Posts Tagged ‘Video Synthesis’

Tempting the Inductance Fairy

Wednesday, April 21st, 2010

So if you don’t know how low I will sink to make a prototype work, now you do :-)

Video Mess Tool Assembly

Wednesday, April 21st, 2010

Yesterday the PCBs for the Mess Tool came in and I populated them. Like a big dummy I forgot to order the LT1203, so testing was relegated to the bench until today. This turned out to be a good thing, since of course there were legit problems which needed attention.
You can see from the pics that there’s a proto area in the mess tool. That ended up being a good thing. Part of the circuit is a set of clamps which let you keep whatever gnarly signal you’re injecting into the poor unsuspecting TV from spilling over into the sync region.

These clamps ended up being the source of some trouble — they were mushy passive clamps which put a 47k in series with a video signal, which it turns out is TOO MUCH, since all that pretty ground plane (and the IC inputs, and whatever other parasitic stuff is going on) contributes significant capacitance at color frequencies. The diodes I was using to clamp to a reference (SD103s) turn out to also have a 50pF capacitance or so at 1MHz, which again, counts for a lot.

This means that the video through the “snarled up” section tended to just get filtered away to nothing. The solution was (well, will be, I’m still building it) an active clamp which lowers all the circuit impedances and generally burns up more current.

The mux came in this morning (and Digikey was out of parts in the correct footprint, grumble) as did the boards for the synth and some low-capacitance diodes. So today will be busy. More soon.

Also. Not to get gooey, but man do I love hardware. Really. I forget that in the good old days it was me and a scope and I never ever wanted to learn to program. You know, when men were men and all that.

Xoxo, TB

Color Me Baddly Gerbers — a companion video tool.

Tuesday, April 20th, 2010

Just for grins, while I was waiting for the PCBs to come in, I decided to lay out a new design for color synthesis that I’d been fooling around with. This, again, is the gerber file.

The sync and blanking circuits in the above are fairly pedestrian — they’re just an AVR running at 14.318 MHz, which controls a 4051 to gate in the correct resistor values to get sync and blanking levels into 75 ohms. This part is pretty much exactly the same as the circuit from Owen Osborn’s old CA synth (which is a really elegant piece of engineering, I think). The AVR generates the colorburst and color carrier also using a hardware timer to divide the crystal frequency by 4. This means this prototype ain’t gonna do PAL. Sorry.

To my mind, the really interesting thing is the way in which color gets generated. Hue is encoded in analog composite video by _PHASE SHIFT_ of a carrier wave. Someone very smart and very good at electronics figured that out a long time ago. I’ve built synths in past which use the AD724 (lame) and varactor diodes to give continuously variable integration.

The varactors are actually a pretty badass way of doing it — they’re fast and kinda nonlinear and totally work, but it requires A LOT of stages of this to get 360 degrees of shift (enough for all the colors a TV can display). Also, (in addition to not being super cheap) the really good varactors are small SMT devices. I personally don’t care, but some of the cave-people with soldering irons who frequent this site occasionally express concern about this sort of thing and their poor tired eyeballs etc etc. Generally when this happens I turn up the Brandenburg Concertos and have my manservant pour me another Campari spritzer, but this time I decided the unwashed masses should have some cake too.


It took a lot of searching and fiddling to find something that I thought would work, that was both elegant and cheap and didn’t require exotic components or a sensitive board layout or weird supply rails or whatever. I had this suspicion that a PLL could do what I wanted, cause you know, its job is to party with phase. PLLs regularly work at or above colorburst frequencies (3.58MHz) which is also good because it means they aren’t on the edge of some spec.

The other idea I had (a voltage controlled all-pass filter) was generally too hard to do at frequency ranges that high (the LM13700 won’t slew anywhere near that fast, for instance). Other than designing an OTA which works at those frequencies (on my list of things to do, along with dating supermodels, designing invincible armor and generally running Stark Enterprises) I wasn’t sure how to implement this in a simple way.

PLLs are not the easiest circuits to understand (for me, anyway). But they are cheap and ubiquitous and many very smart people have written a lot about them. Eventually I stumbled across this circuit in EDN. The description with it is brief, but pithy, and explains the essential details of what I wanted to do.

Armed with this I was able to create a new design, standing on the shoulders of great nerds past. And when UPS shows up, I’ll know how well it (as well as the Mess Tool) works out!

Xoxo, TB

Video Mess Tool, or, Snuggling With Sandin (for a Bent 2010 lecture)

Friday, April 16th, 2010

The pretty pretty gerbers for a new video tool. This device will hopefully allow you to take all the precious bits of composite NTSC (or PAL, I guess) video and keep them safe, while mucking with the rest of the signal. Sync, blanking, and colorburst are separated with a level comparator, and a mux allows you to inject new signals or perturb the old one. It’s analog, and based on the AD828 op amp, the AD8561 comparator, and the LT1203 video mux/buffer. For one, I hope it works. For two, I hope it looks awesome. Stay posted!

I Know What Moms Like

Sunday, December 20th, 2009

Though she probably would have been more interested in a Steven King book and some Calphalon pots, I decided to build a video synthesizer for my Mom for Christmas. I also built one for my girl, who is generally more into blacklights than mom and one for me, because, you know, ho ho ho.

There were lots of reasons to do this. Partially because I think it’s cool to give gifts you make and less altruistically because I was still geeked about the little varactor synth that I’d made and wanted to do a new revision of it.

This synth still generates all the housekeeping signals with an AVR (as well as the colorburst) and switches them around with a 4051 multiplexer. However, it adds a clamp circuit which allowed control of color saturation (the old version was fully saturated color all the time) and a DC offset circuit which controlled brightness (that knob was also at 11). Those circuits worked _OK_ but not great, and I found I generally liked the results when everything was full out. There’s also an option to invert the color carrier, which WAS cool, in an epileptic kind of way. The sync signals on this guy were not _quite_ at the right levels either.

The idea is basically the same as the last varactor synth; except there are now multiple stages of RC phase shift. Each stage uses a SMV1255 “Hyperabrupt Junction Tuning Varactor” as the cap in that RC which is driven by a control voltage. Each stage is also buffered by an XOR gate, to square the signal back up after passing through the RC. One thing I also screwed up in this design was running the chain of buffers NON-INVERTING. This meant that with big phase shifts (which also meant big attenuation from the filter) the color carrier would eventually disappear, I assume because of the asymmetrical thresholds in the gates. On a scope you’d just see the duty cycle getting farther and farther from 50% and then just disappear entirely. The big solder mess here shows the workaround I did to fix this, which helped. Using comparators would help even more. Using a different circuit topology entirely would prove to be the most helpful….

Still this synth was pretty cool. Because it was for Moms and GFs, it was designed to be standalone and not require any inputs. There’s a little prototyping area where I made some function generators to make repetitive waveforms to drive the various color stuff.