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cpsr16

A CircuitPython-based drum machine, suitable for accompanying live performaces.

Uses a 2-button footswitch to advance patterns, select fills, tap tempo.

Inspired by the ancient and venerable Alesis SR-16 - hence the name, CircuitPython SR-16.

Features

  • Two-button foot switch: Start/Stop (we will refer to as "SS", on left) and Count/Fill ("CF", on right).
    • If DM is stopped, pressing CF more than once sets tempo. ** This just uses the last two presses; better would be to average several.
    • If DM is stopped, pressing SS starts pattern MAIN A.
    • If DM is playing, pressing SS stops play immediately.
    • Fill; if DM is playing,
      • As soon as you press CF, FILL X takes over from MAIN X and plays until the end of the fill plus the following fill downbeat.
        • The "following downbeat" is a nice feature of the SR-16; see "Notes from SR-16 manual" below.
      • Normally after playing FILL X, DM will revert back to MAIN X.
      • If a fill is playing and the CF button is pressed again, after playing FILL X it will transition to MAIN Y.
  • Display
    • This is problematic. The Pico doesn't work well with SPI displays - not with what I'm asking of it.
    • A small I2C OLED display seems to work OK. Adafruit product #4440 (128x32) or #326 (128x64).
    • A new feature is a NeoPixel 8-LED strip with a "Christmas tree" display.

Hardware

TO DO

  • Longer measures?
  • Implement hardware config file?
  • "Ready" beep?
  • Overall volume control?

Data file format

The input JSON file is basically just an encoding of a Python object. It's a list of dictionaries, each of which has the following keys:

  • "file comment": An optional string that will be ignored, for documentation, placed in the first dictionary of the list. (Of course, any unknown key will act the same.)
  • "setup": The name that will be displayed on the GUI.
  • "comment": OIptional doc string.
  • "bpm": An integer
  • "ticks_per_measure": An integer, usually 16.
  • "measures_per_pattern": An integer, usually 1 or 2.
  • "pads": A dictionary of
    • {pad name}: {wav file path}
  • "patterns": A dictionary with the keys "main a", "fill a", "main b", "fill b", each of which is a dictionary with one or more (zero or more?) keys of {pad name} (where {pad name} was declared in the "pads" section), with a key value of a string. The string has ({ticks_per_measure} * {measures_per_pattern}) non-blank characters (blanks are ignored) that are either "-", meaning that pad isn't played on that tick, or a digit 0 thru 9, meaning the pad is played at that relative volume. (So "-" and "0" are equivalent.)

Hardware Configuration

Signal Pico Pin # RP2350 Pin # Device
I2C SDA 1 - Display SDA
I2C SCL 2 - Display SCL
GND 3* ? Display GND
AUDIO_OUT_I2S_BIT 11 (board.GP8) ? PCM BCLK
AUDIO_OUT_I2S_WORD 12 (board.GP9) ? PCM WSEL
AUDIO_OUT_I2S_DATA 14 (board.GP10) ? PCM DIN
GND 18* ? PCM GND
BUTTON_B 21 (board.GP16) ? Center menu button
BUTTON_A 22 (board.GP17) ? Left-hand menu button
GND 23* ? (all 3 buttons)
BUTTON_C 24 (board.GP18) ? Right-hand menu button
GND 33* ? Footswitch jack SLEEVE
SWITCH_1 31 (board.GP28) ? Footswitch jack RING
SWITCH_2 32 (board.GP27) ? Footswitch jack TIP
3v3 36 ? PCM VIN
3v3 36 - Display VIN
3v3 ? D13 NeoPixel VIN

*Note: GND is GND - use whatever pin you want! For

Also note: Pico is using "Adafruit UDA1334 I2S DAC" board; does not need XXX line tied to ground, as does the Amazon part.

Credits

Notes

Convert files to appropriate WAV format (mono, 22050 Hz, 16-bit signed) with command:

sox {original_file}.mp3 -b 16 -c 1 -r 22050 {output_file}.wav

Reference

Patterns

Notes from SR-16 manual

This is not actually what we do, but it was the inspiration.

There are 50 of each type of Pattern, numbered 00-49. However, each numbered Pattern actually contains four different "sub-Patterns":

• A pair of independent Main Patterns (A and B, selected by their respective buttons). • A pair of associated Fill Patterns (A Fill and B Fill, selected by pressing the FILL button when either A or B is selected). The Fills primarily provide transitional Patterns between Main Patterns, which makes for more realistic drum parts. The associated Fill Patterns share the same length, Drum Set, and name as their Main Patterns (e.g., if A is 16 beats, A Fill is 16 beats). Otherwise, they are independent.

The reason for pairing the two different A and B Patterns together is simply so that you can switch back and forth between them rapidly in live performance or while improvising. However, A and B Patterns can be treated as completely independent Patterns if desired and can have different lengths, Drum Sets, etc.

Always think of the Main Pattern and its associated Fill as a unit. For example, if you copy a Main Pattern to another Main Pattern, its Fill will travel along with it. Note that even though there are "only" 50 Patterns, the A and B variations double that to 100 Patterns, and the Fills double that again to 200 Patterns. Added to the Preset Patterns, 400 total Patterns are available.

Fill Patterns are the key to creating expressive drum parts. However, Fill is a sophisticated feature that requires some explanation.

...

Remember that Fills are always the same length, and use the same Drum Set, as the associated Main Pattern. This lets you "drop in" a Fill at any time. As soon as you press the FILL button, the Fill takes over from the Main Pattern and starts playing until the end of the Fill.

Generally, Fills are transitional Patterns. Example: Suppose an 8-beat A Main Pattern is playing and you press the FILL button on beat 4. The A Fill Pattern will play the last 4 beats and then automatically transition into the B Main Pattern. Conversely, if B Main is playing and you press Fill, after B Fill has played, the SR-16 will transition to the A Main Pattern. You can select the Fill at any time the Main Pattern is playing.

However, Fills do not have to be transitional Patterns. If you press the FILL button (or footswitch) before the Fill has finished playing and hold it down until after the Fill has played (i.e., past the downbeat of the next Pattern), the SR-16 will return to the original Main Pattern.

Example: Suppose an 8-beat A Main Pattern is playing and you press the FILL button on beat 4 but hold it down past beat 8. The A Fill Pattern will play the last 4 beats, then the SR-16 will return to the A Main Pattern.

Fills cannot start on the downbeat since a Fill, by definition, starts at some point into the Main Pattern. However, anything you record on the Fill downbeat will play on the first downbeat following the Fill (i.e., the downbeat of the next Pattern). To show why this is a useful feature, consider that when coming out of a fill, you'll often want to hit something like a cymbal crash on the downbeat of the next Pattern yet not have that crash repeat every time the Pattern plays. This way of handling Fills lets the downbeat cymbal crash be part of the Fill instead of the Pattern.

Background

This way of handling Patterns explains the logic behind having A, B, and Fill Patterns. In typical pop tunes, A would be the verse and B the chorus. A Fill provides the Fill that transitions from verse to chorus, and B Fill provides the Fill that transitions from chorus to verse. Thus, one of the numbered Patterns may be all you need to put together a tune.

This structure makes it possible to put together songs in minutes using the Preset Patterns. It also makes it easy to play drum parts live. For example, if there's a solo happening over the A Main Pattern, you can keep the Pattern repeating until the solo is about to end, at which point you select the Fill that leads out of the A Main Pattern.

About

CircuitPython-based performance-oriented drum machine.

Topics

adafruit circuitpython raspberry-pi-pico

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License

GPL-3.0 license
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