Sunday, 30 October 2011

video comparator circuits

heres some schematics for the comparators. type 1 is used for sync processing. feed any h or v sync signal in, patch the outputs to the sync input jacks on some vcos, and twiddle the knobs and flick the switch till you get sync lock. easy. no pcb layout for these as i built them on stripboard.

type 2 grounds the negative side of the threshold pot, and takes the +V side through a switching jack, so you can insert an external signal to be used as the threshold voltage. useful for making a pwm squarewave or various other patches. both circuits have hysteresis controls to clean up the output a bit.

both of the above circuits are run from plus and minus 12 volts. the 3k9 pull up resistor means the output swings rail to rail, so it can trigger either of the 2 vco designs (one syncs on a -12 volt spike, the other on a +12) with ease.

Monday, 17 October 2011

new stuff

ive been away from teh internetz for a while. but i have been busy. looky---->
i played a gig, which prompted me to not sleep and solder alot in order to have something worth showing. so the synth is now way bigger. 6 oscillators, mixer, rgb and sync interface, dual comparators and a freshly finished cabinet. and an ms10, all sitting on top of my old diy audio modular. and a bloody great big TV. and a camera. it was at the Vanguard Exhibition in East London, and was it was a great experience. im hoping to get some video from the gig soon, but otherwise here are some stills from some recent home jams:

fractal feedback

comparators and camera feedback:

and some straight RGB mixing of waveforms:
the comparator module is new, and great fun. im still tweaking it, but i hope to have it finished and a schematic up soon. also i have just completed the RGB and Sync interface module, which also needs a bit of work. lots of bleed due to unshielded 'done in a hurry due to impending gig' wiring. doh.
be seeing you

Friday, 27 May 2011

Some Changes to the Plan.

In light of having had the synth for a year now and gained some operational experience with it, Ive come up with some modifications to the system that ill be implementing.
1- ill be removing the sync input jacks from the VCOs, and bussing the various sync signals around internally, with a rotary switch to select sync source. this is because the 2 oscillator designs require sync pulses of different polarities, and thats a bit confusing for live performance.
2 - im adding dedicated, individual modulation oscillators to the 8038 oscillator panels so that more complex RGB synthesis can be achieved. you can never have too many oscillators, and i keep running out.
3 - im adding dedicated H and V timing ramps to the SPG module, for doing vector rescanning. this will avoid having to use 2 vcos to do the job as i currently have to.
4 - im redesigning the 3 by 3 mixer for a wider bandwidth. more on the subject of bandwidth later.
be seeing you,

Tuesday, 19 April 2011

I am Moving House

and moving into temporary accomodation for a bit. there is unlikely to be much happening here for the next month or so, as i am currently living in a shoebox till i move to a Proper House. not really enough space to wave a soldering iron around without setting fire to something. see you later..

Saturday, 2 April 2011

Power Supplies

the spec for the synth's PSU is a s follows:

type: Linear, triple output, regulated.
output 1: 12 volts @ 1 amps
output 2: 12 volts @ 1 amps
output 3: 5 volts at 0.5 amps.

obviously, you could use a +/-12V and another 5 volt supply, or get one that does them all. i reckon a common everyday AT style pc switchmode PSU will also do the job, but i havent tested one. an ATX one will also probably work, but youll have to figure out how to fool it into thinking its plugged into a pc motherboard first. i think you have to short a couple of pins on the big connector together to switch one on.. i leave that up to you, because i dont like switchmode supplies for reliability reasons. salvageing an old linear supply from a junked bit of equipment will also probably be a good route to go down...
be seeing you


Q: but how much does it cost?

A: about 450 pounds. ive been going through my accounts, and thats what it cost me to obtain enough pots, jacks, ics, and boxes etc to build the whole thing. its only 40% complete so far, but at least it wont cost any more to complete..
be seeing you

Thursday, 10 March 2011

the good Video VCO

heres the other VCO circuit ive been using recently. its an EFM design once again, but this time there are some changes from the original; ive got rid of the square wave and changed a cap and a resistor.
heres the schem in roughly edited form:

The original copyright holder of this image is Tom Gamble. i edited it a bit and changed some stuff, but its still essentially his work. 

once again, all rights and respect to Tom Gamble (and Bob Moog!!) for this design, all i did was change a couple of values and get rid of the squarewave output. i tried it and it worked. hooray!

download the full gen on the audio version here

be seeing you


Wednesday, 9 March 2011

Video VCA and mixer by Tom Gamble of EFM

Heres a damn fine module, that i had no part in designing. its an EFM module, designed by Tom Gamble back in 1999. i was lucky enough to buy a load of his PCBs back then and have enjoyed building them, making music with them and endlessly tweaking them ever since.

low res schematic. download the proper PDF below:

anyways, back to the plot.. this is an excellent audio modular synth circuit, its the VCA5B design from toms 1900 series modules. it works very well and in exactly the same way as with audio at video frequencies too. i have no technical specs to back this up, but it works for me..... its also where i borrowed the design for the attenuverting inputs in my synth- its the same circuit but every 47k resistor or pot has been exchanged with 100k because i happened to get a good surplus deal on hundreds of 100k pots. guess what part most of my designs will be featuring for the forseeable future...
the PDF is here
the various corrections from the old EFM forum are here
and both of the above are linked to from this excellent site, which i have mentioned before. go there and read. then go build stuff.
be seeing you

Tuesday, 8 March 2011

VCO updated

ive added  a wiring diagram to the VCO post from several months ago... wow, i actually updated something...
be seeing you

Sync Processor

you should probably build at least 2 of these. they accept positive or negative going sync pulses in any polarity or voltage between -12 and 12 volts and can extract a clean positive or negative trigger for the VCO's. this means you can use pretty much any Sync Pulse generator or VCO circuit with this device. and as its basically just a comparator you can use it for other purposes within a patch too.

a jolly useful circuit in my experience, and one im pretty pleased i came up with. another nice feature is you can set the threshold so that its output sync pulses can lag behind the input slightly, which seems to give some control over colour. which is nice.

be seeing you

3 by 3 Mixer Matrix

theres 3 of these in one module. the attenuverting input channels (theres three on each mixer board, and 3 boards, so 9 inputs in total) are wired so that all 3 inputs on mixer 2 and mixer 3 are normalled to mixer 1s inputs. so you can set up some pretty crazy RGB mixes and feedback loops with only a small number of inputs. each mixer also has a DC bias and an output gain control, (voltage gain of 1 to 10 times) and also a paralleled output for feedback loops. this isnt buffered, and probably should be...

UPDATE 20110310:
I found some more files - this appears to be the PCB image i made mine from. the quote on the PCB is a Coil lyric. most of my PCBs have a Coil reference on them.. I generally design freehand PCB images in MS Paint whilst i think up the schematic in my head, then build them, then draw a schematic. ill have to reverse engineer my own synth in order to draw up a parts placement pic which might take a while, sorry...

be seeing you-

Joystick/Voltage Reference/Distribution Amplifier

er... does what it says in the post title..

be seeing you..

Input Processor

heres the Initial schem for an external camera input processor. as yet, i havent built it. it worked when i modelled it in Spice, so it should be a starter for ten...

ill edit this post as time goes on with updates etc..
be seeing you

oscilloscope experiment

I tried using the vidsynth to provide line and field timing ramps to one of my scopes with a Z axis input, using the mixer matrix as an interface (allowing vert/horizontal scaling, l/r and up down positioning, inversion of all parameters and modulation of same) and a wavetek oscilloscope as the output device. pics taken of the scope screen with my phone cam.... it actually worked! heres some pics..

damn, but i look good in green...

i have also obtained a scanner, and am busy digitising and correcting all my notes and schematics.
be seeing you-

Saturday, 19 February 2011


i use 1.5 metre multicoloured 8 way jack to jack snakes that i bought from Turnkey in the UK (who are sadly no longer around.. boo!). ive also recently started using George L's 1/4 inch jack connectors on standard RG59 cable. they are expensive but super high quality, look great and they mate with the coax cable very securely, and seem to give a better video image. the multicoloured looms are good for patching up all the RGB signal paths to the converters from the synth and i also use it for the H/V sync signals. the george ls stuff i use for all the rest.
be seeing you

Power supplies

the PSU is the most important single part of any electronic design. i use a Power One module. the spec is +/-12 volts at 2.5 amps. it runs alll my modular equipment no worries. its also a linear design rather thatn a more modern switched mode supply. a switchmode supply from an old PC might work, but i havent tested it.

ill edit this post a bit later with links to the prower one site and the unit i use. otherwise, there is always Google.

updated 8th march 2011 : It appears that Power One dont make linear power supplies any more. theyve gone all green instead. oh well. find or build a Bipolar 12 volt psu that gives you about 2 amps or more of VERY CLEAN power.

be seeing you

Friday, 18 February 2011

An Actual Progress Update

Wow! New News!
1. i met up with Chris from the Video Circuits blog for a Vid-Synth jam. he had his (wicked) Chromascope and an ave-5, i had my modular and my ave-3.  we Did Stuff, and Had Fun. and learned some more cool stuff about our gear.
2. this inspired me to take a new look at the Oscillators. i looked at some captures of my original prototypes, that are based on Moog oscillators. the waveforms were much prettier and they didnt seem to suffer from any of the sync problems that the Sandin 8038 based design can, where they reset in antiphase randomly, giving an effect a bit like interlacing (which is cool, but not always desirable). So i went back to the original prototype and tried a mostly discrete design, based on a transistor array and some op-amp buffers. works lovely, looks lovely, no sync issues. the circuit is actually an EFM design, based on VCO4d, for anyone who remembers that far back in time. i miss toms stuff. there was the occasional discrepancy in the docs but it was cheap, interesting, and available back in the days before the current euro-rack modular revolution.. anyways. Here is a link to Fonik's repository of EFM related stuff. he has a very good site, and his synth looks amazing...
3. im working on a schematic for the new osc. theres nothing proprietary in it. its the VCO4d design, with the timing cap ( C4 on the schematic) changed to 250pF and the 100k resistor on the output of the Saw and square waveform changed to 1k. the pulse waveform doesn't seem to work at these speeds. i am still debugging....
4. i will post some vids. this stuff is fun...
5. I have done some more work on the Mixer Matrix. its almost ready, but still has one major bug with the Bias pots that i need to address on the PCB before i publish the design.
6. I have done some more work on the SPG processor. it still seems horribly temperature sensitive though, so investigations are proceeding.
7. coincidentally, i now have a brilliant Cat. he is stripey, likes watching TV, has big feet and a looooong tail and his name is SPG.
8. Im at the building stage for the Joysticks and the Camera/ external input processor. i will draw schematics once ive built them....

none of the above have schematics yet, theyre all still in my head. i should probably write them down somewhere...

be seeing you.