Software
The software is rather complex so I won't go into it in great detail. All the software was written in AVR-GCC using AVR studio 5.  Both of the slave chips have the same exact software on them. Each one is also setup run one coil each. Basically MIDI data from a MIDI instrument (such as a keyboard) or from a computer is sent into the opto isolator in the form of serial data. This data is sent to both of the slave chips. Each slave can be on it's on individual MIDI channel set by the external pot. Depending on the MIDI channel the chip is set to (1 to 16) the processor will ether choose to ignore the data or begin processing it for output. 

Once relevant data is found by one of the uCs it begins by parsing out the rest of the MIDI message and undertaking the action that the MIDI message requires. These actions could be:

• Note On event 
  
• Note Off event
• Pitch Bend event
  
• Control Change event

Note On Event
The controller reacts to this event by performing the necessary calculation  for PW and period of the desired note to be played and loads the data into the hardware timers on board the uC. The hardware timers then take over the processes of waveform generation and create an output that is sent to the fiberoptic link and to the coil. Each uC with play up to 2 notes simultaneously after which point it ignores other Note On events until one is stopped by a Note Off command. The controller automatically makes changes to the PW as more notes are added and/or the frequency changes. The reason for this constant recalculation is to keep the duty cycle within an acceptable range for the DRSSTC. If this operation was not done the coil could end up drawing to much power from the wall and also put unnecessary stress on the coil's electronics.

Note Off Event
The Note Off event turns notes off that are already active in the controllers memory. The note has to match a note already stored in memory for the note off command to take effect. This is so that when playing multiple notes at once it will only shut off the note that is supposed to be turned off and not both.  If two notes are being played and only one is turned off the command also recalculates the PW for the remaining note.

Pitch Bend Event
The pitch bend event modifies the frequency of already playing notes by a max of + or - one octave. Unlike the regular note scales that have set frequencies, the pitch bend event is allowed to slid the frequency in between these set points. This gives an effect that sounds a lot like the mod or bend wheel on a keyboard or a guitar whammy bar. As always every time the frequency is changed  the PW is recalculated using the same algorithm used with the note on and note off commands. 

Control Change Event 
The control change event is mainly used to interact with computer software and keyboard event commands like the stop and play buttons. These would do exactly as they imply and stop all playing notes and reset all pitch bend settings.

Some Physics!
A lot of people ask me about how a Tesla coil can produce music. So I will go into the physics of why Tesla coils can be used to play music.
Tesla Coils do a couple of interesting things that are worth taking note. They create an extremely high voltage high frequency voltage at there output. Note this voltage has enough energy to converter a small part of the air around the coil to a plasma (the purple stuff you can actually see). Now this plasma when it's created forces air out of it's way as it forms. This creates a pressure wave that propagates away from the spark at the speed of sound. 

Now some of you are thinking "Hay isn't that sorta what a speaker dose?" The answer is yes it is!!! When the cone in a speaker is moved foreword or backwards it also creates a pressure wave that propagates through the air at the speed of sound. 

Now a pressure wave is one thing what about about music? I will go back to the speaker for a second. When you move the speaker cone back and forth at a specific frequency it in turn also creates a pressure wave at that same frequency. So if the cone was to move back and forth 330 times a second (330Hz or a middle A note on a keyboard) this would also move the air at 330Hz. This oscillating pressure wave is what we humans can hear with our ears.

With a solid state Tesla coil we have the ability to turn it off and on at a very quick rate of speed. The rate at which we turn the coil on and off also determines how many sparks we make in a given amount of time and also how much air we move out of the way in a give amount of time. So if we turn the coil on and off at 330Hz that is going to displace the air around the coil 330 times a second in turn creating a pressure wave at that exact same frequency! The size of the spark also determines how load the sound will be because the bigger the spark the more air it moves and the loader it is.

    

     

                                                     

                                                     

TeslaSynth by Goodchild Engineering is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Based on a work at http://www.goodchildengineering.com/tesla-synth.

Check out my YouTube Channel for other videos of Tesla Synth.

Question or comments? Drop me a line at egoodchild@goodchildengineering.com