Analog and Digital Sound Synthesis as Relaxation Oscillators

In a recent workshop on e-textile sound for the 2018 e-Textile Spring Camp, I provided a unifying framework for analog and digital sound synthesis and capacitive sensing techniques under the rubric of relaxation oscillators.

The following pages provide context for the workshop and include links to contributions from my co-teacher, Martin De Bie. This includes pointers to various iterations of his 555 timer-based Textilo project.

This project will be expanded ongoingly with further relaxation oscillator schemes.

Introduction to Relaxation Oscillators

Here is a non-electronic relaxation oscillator:

Relaxation Oscillators Generally Speaking

Relaxation oscillators produce nearly periodic variations in a signal by accumulating energy until a certain threshold is reached at which point energy is released making room for subsequent energy accumulation.

This general definition allows us to organize relaxation oscillators we encounter according to the nature of the energy accumulated, the way the relaxation threshold is determined and how energy is released. For example:

Device	Energy	Where	Threshold	Link
Shishi Odoshi	Water Level	Bamboo Tube	Balance Point	https://en.wikipedia.org/wiki/Shishi-odoshi [2]
Old Faithful Guyser	Steam	Rock Tube	Vapor point	http://www.oldfaithfulgeyser.com/learn/geyser-info/ [3]
555 Astable	Charge	Capacitor	Voltage Comparator	https://en.wikipedia.org/wiki/555_timer_IC#Astable [4]
Neon	Charge	Capacitor	Ionization	https://en.wikipedia.org/wiki/Pearson–Anson_effect
Bowed String	Tension	String	Friction	http://newt.phys.unsw.edu.au/jw/Bows.html [5]
UJT	Charge	Capacitor	PN Junction Potential	http://www.circuitstoday.com/ujt-relaxation-oscillator [6]
Schmitt Trigger	Charge	Capacitor	MOS Junction Potential	https://en.wikipedia.org/wiki/Schmitt_trigger#Use_as_an_oscillator [7]
Opamp	Charge	Capacitor	Voltage Comparator	https://en.wikipedia.org/wiki/Relaxation_oscillator#Comparator–based_relaxation_oscillator
BJT Blocking Oscillator	Flux	Inductor	Current Comparator	https://en.wikipedia.org/wiki/Blocking_oscillator [8]
Phase Accumulator (Phasor)	Charge	Register	Modulo operation	https://en.wikipedia.org/wiki/Numerically_controlled_oscillator [9]
Run-up Time ADC	Charge	Capacitor	MOS Junction Potential	https://github.com/adrianfreed/FastTouch [10]

Textile 555 Oscillator ElectroSomatophone for circuit bending

When I saw Martin De Bie's Textilo [11] e-textile 555 oscillator I wondered if it could be extended to allow for body interactions with the pads using CMOS 555 timers in the spirit of the CrackleBox.

I tested this CMOS 555 breadboard version.

This informed suggested improvements to Textilo to Martin who came up with a textile layout which we used to teach during the audio workshop of e-textile Spring Break: http://youtube.com/HdMsk6c3WGw [12]

Alex blogged a great student perspective of this workshop [13].

I also brought along some PCB's which can be sewed to or pressed on Lego baseboards.

To address the need for more interaction room for the fingers I suggested to Martin to look at a radiating octagonal design. He prototyped this at the camp in copper:

Nicole Messier also picked up on this design pattern and made a 555 oscillator based FM Radio transmitter:

A walk through the schematic may be helpful.

We install a capacitor to ground (470pF) from pin 2.

Pin 6 is connected to Pin 2.

A 1Mohm pullup resistor is used on pin 4 (reset).

There is a decoupling capacitor for the speaker output (pin 3)

The diode (from battery to pin 8) is to help you not destroy the 555 timer if you install the battery backwards.

Now what makes things productively confusing is the absence of resistors in the story. These are provided by your fingers or additional LDRs or piezoresistive fabrics. The layout is designed to give you room to put fingers in the useful places for the two popular ways of making the 555 timer oscillator [14]. One way creates square waves. The other makes controllable pulse waves. I capture these possibilities on the following schematic which has a special notation for where you add variable resistance:

555 Timer resources

History

There are few inaccurate histories of the 555 timer invention on the web so a good source is from the designer himself, e.g., http://semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzi... [15]

He also wrote several interesting books. The history and detailed design considerations are covered in a book he kindly made available as a PDF file before his death in 2012: http://www.designinganalogchips.com/_count/designinganalogchips.pdf [16]

Circuits

This is a reasonable compendium of interesting circuits using the IC: http://www.555-timer-circuits.com [17]

Some more:

http://www.nutsvolts.com/magazine/article/using-the-555-timer-ic-in-spec... [18]

Radio Circuits

AM Receiver

https://hackaday.com/2011/02/25/hear-that-its-a-555-timer-am-radio/ [19]

AM transmitter

http://www.circuitsgallery.com/2013/08/555-AM-transmitter-circuit.html [20]

http://streampowers.blogspot.com/2013/07/using-555-as-fm-transmitter-cir... [21]

Audio Circuits

VCO

High Performance VCO : http://www.electro-music.com/forum/topic-54623.html [22] DRAWDIO

https://www.adafruit.com/product/124 [23]

ATARI PUNK CONSOLE http://www.synthrotek.com/products/lo-fi-synthesizer-circuits/atari-punk... [24]

Variations

The CMOS versions of the 555 timer are interesting and lend themselves well to low voltage e-textile applications. My favorite variant is this one: https://www.jameco.com/z/CSS555-ID-Custom-Silicon-Solutions-CSS555-Micro... [25]

Here is the largest variant I know of: https://shop.evilmadscientist.com/productsmenu/tinykitlist/652-555kit [26]

and the smallest (a 1mm x 1mm) package:

http://www.ti.com/product/LMC555/samplebuy [27] https://www.mouser.com/Semiconductors/Integrated-Circuits-ICs/Clock-Time... [28]

FastTouch Open Source Arduino Library for Fast, Portable, Low Fidelity Capacitive Sensing

The fast touch algorithm reimplemented in the library available here [10] was developed for my FingerPhone Instrument [29]. The fast touch library senses touch on any digital I/O pin on many embedded controllers with the Arduino IDE. This includes Atmega 8-bit, SAMD21, and Teensy. This includes most pins on most micro-controllers. A few micro controllers have analog input pins that can't also do digital I/O . The library relies on the ability to enable a built-in pull-up resistor on the pin being sensed. Here is the algorithm:

set the pin to output a LOW value for 1uS or so.

set the pin to output a high via a built-in pulp resistor.

Read the pin regularly and evenly while it reads a low value.

The sensed value is reported as a function of the time it takes for the pull-up to bring the pin to a high value. This depends on whether the pin is touched.

The example code provided with the library uses the Arduino Tone library to sound pitches according to which pins are touched.

Notice that each call to the fast touch library implements a cycle of a relaxation oscillator.

I am indebted to Alice Giordani for exemplifying use of the library so well in this dreamcatcher:

Sound Making Technologies Ordered by Increasing Complexity/Cost/Size

The entries in this table with an * were briefly reviewed in the 3 hour workshop the 2018 eTextile Spring camp. As the table shows, these are in the middle of the affordability/complexity/size axis and were selected for their potential in e-textile and wearable contexts. We had the good fortune to have Bela well represented in another workshop at the camp. Bela is transitioning to a new compact form factor which will make it easier to use with e-textiles.

Approach	Example	Link
non-electronic	Lamello	http://bid.berkeley.edu/papers/chi/lamello_passive_acoustic_sensi/ [31]
*Analog relaxation oscillator	Textilo	http://www.martindebie.com/research_project/textilo/ [11]
Analog harmonic oscillator	self-resonating VCF	http://electronotes.netfirms.com/EN215.pdf [32]
*Digital relaxation oscillator	Arduino Tone	https://github.com/adrianfreed/FastTouch [10]
*Digital relaxation oscillator	Mozzi (LUT)	https://github.com/sensorium/Mozzi/blob/master/examples/01.Basics/Sinewa... [33]
*Digital Modulation Synthesis	Mozzi (FM)	https://github.com/sensorium/Mozzi/tree/master/examples/06.Synthesis/FMs... [34]
*Digital Subtractive Synthesis	Talkie (LPC Speech and singing)	https://github.com/adrianfreed/Talkie [35]
Unit Generator Library	Teensy Audio Library	https://www.pjrc.com/teensy/td_libs_Audio.html [36]
Sampling "Synthesis"	Tone.js (Tone.Player)	https://tonejs.github.io [37]
Dynamically Patched Unit Generators	Bela (libpd)	https://bela.io [38]
Dynamically Patched Unit Generators with Image Synthesis	Max/MSP/Jitter	https://cycling74.com/products/max [39]
Analog Patched Modular	Modular	https://en.wikipedia.org/wiki/Modular_synthesizer [40]

CMOS Inverter Oscillator

This alternative to the 555 timer for making audio oscillators is built from simpler building blocks (inverters) and has more versatility. You can find the schematics that correspond to the build video here: https://www.youtube.com/watch?v=CzO_ZKMSnAU [41]

Links:
[1] http://www.youtube.com/watch?v=zBVHrm83lSI
[2] https://en.wikipedia.org/wiki/Shishi-odoshi
[3] http://www.oldfaithfulgeyser.com/learn/geyser-info/
[4] https://en.wikipedia.org/wiki/555_timer_IC#Astable
[5] http://newt.phys.unsw.edu.au/jw/Bows.html
[6] http://www.circuitstoday.com/ujt-relaxation-oscillator
[7] https://en.wikipedia.org/wiki/Schmitt_trigger#Use_as_an_oscillator
[8] https://en.wikipedia.org/wiki/Blocking_oscillator
[9] https://en.wikipedia.org/wiki/Numerically_controlled_oscillator
[10] https://github.com/adrianfreed/FastTouch
[11] http://www.martindebie.com/research_project/textilo/
[12] http://youtube.com/HdMsk6c3WGw
[13] https://blog.hackster.io/sound-synthesis-with-arduino-etextile-spring-break-45d4cc87e31e
[14] http://www.555-timer-circuits.com/operating-modes.html
[15] http://semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Index.htm
[16] http://www.designinganalogchips.com/_count/designinganalogchips.pdf
[17] http://www.555-timer-circuits.com
[18] http://www.nutsvolts.com/magazine/article/using-the-555-timer-ic-in-special-or-unusual-circuits
[19] https://hackaday.com/2011/02/25/hear-that-its-a-555-timer-am-radio/
[20] http://www.circuitsgallery.com/2013/08/555-AM-transmitter-circuit.html
[21] http://streampowers.blogspot.com/2013/07/using-555-as-fm-transmitter-circuit.html
[22] http://www.electro-music.com/forum/topic-54623.html
[23] https://www.adafruit.com/product/124
[24] http://www.synthrotek.com/products/lo-fi-synthesizer-circuits/atari-punk-console/
[25] https://www.jameco.com/z/CSS555-ID-Custom-Silicon-Solutions-CSS555-Micropower-Timer-DIP-8_2146433.html
[26] https://shop.evilmadscientist.com/productsmenu/tinykitlist/652-555kit
[27] http://www.ti.com/product/LMC555/samplebuy
[28] https://www.mouser.com/Semiconductors/Integrated-Circuits-ICs/Clock-Timer-ICs/Timers-Support-Products/_/N-6j749?P=1z0yr2i
[29] http://adrianfreed.com/content/fingerphone-sustainably-designed-paper-musical-instrument
[30] http://www.youtube.com/watch?v=ZOcHMaCsNfE
[31] http://bid.berkeley.edu/papers/chi/lamello_passive_acoustic_sensi/
[32] http://electronotes.netfirms.com/EN215.pdf
[33] https://github.com/sensorium/Mozzi/blob/master/examples/01.Basics/Sinewave/Sinewave.ino
[34] https://github.com/sensorium/Mozzi/tree/master/examples/06.Synthesis/FMsynth
[35] https://github.com/adrianfreed/Talkie
[36] https://www.pjrc.com/teensy/td_libs_Audio.html
[37] https://tonejs.github.io
[38] https://bela.io
[39] https://cycling74.com/products/max
[40] https://en.wikipedia.org/wiki/Modular_synthesizer
[41] https://www.youtube.com/watch?v=CzO_ZKMSnAU
[42] http://www.youtube.com/watch?v=ot6KIGoI1Uk