Really Tiny Embedded Code

I did a couple of generations of control systems based on 8051 detivatives. These had a bit of ROM - 8K, usually - but only 128 bytes of RAM. The stack used that 128 bytes, and so did your variables. Pretty darn tight fit for interrupt driven code, which needs to use some stack.

I did the If VI Were IX guitar robots on the 8051. It handled MIDI input over the serial port in interrupts, and it had a timer driver interrupt that we used to both update our servo loop and also to derive the pulse chain used to control position on the pluckers - the shaft with a plastic pick or nib that plucks the string when rotated properly.

We put 2 on each shaft - as I said, a pick and a nub, and added support for a MIDI mode switch to select between the two. Based on the requested velocity in MIDI note on commands received from the serial port, we would set the PWM parameters higher (mostly on) for high velocities and lower (mostly off) for low velocities. To make the PWM average out and not be too jerky and noisy, we need to update it as fast as we possibly can. Bear in mind that our 8051 used a 4 MHz clock, and many instruction took more than 4 cycles, so we got less than 1 million instructions per second. Not much power for handling real time updates and asynchronous serial input while playing a guitar in real time.

(Old man ranting at lazy kids mode)
Micro-controller chips today usually have hardware PWM circuits, so we can just load a duty cycle number and provide a really fast MHz+ digital clock and we get a great PWM. Luxury! The 8051 I was using had no PWM hardware, so we implemented it in software using interrupts. Messier, less smooth, lots more code and a few variables in a system that had little room for either. We couldn't even get 1M instructions/sec.

Micro-controllers today also have more RAM - either built in, or external, they just don't make them with 128 bytes much any more. Luxury! (You're supposed to hear the Monty Python bit about having to walk to school uphill both ways, not like the lazy kids today; doesn't come across well in text). Clocks on modern micro-controllers are often in the gigahertz, a thousand or more times faster than the 8051, and also 32 bits wide, so each instruction handles and processes 4 times as much data as the old 8051s could.

So we had all of our local variables - assorted modes (damper tied to notes, or off to allow hammer-on and hammer-off, or on for a damped sound, etc), state (plucking has several states and we need requested and expected positions i order to include the error component in the feedback loop), limit details, channel details, note range details, and more. We also had to have enough left over in the 128 bytes to allow for the registers to be stored during an interrupt (MIDI IO) with enough room for an additional stack frame for an overlapping timer interrupt (servo and position updating).

We managed to squeeze it all in and it works fine. It helps that registers are only 8 bits and there aren't many of them, and the program counter (pushed onto the stack as the return address) is small too - not all that much needs to be pushed on the stack. The upside of little room is that you simply can't have code bloat and variables must be as simple as possible. The result is small enough that you can realistically squeeze all of the bugs out.

The If VI Were IX installation has never crashed due to software fault, and has outlived every single moving part in the system - MIDI sources had to be replaced with a solid state source, pluckers and strings replaced, yet the 8051 micro-controllers are still fine a decade later.

If I was doing this over again from scratch today, I'd probably base it on a Raspberry Pi system with gigabytes of memory and a flash hard drive with tens of gigabytes more. Luxury!

Guitar Robot Revisited

One of the cooler projects I've ever done was for Trimpin's "If VI Were IX" robot guitar installation at the EMP, which is now call the Museum of Pop or MoPop.

Photo is by Thomas Upton who was nice enough to license it under the Creative Commons license. A higher resolution version from Mr. Upton is available on flickr.

The giant whirlwind looking collection of instruments in MoPop shown in the picture above is actually a guitar robot. You can see various guitar-ish looking items with rows of devices lined up on either side of the guitar fret board. These are the guitar robot elements. A little above the center, slightly to the left, the purple and blue-ish guitars are both Trimpin's custom build guitar robot elements.

Each element has a single string, a "plucker," and "frettles" which are the devices lined up along the fretboard. These are solenoids and when you apply voltage to their coils they pull the solenoid closed, which pushes a pad down on the guitar string, one device per fret.

Each string has a controller - well, two for now, one to control frettles and one to control the plucker. A MIDI input is routed to all of the controllers, and groups of 6 pluckers and their associated frettles are assigned to a single MIDI channel as a logical guitar. The mapping from MIDI notes to guitar frets is odd since we skip octaves between strings so that each fret on each string is a unique note. This means that the MIDI played on the robot guitar has to be processed to put notes into artificial octaves (incorrect musically) so that each note can only be played on one specific string, making the control easier to implement.

Trimpin made a second generation version for his own use that was simplified and improved. The plucker controller now also controls the frettles fot the string it plucks. The quality of the sound degrades as youi move up the frets - by 7 it's tinnier, by the 12th fret it sounds lousy - so in the second version Trimpin only provides 5 frets per string, and adds more strings to handle multiple notes simultaneously.

We also added a damper - a soft pad that rests on the strings, damping them so that a plucked note ends very quickly. When a note goes on, we lift the damper for that string, and when we get the note off command we drop the damper back onto the string. If a note-on is followed by a different note-on for the same string, we switch frettles and keep the damper up.

 We modified our response to MIDI note-on commands so that low velocity notes (below 12% of full velocity) do not actually use the plucker, they just keep the damper up and deploy the correct frettle. This allows for hammer on and hammer off techniques - playing notes by hitting frets with the left hand, rather than using the right hand to pluck.

We also added a mode where you can switch off the dampers, so they stay damped the whole time. This allows for a different sounding result - notes are brief and a little muffled,

Using MIDI commands the new generation control has a wider variety of options and sounds available. Trimpin has used the second generation guitar robot control in live performances from time to time, but I dob't think there's a permanent installation using them yet.

We're hoping that the EMP will want to replace the current first generation controls with a 2nd generation control (or by then probably a 3rd generation since we'll most likely add a few more tweaks). We've looked into the devices needed - our original microcontrollers are no longer available new, and using surplus used devices is dicey at best, so we'll be changing microcontrollers and tool sets if we get to do this work.

I hope we get to do it, I enjoy working on robots and robotic guitars are one of the more interesting robots I've ever worked on.