Controlling Your Sound

So far during this series we’ve focussed on triggering sounds. We’ve discovered that we can trigger the many synths built into Sonic Pi with play or synth and how to trigger pre-recorded samples with sample. We’ve also looked at how we can wrap these triggered sounds within studio FX such as reverb and distortion using the with_fx command. Combine this with Sonic Pi’s incredibly accurate timing system and you can produce a vast array of sounds, beats and riffs. However, once you’ve carefully selected a particular sound’s options and triggered it, there’s no ability to mess with it whilst it’s playing right? Wrong! Today you’re going to learn something very powerful - how to control running synths.

A Basic Sound

Let’s create a nice simple sound. Fire up Sonic Pi and in a fresh buffer type the following:

synth :prophet, note: :e1, release: 8, cutoff: 100

Now press the Run button at the top left to hear a lovely rumbling synth sound. Go ahead, press it again a few times to get a feel for it. OK, done? Let’s start controlling it!

Synth Nodes

A little known feature in Sonic Pi is that the fns play, synth and sample, return something called a SynthNode which represents a running sound. You can capture one of these SynthNodes using a standard variable and then control it at a later point in time. For example, let’s change the value of the cutoff: opt after 1 beat:

sn = synth :prophet, note: :e1, release: 8, cutoff: 100
sleep 1
control sn, cutoff: 130

Let’s look at each line in turn:

Firstly we trigger the :prophet synth using the synth fn as normal. However we also capture the result in a variable called sn. We could have called this variable something completely different such as synth_node or jane - the name doesn’t matter. However, it’s important to choose a name that’s meaningful to you for your performances and for people reading your code. I chose sn as it’s a nice short mnemonic for synth node.

On line 2 we have a standard sleep command. This does nothing special - it just asks the computer to wait for 1 beat before moving onto the next line.

Line 3 is where the control fun starts. Here, we use the control fn to tell our running SynthNode to change the cutoff value to 130. If you hit the Run button, you’ll hear the :prophet synth start playing as before, but after 1 beat it will shift to sound a lot brighter.

Modulatable Options

Most of Sonic Pi’s synths and FX opts may be changed after being triggered. However, this isn’t the case for all of them. For example, the envelope opts attack:, decay:, sustain: and release: can only be set when triggering the synth. Figuring out which opts can and can’t be changed is simple - just head to the documentation for a given synth or FX and then scroll down to the individual option documentation and look for the phrases “May be changed whilst playing” or “Can not be changed once set”. For example, the documentation for the :beep synth’s attack: opt makes it clear that it’s not possible to change it:

• Default: 0
• Must be zero or greater
• Can not be changed once set
• Scaled with current BPM value

Multiple Changes

Whilst a synth is running you’re not limited to changing it only once - you’re free to change it as many times as you like. For example, we can turn our :prophet into a mini arpeggiator with the following:

notes = (scale :e3, :minor_pentatonic)
sn = synth :prophet, note: :e1, release: 8, cutoff: 100
sleep 1
16.times do
  control sn, note: notes.tick
  sleep 0.125
end

In this snippet of code we just added a couple of extra things. First we defined a new variable called notes which contains the notes we’d like to cycle through (an arpeggiator is just a fancy name for something that cycles through a list of notes in order). Secondly we replaced our single call to control with an iteration calling it 16 times. In each call to control we .tick through our ring of notes which will automatically repeat once we get to the end (thanks to the fabulous power of Sonic Pi’s rings). For a bit of variety try replacing .tick with .choose and see if you can hear the difference.

Note that we can change multiple opts simultaneously. Try changing the control line to the following and listen for the difference:

control sn, note: notes.tick, cutoff: rrand(70, 130)

Sliding

When we control a SynthNode, it responds exactly on time and instantly changes the value of the opt to the new one as if you’d pressed a button or flicked a switch requesting the change. This can sound rhythmical and percussive - especially if the opt controls an aspect of the timbre such as cutoff:. However, sometimes you don’t want the change to happen instantaneously. Instead, you might want to smoothly move from the current value to the new one as if you’d moved a slider or dial. Of course, Sonic Pi can also do this too using the _slide: opts.

Each opt that can be modified also has a special corresponding _slide: opt that allows you to specify a slide time. For example, amp: has amp_slide: and cutoff: has cutoff_slide:. These slide opts work slightly differently than all the other opts in that they tell the synth note how to behave next time they are controlled. Let’s take a look:

sn = synth :prophet, note: :e1, release: 8, cutoff: 70, cutoff_slide: 2
sleep 1
control sn, cutoff: 130

Notice how this example is exactly the same as before except with the addition of cutoff_slide:. This is saying that next time this synth has its cutoff: opt controlled, it will take 2 beats to slide from the current value to the new one. Therefore, when we use control you can hear the cutoff slide from 70 to 130. It creates an interesting dynamic feel to the sound. Now, try changing the cutoff_slide: time to a shorter value such as 0.5 or a longer value such as 4 to see how it changes the sound. Remember, you can slide any of the modifiable opts in exactly this way and each _slide: value can be totally different so you can have the cutoff sliding slowly, the amp sliding fast and the pan sliding somewhere in between if that’s what you’re looking to create…

Bringing it all together

Let’s look at a short example which demonstrates the power of controlling synths after they’ve been triggered. Notice that you can also slide FX just like synths although with a slightly different syntax. Check out section 7.2 of the built-in tutorial for more information on controlling FX.

Copy the code into a spare buffer and take a listen. Don’t stop there though - play around with the code. Change the slide times, change the notes, the synth, the FX and the sleep times and see if you can turn it into something completely different!

live_loop :moon_rise do
  with_fx :echo, mix: 0, mix_slide: 8 do |fx|
    control fx, mix: 1
    notes = (scale :e3, :minor_pentatonic, num_octaves: 2).shuffle
    sn = synth :prophet , sustain: 8, note: :e1, cutoff: 70, cutoff_slide: 8
    control sn, cutoff: 130
    sleep 2
    32.times do
      control sn, note: notes.tick, pan: rrand(-1, 1)
      sleep 0.125
    end
  end
end