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DIY Schematics: Simple Video VCA, Crossfader and Mixer
MUFF WIGGLER Forum Index -> Video Synthesis Goto page 1, 2  Next [all]
Author DIY Schematics: Simple Video VCA, Crossfader and Mixer
lizlarsen
Here's something for you DIY'ers. This is a dead simple video-bandwidth VCA using the LT1251/LT1256 DC video crossfader chip, which you can get for $6-$7 (or free samples are available from Linear's site still I believe, if you register). You could build this easily on protoboard. This chip is begging for DIY circuits because as you can see it can do so much with just a handful of external resistors.

Adding an attenuator and bias control on the CV input expands the circuit quite a bit, since you need high bandwidth buffering before attenuation, so that's why that's not included. This is just meant to be super simple.

Hope you find it useful!

Edit: Added variation for CV crossfader.

Important Note for V1.0 schematics: I've just realized R2 in the VCA & Crossfader schematics should be 10K instead of 1K. I'll fix and re-upload soon.
lizlarsen
And one more, while I'm at it. Here's a simple wide bandwidth summing mixer with 4 inputs (one of them is inverted.)

Dead simple, but this demonstrates the building blocks of how we approach something simple like a summing op-amp circuit in the LZX modules in terms of the buffered inputs, chips used, resistor values, I/O termination, etc. So hopefully it serves as a gateway to adapting other DIY synth circuits for this purpose.

Hint: With two of these mixers, and the outputs connected to the CV crossfader circuit posted above, you could make a clone of the Sandin Adder/Multiplier module.
snufkin
This is all amazing Lars and will go on my to do list
lizlarsen
Here's a quick block diagram of a useful module/application idea for driving vector monitors that can be made with the "simple" schematics. It's based on the Rutt-Etra video synthesizer and contains 4 signal summers and 2 vcas.

The ramp inputs are meant to be connected to the Video Ramps module.

Other ideas on expanding this would be bias controls summed with the original ramp signals to adjust the offset points at which the image anchors from (bipolar ramps will zoom from center, unipolar ramps will zoom from one corner).

This could also be implemented as a patch with a couple Triple Video Processors (as 2-input summing amps) and Triple Video Fader & Key Generator (as the vcas.)

wcfields
Just ordered a few samples of the LT1251/LT1256. Hope to bread-board them up soon.

Lars, do you have any recommendations of the miscellany parts, specifically the 3.5 MM jacks?

Also, are there any other interesting, straight-forward IC's that you would recommend for novices to play around with?
lizlarsen
Quote:
Just ordered a few samples of the LT1251/LT1256. Hope to bread-board them up soon.

Breadboards can be bad for video circuits due to stray capacitance in the sockets. I would recommend building these on protoboard instead.

Quote:
Lars, do you have any recommendations of the miscellany parts, specifically the 3.5 MM jacks?


A very common part are the Kobiconn 16PJ138 jacks, available from Mouser. That's what I've been using so far.

There are also the PCB-mount variety supplied by Erthenvar, which I may be moving to in the future. Mechanically they are very similar.

There are some prototyping kits for Euro modules available from Synovatron that may be really good for these projects:
http://synovatron.blogspot.com/p/products.html

Quote:
Also, are there any other interesting, straight-forward IC's that you would recommend for novices to play around with?


As far as stuff available in DIP format there's not a lot. We build 90% of our circuits with LM6172 op-amps because we want to keep things away from proprietary chips that may go out of production when possible. Some other chips you can check out are the LT1203 video-rate switch and LT1228 OTA (which is similar to the LT1251 in some ways but not as straightforward). LM319 is cheap and makes a decent but not super-fast comparator. LM1881 is an essential, inexpensive chip for extracting sync from an external signal.

A good circuit for me to add to this collection would be something demonstrating a front/backend I/O for interfacing with 74HC series logic chips (typically an input with an analog comparator and any voltage protection, the logic circuit, then a 5V>1V level attenuator followed by an op-amp output buffer). With that information, you can build all kinds of boolean logic, dividers/counters, etc.
fletch
This is just what I was looking for. Thanks Lars!!
wcfields
Found this when searching for LT1251:

Video Dissolve Unit from the Senior Design Group Four (pdf). They got a 100 on the paper as shown on the scanned PDF so at least it isn't half-done.

The general idea is outlined in the PowerPoint
Quote:
The Video Dissolve Unit is a foot controlled video switcher and cross-fader. It has the ability to input eight video signals into two independent arrays. Each array outputs a single active signal to a dissolve circuit where they are blended together and output on a screen.


They budget it out ($670! but nearly $500 of that is a Datavideo TBC).
lizlarsen
Neato! Sounds like they need a modular video synthesizer. smile
4mspedals
Cool, I ordered some samples as well. I see Digikey carries the SOIC14 version of the LT1256, but I think I'd rather have the LT1251 because it automatically cuts off a channel if the CV is less than 2% (so you can do complete fades with having to carefully trim your CV range). Know of any other suppliers that have the 1251? I might want to make a mini Video VCAMatrix for my own use...
lizlarsen
Yep, that's the only difference between the 1251 and 1256.
I always order these directly from Linear Tech! I think it ends up cheaper that way.

Quote:
I might want to make a mini Video VCAMatrix for my own use...

Yeah! Some powerful possibilities, there.
johnnywoods
4mspedals wrote:
I might want to make a mini Video VCAMatrix for my own use...


screaming goo yo screaming goo yo screaming goo yo
BRIZ

oh-my .... started the diy cv crossfade project at 2am ... took a nap after sun up ... asked Lars what the hell the F1 & F2 on the schematic was ... finished ... plugged it in ... and IT WORKS! a tear almost came from my eye. almost.
THANKS LARS!
zombie
lizlarsen
Sweeet!! Good job.
nickciontea
awesome!! the chip is expensive but very easy to work with!
fletch
Hey Lars, just checking out these schematics, got a working crossfader. On the summing amplifier, I'm just wondering what the purpose of the 1k feedback resistors and the 499R resistors on the non-inverting input are. I'm assuming the input op-amps act as a simple buffer stage?

thanks!
lizlarsen
Quote:
On the summing amplifier, I'm just wondering what the purpose of the 1k feedback resistors and the 499R resistors on the non-inverting input are. I'm assuming the input op-amps act as a simple buffer stage?


Yes, IC1 & IC2 buffer the signals before mixing. You could probably just put them on the top of the 10K pots instead in this case, but buffering is definitely best practice to prevent extra loading attenuation.

The 499R input R on the non-inverting input and 1K feedback resistor are best practice for these op-amps (LM6172). The circuit is identical to removing those, in the case you were using TL072 op-amps.

The bandwidth of this module should be more than sufficient, but if you wanted to push it a little more, you could use 1K pots and resistors instead of every 10K value in this schematic. In that case, the input buffers would definitely be necessary.
neon
Thanks for posting these! I've been trying to kludge my way through DIY video electronics projects for a while now, and have been struggling with the relative scarcity of schematics in comparison to audio synth DIY. This info is really helpful.
Eli
Awesome! Thanks Lars. Going for it...
will post results here.
salik
where are the schematics for this? I don't see them in the thread?

EDIT: nvm! sorry, go about your day.
raveboyy
lizlarsen wrote:
And one more, while I'm at it. Here's a simple wide bandwidth summing mixer with 4 inputs (one of them is inverted.)

Dead simple, but this demonstrates the building blocks of how we approach something simple like a summing op-amp circuit in the LZX modules in terms of the buffered inputs, chips used, resistor values, I/O termination, etc. So hopefully it serves as a gateway to adapting other DIY synth circuits for this purpose.

Hint: With two of these mixers, and the outputs connected to the CV crossfader circuit posted above, you could make a clone of the Sandin Adder/Multiplier module.


Dear Lars,

would you provide a PCB print and partslist for this please ?smile
Jonas Bers
BRIZ wrote:
... asked Lars what the hell the F1 & F2 on the schematic was ...


Do "F1 & F2" on the schematic refer to a fuse?
daverj
Jonas Bers wrote:
BRIZ wrote:
... asked Lars what the hell the F1 & F2 on the schematic was ...


Do "F1 & F2" on the schematic refer to a fuse?


I would guess they are ferrites. (though usually ferrites on a schematic are given "L" numbers like inductors, while "F" is used for fuses)
Jonas Bers
Ahh, that makes sense. Thanks!!
04tm34l3
Hey Liz thanks for posting these. Real interested in making some of these.

Just a few questions. Numbers correspond to points on attached schematic.

1. Not sure what this is, does this mean 1/8 jack, shield to ground and the input to the 100k?

2. Whats 100R is that a 100 ohm resistor.

3. Whats F1 and F2

siiiiiiiiiiiickkk screaming goo yo screaming goo yo screaming goo yo screaming goo yo
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