13 November 2022: A major improvement to allow tables to be in Flash ROM memory when the polling interval is known at compile time. That saves about 1200 bytes of RAM, which is a big deal on a Nano with 2K! We do this when DO_CONSTANT_POLLTIME is set to 1, which is now the default.
Playtune_poll
An Arduino Tune Generator library that uses interrupt polling
to play a polyphonic musical score.
About PlaytunePlaytune is a family of music players for Arduino-like microcontrollers. They each intepret a bytestream of commands that represent a polyphonic musical score, and play it using different techniques.
The original Playtune that was first released in 2011 uses a separate hardware timer to generate a square wave for each note played simultaneously. The timers run at twice the frequency of the note being played, and the interrupt routine flips the output bit. It can play only as many simultaneous notes as there are timers available. https://github.com/LenShustek/arduino-playtune
This second ("polling") version uses only one hardware timer that interrupts often, by default at 20 Khz, or once every 50 microseconds. The interrupt routine determines which, if any, of the currently playing notes need to be toggled. https://github.com/LenShustek/playtune_poll
This third version also uses only one hardware timer interrupting frequently, but uses the hardware digital-to-analog converter on high-performance microntrollers like the Teensy to generate an analog wave that is the sum of stored samples of sounds. The samples scaled to the right frequency and volume, and any number of instrument samples can be used and mapped to MIDI patches. It currently only supports Teensy. https://github.com/LenShustek/playtune_samp
The fourth version is an audio object for the PJRC Audio Library. https://www.pjrc.com/teensy/td_libs_Audio.html It allows up to 16 simultaneous sound generators that are internally mixed, at the appropriate volume, to produce one monophonic audio stream. Sounds are created from sampled one-cycle waveforms for any number of instruments, each with its own attack-hold-decay-sustain-release envelope. Percussion sounds (from MIDI channel 10) are generated from longer sampled waveforms of a complete instrument strike. Each generator's volume is independently adjusted according to the MIDI velocity of the note being played before all channels are mixed. www.github.com/LenShustek/Playtune_synth
The fifth version is for the Teensy 3.1/3.2, and uses the four Periodic Interval Timers in the Cortex M4 processor to support up to 4 simultaneous notes. It uses less CPU time than the polling version, but is limited to 4 notes at a time. (This was written to experiment with multi-channel multi-Tesla Coil music playing, where I use Flexible Timer Module FTM0 for generating precise one-shot pulses. But I ultimately switched to the polling version to play more simultaneous notes.) www.github.com/LenShustek/Playtune_Teensy
***** Details about this version: Playtune_poll
The advantage of this polling scheme is that you can play more simultaneous notes than there are hardware timers. The disadvantages are that it takes more CPU power, and that notes at higher frequencies have a little bit of jitter that adds some low frequency "undertones". That problem disappears if you use a microcontroller like the Teensy, which uses a fast 32-bit ARM processor instead of the slow 8-bit AVR processors used by most Arduinos so the interrupt frequency can be higher.
The interrupt routine must be really fast, regardless of how many notes are playing and being toggled. We try hard to be efficient by using the following techniques:
Unroll the loop for checking on channel transitions at compile time. (If you are unfamiliar with loop unrolling, see https://en.wikipedia.org/wiki/Loop_unrolling)
Once a score starts playing, all of the processing happens in the interrupt routine. Any other "real" program can be running at the same time as long as it doesn't use the timer or the output pins that Playtune is using.
The easiest way to hear the music is to connect each of the output pins to a resistor (500 ohms, say). Connect other ends of the resistors together and then to one terminal of an 8-ohm speaker. The other terminal of the speaker is connected to ground. No other hardware is needed! An external amplifier is good, though.
By default there is no volume modulation. All tones are played at the same volume, which makes some scores sound strange. This is definitely not a high-quality synthesizer.
There are two optional features you can enable at compile time:
(1) If the DO_VOLUME compile-time switch is set to 1, then we interpret MIDI "velocity" information and try to modulate the volume of the output by controlling the duty cycle of the square wave. That doesn't allow much dynamic range, though, so we make the high pulse of the square wave vary in duration only from 1 to 10 interrupt-times wide.
There's a mapping table from MIDI velocity to pulse width that you can play with. The MIDI standard actually doesn't specify how velocity is to be interpreted; for an interesting emprical study of what synthesizers actually do, see this paper by Roger Dannenberg of CMU: http://www.cs.cmu.edu/~rbd/papers/velocity-icmc2006.pdf
In order for volume modulation to work, the bytestream must include volume information, which can be generated by Miditones with the -v option. (It's best to also use -d, so that the self-describing header is generated.)
(2) If the DO_PERCUSSION compile-time switch is also set to 1, then we interpret notes on MIDI channel 9 (10 if you start counting with 1) to be percussion sounds, which we play with a high-frequency chirp that is dithered with a pseudo-random number. Tables set the frequency and duration of the chirp differently for the various notes that represent different percussion instruments. It will only play one percussion sound at a time.
In order for this to work, the bytestream must have the notes on the percussion track relocated from 0..127 to 128..255, which Miditones will do with the -pt option in addition to the -v option. (Again, it best to also use -d.)
** Using Playtune_poll *****
Unlike the original Playtune, this is not configured as a library because we must make compile-time changes for pin assignments. You should create a sketch directory with the following files in it:
Playtune_poll.ino This file, which has most of the code
Playtune_poll.h The header file, which defines the output pin configuration
Playtune_poll_test.ino The main program, which contains the score(s) you wish to
play, and any other code you want to run.In order to assign the output pins at compile time, you must add lines to the Playtune_poll.h file. How you do that depends on the kind of microcontroller you are using; see the instructions and examples in that file.
You can define up to MAX_CHANS channels, which is 8 by default. You can increase MAX_CHANS up to 16. There is some inefficiency if MAX_CHANS is much larger than the number of channels actually being used.
We also use the TimerOne library files, which you can get at http://playground.arduino.cc/Code/Timer1 and put into your Arduino library directory, or just put in the directory with the other files.
There are seven public functions and one public variable that you can use in your runtime code in Playtune_poll_test.ino.
void tune_start_timer(int microseconds)
This is optional, and is available only if DO_CONSTANT_POLLTIME is set to 0.
Call it to set how often notes should be checked for transitions,
from 5 to 50 microseconds. If you don't call it, we'll pick something that seems
appropriate from the type of microcontroller and the frequency it's running at.void tune_playscore(byte *score)
Call this pointing to a "score bytestream" to start playing a tune. It will
only play as many simultaneous notes as you have defined output pins; any
more will be ignored. See below for the format of the score bytestream.boolean tune_playing
This global variable will be "true" if a score is playing, and "false" if not.
You can use this to see when a score has finished.void tune_speed(unsigned int percent)
This changes playback speed to the specified percent of normal.
The minimum is percent=20 (5x slowdown),
and the maximum is percent=500 (5x speedup).void tune_stopscore()
This will stop a currently playing score without waiting for it to end by itself.void tune_stop_timer()
This stops playing and also stops the timer interrupt.void tune_resumescore (bool init_pins)
The resumes playing a score that was stopped with tune_stopscore() or tune_stop_timer().
If the I/O pins need to be reinitialized because they were used by something else in the
meantime, pass TRUE for init_pins.The score bytestream
The bytestream is a series of commands that can turn notes on and off, and can start a waiting period until the next note change. Here are the details, with numbers shown in hexadecimal.
If the high-order bit of the byte is 1, then it is one of the following commands:
9t nn Start playing note nn on tone generator t. Generators are numbered
starting with 0. The notes numbers are the MIDI numbers for the chromatic
scale, with decimal 60 being Middle C, and decimal 69 being Middle A
at 440 Hz. The highest note is decimal 127 at about 12,544 Hz. except
that percussion notes (instruments, really) range from 128 to 255.
[vv] If DO_VOLUME is set to 1, then we expect another byte with the velocity
value from 1 to 127. You can generate this from Miditones with the -v switch.
Everything breaks if vv is present with DO_VOLIUME set 0, and vice versa!
8t Stop playing the note on tone generator t.
Ct ii Change tone generator t to play instrument ii from now on. This version of
Playtune ignores instrument information if it is present.
F0 End of score: stop playing.
E0 End of score: start playing again from the beginning.If the high-order bit of the byte is 0, it is a command to wait. The other 7 bits and the 8 bits of the following byte are interpreted as a 15-bit big-endian integer that is the number of milliseconds to wait before processing the next command. For example,
07 D0would cause a wait of 0x07d0 = 2000 decimal millisconds or 2 seconds. Any tones that were playing before the wait command will continue to play.
The score is stored in Flash memory ("PROGMEM") along with the program, because there's a lot more of that than data memory.
Where does the score data come from?
Well, you can write the score by hand from the instructions above, but that's pretty hard. An easier way is to translate MIDI files into these score commands, and I've written a program called "Miditones" to do that. See the separate documentation for that program, which is also open source at https://github.com/lenshustek/miditones
Nostalgia from me
Writing Playtune was a lot of fun, because it essentially duplicates what I did as a graduate student at Stanford University in about 1973. That project used the then-new Intel 8008 microprocessor, plus three hardware square-wave generators that I built out of 7400-series TTL. The music compiler was written in Pascal and read scores that were hand-written in a notation I made up, which looked something like this: C Eb 4G 8G+ 2R + F D# This was before MIDI had been invented, and anyway I wasn't a pianist so I would not have been able to record my own playing. I could barely read music well enough to transcribe scores, but I slowly did quite a few of them. MIDI is better!
Len Shustek, originally 4 Feb 2011, but updated for the polling version in July 2016.