Telmatron

The project started with an idea for a plasma-based audio component, that could allow sound to be manipulated by magnetic fields. Magnetic control turned out to work particularly well for signals generated within the plasma, at conditions created and controlled by the machine we call a telmatron.

This page is a bit of a project scrapbook, with technical details, demos of the 'TM01' prototype in development, and other background stuff.

Technical details

Electrical signals can easily be produced by fluctuations that occur naturally as electric current passes through a plasma, over a wide frequency band including the audible.

Making audio with plasma is not new in itself, but our machine controls the stability of current (and so the sound) by varying both the voltage and excitation of the plasma - producing a range of sound output at different conditions.

Increasing tube voltage generally raises pitch, which is also subject to sudden changes as the plasma state changes mode. Increasing plasma excitation generally causes a progressive filtering and distortion of the signal, until the plasma stabilises and sound disappears.

Magnetic fields cause electron paths to curve, which tends to extend their lifetime in the plasma and increase the local efficiency of the discharge. This means a hand-held magnet can have a significant influence upon the sound, that varies with proximity and location along the tube.

Modulation of current and excitation affect the operating point of the instrument differently, and combinations of these cause a surprisingly vocal output.
Experiments so far with T5 fluorescents have often sounded like a zoo - distorted howls and growls, shrieks, squeaks and whistles... also like brass, string or woodwind instruments, speech and birdcalls. We hadn't expected the kind of variation in sound produced by different tubes - some can operate in modes down to bass frequencies, whilst others mainly shriek or whistle across the operating range.

We do not expect to gain a precise understanding of the plasma fluctuations that create the audio output - plasmas are subject to many different waves and instabilities (though most occurring at frequencies far above audible), and have complex dependences upon physical environment. The low frequencies and voltage dependence we observe are characteristic of 'relaxation' oscillations, in which a DC discharge ignites and extinguishes rapidly, and this mechanism is likely to play a principal role in causing the audio signal. The effect of auxiliary excitation might then be explained as maintaining the discharge to a varying extent during the extinction part of the cycle. Acoustic waves also occur commonly in plasmas and might also contribute, the regimes of current (in)stability across our operating range perhaps related to diverse damping effects by different background plasma properties. Regardless of the mechanism of production/propagation, these fluctuations will be influenced by interactions as current crosses boundaries within the plasma, such as sheath regions around electrodes. Plasma properties are highly dependent on gas species/pressure and electrode material, and the observed variation between tubes are likely to reflect quite minor physical differences.

Project direction

In the short term Telmatronics are making a simple fluorescent tube-driver machine available, that can be used with commercial T5 tubes for experimental noise-making. Fluorescent tubes are being phased out in the next year or so, but we are securing adequate supplies for now, and in the longer term we aim to develop purpose-built tubes.

Prototypes and demos

The short film below shows our first dual-modulated prototype:

And this video demonstrates some various sounds and basics of how they are controlled:

Different fluorescent tubes can produce quite different sounds. This video has clips of six types of tube making a range of noises:

Clips below show experiments sending control signals to the prototype from other instruments.

This video shows what happened when we plugged in a CV signal from an Arturia sequencer to a modulatuion channel of the TM01. The keyboard is also sending a timing signal to a Korg Monotribe.

Another experiment, using the gate signal from the KeyStep to send a step waveform... making some crazy noises! 

Trivia

Why telmatron?
Telmatron means something like 'swamp tube' - this was chosen to describe the effect of the plasma excitation control which does progressively swamp the audio signal. It also seems appropriate, given the very wet noises some tubes produce.

What about the earlier instruments?
The telmatron is not the first instrument to generate sound using a gas discharge - various early electronic instruments/electrophones such as the Trautonium also relied upon current fluctuations in small neon tubes and thyratrons. The non-linearities of gas discharges can be problematic for maintaining a consistently reproducible pitch, and they were not widely used beyond the mid-20th century.

Links

These are a few sites that have been interesting and helpful for this project. Much other information about plasmas and synthesisers is also online, wikipedia is brilliant.

Fascinating history of electronic instruments
https://120years.net/wordpress/

Great collection of unusual instruments and resources
http://www.oddmusic.com/

Anarchic site featuring some crazy synthesisers
http://www.deviantsynth.com/

Fantastic resource for valve circuit design
http://www.valvewizard.co.uk/

Another great site for valve electronics
https://www.angelfire.com/electronic/funwithtubes/

GBVP - a project to restart valve manufacture in the UK
https://brimaruk.com/menugbvp/great-british-valve-project-2/