Tuesday, October 25, 2016

Craig Anderton's Multiple Identity Filter

Craig Anderton's Multiple Identity Filter™ originally appeared in a series of ariticles in Contemporary Keyboard in 1979. It is a CEM3320-based VCF with each of the four stages operating semi-independently. Each stage has...
  • Three Filter Modes: Lowpass, Highpass, Allpass
  • Two frequency ranges (10:1 ratio)
  • Series/Parallel Switching
Add to that voltage-controlled resonance, straight/filtered signal mixer w/phase inverter (for phasing, changing bandpass to notch, etc.) and more. Craig was in top form—this is one tricked-out VCF.

Here is the article: MIF.pdf

Here is a mirror on Mediafire in case the link above doesn't work for you. I've been told that some web browsers (or their extensions) warn that Mediafire is a dangerous site. It's not but the page does have a lot of extraneous links and other junk that can make it confusing to navigate. To start the actual download of the pdf file click on the big green "DOWNLOAD" button that also has the file size in smaller letters: MIF pdf

Update 2017.2.26

Note that this design is not optimal. More work has to be done to get the stages to "behave" properly. This has been discussed on the Synth DIY list on multiple occasions. There is a searchable archive here: SDIY Archive

And there's some very good news: a clone of the CEM3320 has been fabbed. It's available here and the price is very reasonable. The datasheet is here.

Wednesday, February 24, 2016

The Oscilloscope Artist

The Oscilloscope Artist originally appeared in the November, 1975 issue of Popular Electronics. It creates all sorts of fascinating moving geometric patterns on an oscilloscope screen.

All you need is a low bandwidth 'scope with horizontal and vertical (XY) inputs.

Updates (2018/02/16): David Dixon spotted a really stupid typo in the original pdf file that was shared below. This has been fixed and the latest version is now 1.2.1. All other (unattributed) copies floating around the Internet are of the earlier, incorrect version.

(2020/04/10): A new PC board layout using TL074 quad op-amps is now available as well as etched boards! Message me.

A reconstruction of the complete article is in a pdf file. All of the original parts are still available!

The original block diagram is a bit confusing so I redrew it. Oscillators A & D create a Lissajous figure baseline from triangle/square waves. Oscillators B & C are multiplied and summed with A & B.

To make things interesting, C is 90° out of phase between the multipliers. All the oscillators are sync'd to A, which is ~60Hz. All of this produces complex patterns which move and shift.

I've also used it with a laser projection system with X and Y galvos and it works although the corners aren't too sharp.