: > > >>Thanks for the reply. As I explained, I'm converting through > >>a 1000pF capacitor. This have been sufficient for me for over 6 > >>years, and anyway I have the same problem feeding into a > >>S-Video input to the monitor. > > > That may be too much capacitance. You are working with siglas in the > >3.5 MHz range and may be coupling too much of the Luminance signal into > >the chroma channel. I've seen as little as 47 pF used. > > I did experiment years ago and found 1000pF to be the best compromise in > practice from the values I normally keep in stock. I have never had the > need to revisit the situation. I already /know/ that the converter is > not the problem here because I get the same fault if I plug directly > in to the S-Video input of the monitor. > > -- > - Mike well I can't see your pics for some reason but the problem has got to be either freq or amplitude of the chroma info... It sounds like you are using a small cap for a crude combiner, but consider this, the cap will pass the chroma signal and sum it with the luma, but it depends on what the output Z of the luma signal is. so instead of using just a cap, you may want to insert some small Z like a resistor or inductor in series with the luma output, like a crude diplexer. Regarding the freq, the color bust freq and the V and H should all be locked together. you can probably count the H and it should be 15.734. kHz for NTSC. Another trick is to take a known good video signal and loosly couple the UUT video to it and you will see the video sync drift through. If the H sync drift less then 1 frame per sec or so, it is probably close enought that a monitor should lock to it. The FCC spec for TV stations is a lot tighter then most monitors require, consider they work with cheap VCRs. Please do come back and let us know what you find when you solve it. Mark
From: Phil Allison on 18 Jan 2010 23:01 "Mark" Regarding the freq, the color bust freq and the V and H should all be locked together. you can probably count the H and it should be 15.734. kHz for NTSC. ** The OP is in Australia an so using PAL monitors. Another trick is to take a known good video signal and loosly couple the UUT video to it and you will see the video sync drift through. If the H sync drift less then 1 frame per sec or so, it is probably close enought that a monitor should lock to it. ** Horizontal synch is NOT the problem. Colour sub carrier synch IS !! ...... Phil
From: bg on 19 Jan 2010 01:06 Phil Allison wrote in message <7rk966Fnf0U1(a)mid.individual.net>... > >"bg" > >> If the frequency of the burst was off, the color hue would shift, but you >> would still have color. > >** Completely WRONG. > >The phase of the colour sub carrier is what produces hue shifts. > >Explains why NTSC is so prone to hue shifts in transmission and why PAL was >invented to solve the problem. > >If the TV receiver or monitor cannot phase lock onto the sub carrier burst, >then normally no colour will appear. > > > >.... Phil If the frequency was off but close enough to allow the oscillator to partially lock to it, wouldn't that cause the hue to alternate?
From: Phil Allison on 19 Jan 2010 01:38
"bg" > > Phil Allison >"bg" >> >>> If the frequency of the burst was off, the color hue would shift, but >>> you >>> would still have color. >> >>** Completely WRONG. >> >>The phase of the colour sub carrier is what produces hue shifts. >> >>Explains why NTSC is so prone to hue shifts in transmission and why PAL >>was >>invented to solve the problem. >> >>If the TV receiver or monitor cannot phase lock onto the sub carrier >>burst, >>then normally no colour will appear. >> > > If the frequency was off but close enough to allow the oscillator to > partially lock to it, wouldn't that cause the hue to alternate? ** Oh dear.... Go look up PLL on Google an learn something. ..... Phil |