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subcarrier strangeness
MUFF WIGGLER Forum Index -> Video Synthesis  
Author subcarrier strangeness
smrl
So I'm still trying to work some flaws out of my system. I've had some problems with chrominance throughout my system for a while - i can get it to work in some configurations but I haven't been able to explain why/how it's partially functional.

I finally acquired a better video waveform monitor and a vectorscope. And here's what I discovered --



This is what it looks like when I have the LZX or another video source genlocked to my Panasonic SEG. Color is functioning properly here but the chroma subcarrier doesn't appear in phase at all - doesn't look like it's at the right freqency!



And this is what it looks like without genlocking externally. That's what I expected to see there. So, what's going on here? And how is color working at all?
lizlarsen
Can you explain the signal connections a bit more? We're looking at the Panasonic SEG's output directly here, right?
daverj
The second one has the subcarrier locked to the horizontal, which is normal for broadcast signals, and better quality video generators. Or at least it is moving slow enough that when you took the picture it looked clean. Which would mean that it's frequency is very close to the correct multiple of horizontal frequency.

The first one looks like the subcarrier is free running, which is what I would expect from the LZX encoder, since it's subcarrier is a free running oscillation on the encoder board with no lock to external or to it's own horizontal. The first one might also be further off in frequency from the expected multiple of horizontal.

Either one works fine for making color images As long as the frequency isn't too far off. The monitor, VCR, capture card, or whatever decodes the color by comparing the subcarrier in the video portion with the subcarrier in the burst/sync section. As long as those relate to each other then the color decoding works. It might have a bit more chroma crawl with free running subcarrier, but it will still make color.
smrl
Oh, I see. Yeah, that makes sense, so /technically/ the subcarrier is an exact multiple of the horizontal refresh rate, and if it were precisely on point would appear as a static waveform at the beginning of each line, in exactly the same phase. And that's what the SEG does on its own, but the LZX actually just switches in the subcarrier for the correct portion, which is just a free-running oscillator and unless I can get it precisely timed to that 'magic number' of 4.5 × 455/572, it will look like that...

And yes, lars, what you're looking at is the program out of the SEG. It doesn't move at all. The difference between the photos is with the internal sync of the SEG and externally synching to the LZX output. I patched my scaler into the genlock in and got the same result -- incidentally, the scaler is normally hooked up to the video input of the LZX and is where I derive my master sync, since it's the only device I'm using that can't lock at all to an external source.

This helps. I imagine I couldn't just take the SC out of the SEG and patch it into the CVE at the output of the oscillator, since when I genlock the SEG to the LZX it's grabbing SC from there and reproducing it at that output? I guess I could just hook up the waveform monitor to the SC out and see.
lizlarsen
Yes, that's all correct.

It all goes back to the fact that the LZX is natively an RGB (or "triple colorspace") synth with a Composite output, rather than a Composite synth natively. The assumption is that in almost all cases, you'll be sending the output to a display or a recording device, so we didn't opt for the more expensive solution of doing black burst distribution, etc. And in the cases where you do need that, it's typically in an old gear studio where you'll have TBCs available.

Does the SEG have RGB or YUV inputs?

Does the SEG have subcarrier output? If you want to get DIY on this, yes, I think the AD724 encoder chip can accept an external FSC clock. It might need to be TTL logic levels. Only thing is your encoder wouldn't work unless you were sync-locked to the SEG (or had a switch to change modes.)
daverj
smrl wrote:
I imagine I couldn't just take the SC out of the SEG and patch it into the CVE at the output of the oscillator, since when I genlock the SEG to the LZX it's grabbing SC from there and reproducing it at that output? I guess I could just hook up the waveform monitor to the SC out and see.


No. If you replaced his oscillator (assuming it's even a subcarrier rate oscillator and not a 4x oscillator) with the output of the SEG subcarrier, while locking the SEG to the encoder, you would end up with a feedback loop that would create an unknown frequency (unlikely to be the right frequency).

It's not a real problem for a single encoder system. Many VCRs and older video games had free running subcarrier and looked fine. It would be a problem if you added a second CVE, since the subcarrier of the two wouldn't match and you'd have to TBC one of them to mix them in an SEG.

Or once my system is available you could feed his RGB signals over to my encoder/colorizer module and out through my Output Amps. The subcarrier of my system is common through all modules since my system is designed for multiple color outputs that are all in phase with each other.
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