I made a camera obscura as a gift this holiday season. While I was researching the principles behind the contraption I was drawn in by its simplicity and peculiar hybrid relationship with photography and fine art. Think of it as a camera, but instead of film or a digital sensor recording the image, you the artist with your pencil record it on a piece of trace-paper. I’m tempted to put it under the heading of “alternative photographic processes”, especially with all the recent interest in nonlinear tone mapping algorithms in photography, some of which are even based on brain neurology and old painting techniques. It’s also a good use for that weird beautifully patinad box you found at a junk sale.
I built the saw-tooth oscillator out of a 555. It can be done, contrary to what I said in a previous post. The trick is to supply a regulated current to the timing capacitor, which creates a linear ramp. This is done simply with a single transistor configured as a current source. Unfortunately you also have to isolate the output with a high impedance buffer. For the experiment I used a little Radioshack amplified speaker, shown in the video, which seems to have a very high input impedance as it didn’t effect the frequency of the oscillator at all. For a stand alone design an op-amp buffer stage would be required. So it’s a toss up as to whether this design is any simpler than the one I posted earlier which uses two op amps and a JFET.
I made a three string fiddle out of an old vodka bottle box. It sounds great – like a fiddle, and it’s loads of fun to play. I’m getting pretty good at chopping along to country and dance tunes. Everyone makes playing one out to be so hard.
For this project I challenged myself to resist the urge to buy parts. I could have used more appropriate tuners, but I didn’t. I could have bought a tailpiece but I used some scrap metal instead. It forced me to rethink my approach I think for the better. I’m really pleased with the final result. I carved the finger board myself from a piece of mahogany. It wasn’t too hard, though I can say it sounds fine when bowing when plucking however there are certain notes that buzz a little. I used a sound post but I did not install a bass bar. Maybe I’ll try a bass bar next time for reference. The bow is literally a bow as in the medieval style. It is made with fishing line. It works well for me but I’ve never had the luxury of a quality bow so I wouldn’t know the difference.
The box I used turned out to be very good acoustically. It’s made of solid straight grain soft wood of some kind. I used a set of tuners from a broken classical guitar rescued from a dumpster. The neck is a piece of mahogany rescued from a dumpster behind a furniture shop (where I get all my wood). The tail piece is cut from an olive oil can rescued from the trash behind a french restaurant in town (they throw a lot of them away). The bridge is carved from a thin piece of oak. The nut from a piece of maple. I closed the back up with a thin piece of oak. I finished the whole thing with raw linseed oil. I found this to be a great option for a simple and attractive no fuss finish. It also allows the wood to darken naturally which I like.
I recently stumbled upon a document from the annals of 70s electronic music enthusiasm dedicated to the Steiner Filter. It’s a three mode (LP, HP, BP) resonant filter designed by the creators of the Synthacon, an obscure model of subtractive style synthesizer out of Utah USA that never went into large scale production. The synth itself never made a name for itself, and could have been lost to the tides of time and never to be spoken of again, if… it weren’t for its filter design, which due to its simplicity and unique sound has been floating around the DIY synth and guitar effects forums for quite some time. Here is the PDF N-Steiner VCF 1974.
The design is built around standard off the shelf parts – no special matched transistors are called for, or temperature compensated or precision resistors, or fancy transconductive op-amps from limited and deceased runs. And what’s more, the great simplicity and practical efficiency of the circuit in no way hinders it from producing some of the richest and most interesting tambers I’ve ever heard out of a VCF. This is I believe due in part to the influence on the Q or feedback of the circuit by the frequency control. That paired with a general unwieldy tendency to snap into oscillation makes it pretty grungy and slightly unpredictable – it’s almost instrument unto itself.
The Steiner filter is a perfect fit for my bigger over-arching project outlined here, which calls for simple designs using a minimum of components that are redly available off the shelf, and that highlight and carefully cultivate nonlinearities and manifold interactions, producing richer more interesting sounds, rather than compensating for and attenuating them into submission producing an auditory expression of perfected boring domestic sterility. But I digress…
I was especially interested in the concept circuit in the first part of the paper, because it is even simpler than the full fledged voltage controlled circuit. I fleshed it out in a circuit simulator to see how it worked, and it worked great. Here it is with a sawtooth signal going into the low-pass input. Note the ringing caused by a high Q.
And here it is with the sawtooth signal going into the high-pass input.
And here it is in oscillation with the Q set just at the point before it would start clipping as the signal maxed out at VCC.
I built the concept circuit presented in the first part of the paper, but modify it for light control so it could go with the light controlled saw tooth oscillator I still had kicking around from last time. Though the two variable resistors will change to vary the cutoff frequency of the filter they must be the same value. Whereas Moog or Steiner put a bunch of reversed biased transistors or diodes in place of the two resistors to make it voltage controlled, instead to make it light controlled, I replaced the resistors with a pair of CDS cells. The filter is then controlled with the shadow of the hand covering both CDS cells. If they are covered unequally interesting things happen. If R1 is covered mored than R2 the Q is increased. If R2 is covered more than R1 the Q is decreased. Here is the final schematic:
I added one more modification to the circuit consisting of two diodes set up in a clipping arrangement, which squelches the feedback when it reaches a certain magnitude (-+7V to be precise, luckily just a little within the bounds of the normal magnitude of the feedback signal) preventing the volume from increasing dramatically when the filter slips into oscillation. A side effect of this method of squelching is a rich harmonic distortion which sounds, well… awesome. The filter now also recovers from oscillation more quickly, making it more useful for playing “music.” Here it is shown in simulation:
The final result sounds pretty much like classic analogue synthesizer (for $10 in parts!). It sounds pretty great! Take a listen.
During performance the light controlled synthesizer exceeded all expectations. It turned out to be a powerful instrument suitable for beautiful music making. Please listen to this improvisation featuring the light controlled synthesizer accompanying an electric harmonium and electric guitar. The filter really starts coming in at around 2 minutes.
And if for some reason you can’t get enough, here’s another cut:
I think my next project will be to build an exponential converter which will make playing the light controlled synth a little easier.
There are many 555 based noise making circuits out there in the DIY noise and circuit bending community. Its popularity is partly due to the fact that it is a simple chip to use; it provides all the facilities for a square wave oscillator. But it is also due to the fact that there are countless 555 schematics out there churned over and over from book to book and now blog to blog who’s first incarnation begin probably in the era of ham radio or on the graph paper pages of a Forrest Mims book. It makes one wonder if the 555 timer is so used not because it’s the right tool for the job but more because it’s what’s in the prevalent circuit designs, leading to a kind of stagnant force of habit in the community. My main complaint about these 555 circuits is that they can only generate square waves. Saw tooth waves sound much meatier especially when run through a low pass filter. They are the perfect wave form for subtractive synthesis. Further a sawtooth waveform can easily be converted to a square wave or a triangle or even a sine wave. This is not the case for a square wave. At the heart of your typical electronic synthesizer you won’t find a 555. You will find instead a voltage controlled sawtooth oscillator. But these synth schematics are sophisticated and inaccessible to someone just getting into electronics so they don’t lend themselves to the kind of viral proliferation of say the Atari Punk Console. I decided to see how hard it would be to design a simple sawtooth relaxation oscillator using a common TL082 dual op-amp chip, which like the 555 can be found at RadioShack. The only other active components are a diode and an MPF102 JFET transistor, also available at RadioShack.
I used whatever components I had lying around and scavenged others off boards from my junk pile. Here is the final schematic:
The theory of operation is as follows: The first op-amp (left) charges the capacitor in its feedback loop. The output of the op amp is positive going and increases from 0v to 15v linearly, at a rate dependent on the negative current through the input resistor. But before it gets to 15v the second op amp, acting as a comparator, discharges it. The moment the output of the first op amp exceeds 7.5v (half the supply voltage) the second op amp switches its output from -15v to 15v. This fires the JFET which shorts out the capacitor, reseting the circuit to its original state. The diode prevents the input of the JFET from going above 0v and the resistor to ground pulls it to ground when it’s not being pulled to -15. The capacitor in the second op amp keeps the comparator from changing state before the main capacitor is fully discharged.
Here’s a video of it in operation. The meter is connected to the main capacitor, showing it charging up and then quickly discharging when it reaches the 7.5v threshold. I have a potentiometer hooked up between -15v and ground with the wiper connected to the input resistor – a variable voltage divider basically. This allows me to input any voltage between 0 and -15v to the circuit, which gives a corresponding frequency tone.
Here is an example of the circuit put to practical use as a light sensitive theremin. I replaced the 10k input resistor with a CDS cell (light sensitive resistor) allowing me to vary the frequency with the shadow of my hand
And another one of the light theremin at a lower frequency.
So this circuit was pretty easy to build. It is a little more complicated than a 555 oscillator, but it’s also little more interesting. A 555 is actually a relatively complex device but for the beginner it is usually treated like a black box. This circuit however has all the mechanisms exposed, making it perhaps a better education tool than the 555. The dual supply might be a little off-putting, requiring either two wall warts or two 9v batteries. Could this be modified to work with a single supply? One thing is for sure though, it sounds good, I dare say better than a 555 circuit. One more important point, with this circuit frequency modulation is possible. We could build a few of these circuits and be able to generate very rich FM sounds. We could also build a slew limiter for the input, or a random signal generator. The possibilities are virtually boundless with voltage control. To fully realize these options we would want to invert the input, and perhaps while we are at it scale the input exponentially. This unfortunately calls for another op amp.
All classic analogue synthesizers have exponential converters built into their oscillators to convert the linear voltage scale we use to control them into the exponential scale we hear. The 1 volt per octave standard has this transformation built into it. This is a critical part of the synthesizer system because, without it, pitches would sound flat for a given voltage interval or fall entirely outside the control voltage range. This has to do with the fact that we hear pitches in an exponential scale. This can be heard using a slew limiter switched to linear on a classic synth. Exponential control of filter frequencies is just as important for the same reasons. So controlling the pitch in a synthesizer with an exponential function is important. Now, to date, the common way of achieving such a conversion uses some of the most uncommon parts and very sophisticated circuit designs that utilize matched components and little heater ovens and very fine adjustments with precision manufactured resistors and precision voltage and current references. This is all for the sake of accuracy, and elimination of drift caused by changes in temperature. We’re going to blow that all away and start with the simplest most accessible approach using the central mechanism at the heart of these systems.
At the heart of an exponential converter is a simple lowly diode. That’s it. A lesser known fact about diodes is that they have a nonlinear current to voltage relationship that pretty much matches an exponential function. See the exponential curve in the top right quadrant of the graph when V approaches Vd.
So we have at our disposal a linear voltage to exponential current converter. This turns out to be an advantage because the typical sawtooth oscillator is actually current controlled rather than voltage controlled, because the frequency depends on how fast the timing capacitor charges, and that rate of charge is dependent on the rate of electron accumulation at the plate, i.e. current. More theory on relaxation oscillators can be found on my blog post.
The problem with the diode approach for classic synthesizer designs is that the V to I function is heavily temperature dependent. According to Hal Chamberlin (my elder in these matters) for a typical VCO design a 1 degree Celsius change in temperature would result in nearly a half step change in pitch. Pretty significant if you consider that the ambient temperature in your average terrestrial environment can vary between 4 and 40 degrees Celcius, which would result in a possible pitch drift of roughly 1 1/2 octaves. Is this too much drift for a chaotic noise box, or is it the range of instability that we find actually advantageous? If the pitches drifted into an inaudible range it would render the device useless at certain temperatures. If you wanted the device to function in outer space it could be a problem, but it looks like here on earth it would produce meaningful values in any condition short of a sub zero meat locker. Good to know.
If it turns out that the drift is too much there is a pretty simple approach to eliminating the bulk of the drift using a second diode which cancels out the changes. There will still be nonlinearities and since we won’t bother to source matched components it won’t be perfect, but the bulk of the drift will have been eliminated.
If you’re interested in this topic I recommend checking out Hal Chamberlin’s book “Musical Applications of Microprocessors.” It’s out of print so you’ll have to search the library system, but it will be well worth the effort as it has an extensive section on classic analogue synthesizer circuit design. If you are thinking about building a synthesizer I also recommend checking out the ASM-1 site as they have a whole modular synthesizer design that is open source, developed and well tested.