Tracking the Beat

Last month in this series we took a deep technical dive into the randomisation system underpinning Sonic Pi. We explored how we can use it to deterministically add new levels of dynamic control over our code. This month we’re going to continue our technical dive and turn our attention to Sonic Pi’s unique tick system. By the end of this article you’ll be ticking your way through rhythms and riffs on your way to being a live coding DJ.

Beat Counting

When making music we often want to do a different thing depending on which beat it is. Sonic Pi has a special beat counting system called tick to give you precise control over when a beat actually occurs and even supports multiple beats with their own tempos.

Let’s have a play - to advance the beat we just need to call tick. Open up a fresh buffer, type in the following and hit Run:

puts tick #=> 0

This will return the current beat: 0. Notice that even if you press the Run button a few times it will always return 0. This is because each run starts a fresh beat counting from 0. However, whilst the run is still active, we can advance the beat as many times as we want:

puts tick #=> 0
puts tick #=> 1
puts tick #=> 2

Whenever you see the symbol #=> at the end of a line of code it means that that line will log the text on the right-hand-side. For example, puts foo #=> 0 means the code puts foo prints 0 to the log at that point in the program.

Checking the Beat

We’ve seen that tick does two things. It increments (adds one) and returns the current beat. Sometimes we just want to look at the current beat without having to increment it which we can do via look:

puts tick #=> 0
puts tick #=> 1
puts look #=> 1
puts look #=> 1

In this code we tick the beat up twice and then call look twice. We’ll see the following values in the log: 0, 1, 1, 1. The first two ticks returned 0, then 1 as expected, then the two looks just returned the last beat value twice which was 1.

Rings

So now we can advance the beat with tick and check the beat with look. What next? We need something to tick over. Sonic Pi uses rings for representing riffs, melodies and rhythms and the tick system has been specifically designed to work very closely with them. In fact, rings have their own dot version of tick which does two things. Firstly, it acts like a regular tick and increments the beat. Secondly it looks up the ring value using the beat as the index. Let’s take a look:

puts (ring :a, :b, :c).tick #=> :a

.tick is a special dot version of tick which will return the first value of the ring :a. We can grab each of the values in the ring by calling .tick multiple times:

puts (ring :a, :b, :c).tick #=> :a
puts (ring :a, :b, :c).tick #=> :b
puts (ring :a, :b, :c).tick #=> :c
puts (ring :a, :b, :c).tick #=> :a
puts look                   #=> 3

Take a look at the log and you’ll see :a, :b, :c and then :a again. Notice that look returns 3. Calls to .tick act just like they are regular calls to tick - they increment the local beat.

A Live Loop Arpeggiator

The real power comes when you mix tick with rings and live_loops. When combined we have all the tools we need to both build and understand a simple arpegiator. We need just four things:

1. A ring containing the notes we want to loop over.
2. A means of incrementing and obtaining the beat.
3. The ability to play a note based on the current beat.
4. A loop structure to keep the arpegiator repeating.

These concepts can all be found in the following code:

notes = (ring 57, 62, 55, 59, 64)

live_loop :arp do
  use_synth :dpulse
  play notes.tick, release: 0.2
  sleep 0.125
end

Let’s look at each of these lines. First we define our ring of notes which we’ll continually play. We then create a live_loop called :arp which loops round for us. Each time round the live_loop we set our synth to :dpulse and then play the next note in our ring using .tick. Remember that this will increment our beat counter and use the latest beat value as an index into our notes ring. Finally, we wait for an eighth of a beat before looping round again.

Multiple Simultaneous Beats

A really important thing to know is that ticks are local to the live_loop. This means that each live_loop has its own independent beat counter. This is much more powerful than having a global metronome and beat. Let’s take a look at this in action:

notes = (ring 57, 62, 55, 59, 64)

with_fx :reverb do
  live_loop :arp do
    use_synth :dpulse
    play notes.tick + 12, release: 0.1
    sleep 0.125
  end
end

live_loop :arp2 do
  use_synth :dsaw
  play notes.tick - 12, release: 0.2
  sleep 0.75
end

Clashing Beats

A big cause of confusion with Sonic Pi’s tick system is when people want to tick over multiple rings in the same live_loop:

use_bpm 300
use_synth :blade
live_loop :foo do
  play (ring :e1, :e2, :e3).tick
  play (scale :e3, :minor_pentatonic).tick
  sleep 1
end

Even though each live_loop has its own independent beat counter, we’re calling .tick twice within the same live_loop. This means that the beat will be incremented twice every time round. This can produce some interesting polyrhythms but is often not what you want. There are two solutions to this problem. One option is to manually call tick at the start of the live_loop and then use .look to look up the current beat in each live_loop. The second solution is to pass a unique name to each call to .tick such as .tick(:foo). Sonic Pi will then create and track a separate beat counter for each named tick you use. That way you can work with as many beats as you need! See the section on named ticks in 9.4 of the built-in tutorial for more information.

Bringing it all together

Let’s bring all this knowledge of ticks, rings and live_loops together for a final fun example. As usual, don’t treat this as a finished piece. Start changing things and play around with it and see what you can turn it into. See you next time…

use_bpm 240
notes = (scale :e3, :minor_pentatonic).shuffle

live_loop :foo do
  use_synth :blade
  with_fx :reverb, reps: 8, room: 1 do
    tick
    co = (line 70, 130, steps: 32).tick(:cutoff)
    play (octs :e3, 3).look, cutoff: co, amp: 2
    play notes.look, amp: 4
    sleep 1
  end
end

live_loop :bar do
  tick
  sample :bd_ada if (spread 1, 4).look
  use_synth :tb303
  co = (line 70, 130, steps: 16).look
  r = (line 0.1, 0.5, steps: 64).mirror.look
  play notes.look, release: r, cutoff: co
  sleep 0.5
end