Thursday, 10 March 2016

Waveform Building Strategies

I decided that using a separate oscilloscope and feedback set-up (as per Oram's design) was too much of a rigmarole for an 'intuitive interface' and that due to the variety in analogue scopes it would have required quite fine calibration and frequent adjustments and also may have not ever worked on some types of oscilloscopes. 

This left two options for my design:

  • Build the whole CRT / Photomultiplier set-up into Mini-Oramics like Oram's original machine.
  • Come up with another strategy.
The first option had a few issues, the main one being that small CRTs are only now available as 'new old stock' and are expensive and limited in supply - I wanted my new design to be a bit future proof. Also I have little existing expertise in valve technology or associated high voltage analogue electronics, so I opted for a totally different strategy.

The new strategy had to be in keeping with my design goals of building something which would have been technically feasible in the early/mid 1970s. 

After some discussion with my supervisor Mick Grierson I opted for the 'sped up analogue sequencer' type of wave shaper. This works by sequentially switching through a series of adjustable voltages very quickly creating a stepped waveform. This type of waveshaping strategy has been described in Peter Manning's Electronic and Computer Music. And also in Forrest Mims' Circuit Scrapbook Volume II

In fact when I started prototyping my wave-shaping oscillator I hadn't realised you could buy one off the shelf, so I must give fair mention to Ian Fritz's Double Deka VCO

This video shows my early ten step prototype in action:

unfiltered scans from Tom Richards on Vimeo.

And this video shows the same prototype with sound, played through a basic low pass filter in order to reduce the 'stepping' or aliasing noise:

first wave scans from Tom Richards on Vimeo.

Shortly after making those videos I performed with the new system, nerve-wrackingly still in breadboard  for the Nonclassical label's Pioneers of Electronic Music festival. If you look closely I am adjusting tiny preset pots with a screwdriver live on stage at the Macbeth in Hoxton!

Picture courtesy of Nonclassical

Sound from that gig here:

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