Poly

Poly is a command-line assistant designed to generate MIDI file from the description of drum parts. It provides a convenient way to input a DSL (Domain-Specific Language) in the command line, then it calculates when the drum parts will converge together, making it easy to compose polyrhythimic parts with frequent shifts over the bar lines. Additionally, it has the capability to generate a bass MIDI track that follows the kick drum.

Poly is specifically designed to assist musicians and composers working in genres such as modern progressive rock, metal, djent, fusion, and Indian Carnatic music. It aims to simplify the process of creating complex polyrhythmic drum patterns, enabling users to focus on the creative aspects of their compositions.

Motivation

I'm a guitar player, and I use tablature notation editors such as Guitar Pro a lot. However, it gets complicated fast to write a polyrhythmic/polymetric drum parts as shifts tend to go over the bar lines. The other property of such parts is: it tend to unfold in a simple way from simple ideas such as "I want to create a drum part that will have a 3 against 4 feel with a kick drum against a snare drum". The other way to think about it is that it has a simple blueprint, but it's tricky to express in the western musical notation. This is why Poly exists. I wanted to have a simple tool to workshop/brainstorm rhythmic ideas and evaluate them by having a MIDI playback. I'm into modern progressive rock/metal music, fusion, so it all applies very well. I have an impression it may be useful for indian carnatic music as well, but I would like to get some insightful confirmation on that.

Usage

Poly runs as an executable with the desired command line options. The available options are as follows:

Usage: poly [OPTIONS]

Options:
  -K, --kick <KICK>
          Kick drum pattern
  -S, --snare <SNARE>
          Snare drum pattern
  -H, --hi-hat <HIHAT>
          Hi-Hat pattern
  -C, --crash <CRASH>
          Crash cymbal pattern
  -t, --tempo <TEMPO>
          Tempo value [default: 120]
  -s, --time-signature <TIME_SIGNATURE>
          Time signature [default: 4/4]
  -o, --output-file <OUTPUT>
          Output file path, make a dry run if omitted
  -B, --follow-kick-drum-with-bass
          Generate a second MIDI track for the bass following the kick drum
  -h, --help
          Print help
  -V, --version
          Print version

Poly uses a simple DSL (Domain-specific language) for drum patterns. For a more detailed explanation, go to DSL Overview.

Let's say you want to tell if two patterns will converge and how soon. We'll start with a 3 against 4. The first pattern would be a series of 8th notes on the kick drum

poly  --kick '8x--x--' --snare '4-x'

Output

No output file path was supplied, running a dry run...
Converges over 3 bars

We haven't provided an --output-file / -o parameter, so poly made a dry run. It tells us it will converge in 3 bars. Let's see how it will look in the MIDI file by adding an output.

poly  --kick '8x--x--' --snare '4-x' -o out.mid

Output

Converges over 3 bars
out.mid was written successfully

Poly operates under an assumption, that it's easy to replicate a pattern that converges over a finite number of bars in the DAW or tablature editor, so it only generates 3 bars of drums in this case. On Mac OS, I usually do something in lines of poly <OPTIONS> -o out.mid && open out.mid or poly <OPTIONS> -o out.mid && open -a 'Guitar Pro 7' out.mid.

This way it defaults to 4/4 as a time signature, but we may want to interpret this rhythmic pattern in 3/4 for example. Let's try it:

poly --time-signature '3/4' --kick '8x--x--' --snare '4-x' -o out.mid && open -a 'Guitar Pro 7' out.mid

Output:

Converges over 2 bars
out.mid was written successfully

Now we can see it converges in 2 bars, not 3. Honestly, I like the 4/4 host time signature better. Let's get back to it. Also, we can add a crash cymbal and hi-hat patterns too, also we can make it just a bit livelier by increasing the tempo:

poly --time-signature '4/4' --tempo 138 --crash '4x---' --hi-hat '8-xxx' --kick '8x--x--' --snare '4-x' -o out.mid

That's cool, but let's make it even more useful by adding a blueprint for the bass track. Simple way of doing that is to make bass follow the kick drum. Poly has an option to do this called -B/--follow-kick-drum-with-bass. Let's add it to the previous command to add the bass track to the output file:

poly --time-signature '4/4' --tempo 138 --crash '4x---' --hi-hat '8.t-xxx' --kick '8x--x--' --snare '4-x' -o out.mid -B

Now we have two tracks in the output file and you can change the bass notes to create an expected harmonic context.

Let's try one more thing:

poly -t 115 -K '32xx16xx' -H '8x' -S '4--x-' -B -o bleed.mid

Congratulations, now you have a basic version of "Bleed" by Meshuggah!

DSL overview

Any pattern can be described by a series of note groups. All note in the note group have the same length. Possible lengths are:

• 1 - Whole note
• 2 - Half note
• 4 - Fourth note
• 8 - Eighth note
• 16 - Sixteenth note
• 32 - Thirty secondth note
• 64 - Sixty Fourth note
• . - dotted note (meaning it has 1.5 length of the unmodified duration). Dot should be applied after the basic length like this: 8.
• t - Triplet notes, should be applied after basic lengths and dots. e.g. 4.t means triplets of dotted fourth notes.

Now let's talk about the drums. Poly has a logic similar to a drum machine, so we only concern ourselves with drum hits and rests:

• x - Hit
• - - Rest

Let's compose a few simple note groups:

• 4x - a group of one fourth note.
• 8.-x a group of a rest and a drum hit. Both rest and hit have a length of 8th dotted note each.

It's possible to repeat a group of notes of same length with the following syntax:

• (3,8x-x) means repeat three times a series of hit, rest, hit in eighth notes

Now that we know that, we may sequence multiple groups like this:

• 32xx16xx - Kick pattern from "Bleed" by Meshuggah

...

Guitar pro remarks

Don't forget to quantize MIDI imports to 64th notes as it gets increasingly crazier as we get into the wilder note groupings: