Squawk

Squawk is a minimalistic 8-bit software synthesizer & playroutine library for Arduino.

The approach to generating sound is very simple - much akin to 80's game consoles.
It follows that Squawk sounds very much like an old-school 8-bit video game (http://youtu.be/sFPqoVnI8sY).

Why? Because simple is also fast.

The library is designed to play music in the background - simply tell Squawk to start playing, then go about your business and do whatever else you want your sketch to do. The music will keep playing until you tell it to stop.

Depending on the sample rate (quality), Squawk will use between 10% and 40% of the average Arduino CPU time.
That leaves you with plenty of room to run your other tasks.

Contributors:

• Davey Taylor - squawk
• Xun Yang - small footprint implementation using fat16-library
• Niklas "Salkinitzor" Ström - music
• Philip "Rubberhands" Linde - mod conversion, music

Thanks to:

• Xun Yang
• Fredrik Ericsson
• Linus Åkesson
• David Cuartielles

Making music for Squawk

Music is made using a tracker that can handle ProTracker files.

• A tracker is a type of music composing software.
• ProTracker is a tracker that was popular on the Amiga 500.

If you are unfamiliar with trackers, you will need to refer to the manual for the tracker you choose.

Tracker suggestions

There are many modern trackers that are capable of composing music in this format. These are a few:

• OpenMPT http://openmpt.org/
• MilkyTracker http://www.milkytracker.org/

Tracking rules

When tracking your music, start with template/template.mod, and follow a few additional rules.

1. Do not modify the template instruments/samples
2. Only use instrument/sample 1 in channel 1, 2 in ch. 2, and so on
3. Only use even numbers for parameters on volume related commands
4. Don't use advanced looping (regular jumps/breaks are ok)
5. Don't change tempo (changing speed is ok)
6. Put no more than 64 patterns in your pattern list

Have a look at the music in convert/music for example well-formed ProTracker modules.

Getting your music onto your Arduino

Squawk requires your ProTracker files to be converted into Squawk melodies.

Melodies come in two flavors:

• As data you put in your sketch to download with your code.
  sketches/Squawk_player
• As a file on an SD card connected to your Arduino.
  sketches/SquawkSD_player

Both formats can be generated by converter in the convert/ directory.

If you have an SD card connected to your Arduino, you can also have the Arduino convert your files directly on the SD card.
sketches/SquawkSD_convert

Building the hardware

Using PWM / pulse width modulation

The simplest way of testing things out is to connect a piezo speaker between GND and the chosen output pin (default 3).
The sketches/Squawk_player sketch is tuned to frequencies suitable for a small boxed piezo speaker.

It is also possible to connect output to a speaker system, but this requires a few additional components:

PIN 3 ____/\/\/\________| |____ AUDIO OUT
           1kΩ     _|_  | | 1µF
               20nF___
GND   ______________|__________ AUDIO GND

This is a first order ~8kHz low-pass filter to "smooth out" the PWM carrier frequency.

Using a resistor ladder

The maximum PWM frequency for 8-bit output on a 16MHz Arduino is 62.5kHz.
This is relatively close to audible frequencies (0-20kHz) which makes it difficult to filter out.
There is also some distortion due to interference between the sample rate and carrier frequency.

Both of these problems can be alleviated by using an R-2R resistor ladder.
See http://en.wikipedia.org/wiki/Resistor_ladder for more information.

At this time, this is supported on ATmega168 & ATmega328 devices only.

AUDIO OUT __| |_________              2kΩ
        1µF | |    1kΩ  |___________/\/\/\__ PIN 7
         2kΩ    _/\/\/\_|    1kΩ
PIN 6 __/\/\/\_|___________/\/\/\_    2kΩ
                   1kΩ   _________|_/\/\/\__ PIN 5
         2kΩ    _/\/\/\_|    1kΩ
PIN 4 __/\/\/\_|___________/\/\/\_    2kΩ
                   1kΩ   _________|_/\/\/\__ PIN 3
         2kΩ    _/\/\/\_|    1kΩ
PIN 2 __/\/\/\_|___________/\/\/\_    2kΩ
                   1kΩ   _________|_/\/\/\__ PIN 1
         2kΩ    _/\/\/\_|    2kΩ
PIN 0 __/\/\/\_|___________/\/\/\__ GND +AUDIO GND

This will give you a cleaner signal, with very crisp treble.

To configure your Squawk sketch for this set-up, use the line
SQUAWK_CONSTRUCT_ISR(SQUAWK_RLD_PORTD)

Digital output from a microcontroller is usually not perfectly clean:

• Glitches - differences in switching time between pins
• Ringing - swings in output voltage following level transition
• Noise - "random" voltage fluctuations due to internal operation

Therefore it is not a bad idea to add low-pass filtering even when using a resistor ladder. This will reduce glitches and ringing. Noise is harder to deal with.

Reducing noise

Any noise in the power supply or from the microcontroller itself will propagate to the audio output.

Power supply noise can be reduced by using a clean power supply.

Producing a perfectly clean output signal is not covered by this project, but basically involves:

• Having a separate clean power supply for...
• ...buffering the digital outputs
• Careful filtering of the analog output