# Amplitude Modulation

This month we’re going to take a deep dive into one of Sonic Pi’s most powerful and flexible audio FX - the `:slicer`. By the end of this article you will have learned how to manipulate the overall volume of parts of our live coded sound in powerful new ways. This will allow you to create new rhythmic and timbral structures and broaden your sonic possibilities.

## Slice that Amp

So, what does the `:slicer` FX actually do? One way to think about it is that it’s just like having someone play around with the volume control on your TV or home hi-fi. Let’s take a look but first, listen to the deep growl of the following code which triggers the `:prophet` synth:

    synth :prophet, note: :e1, release: 8, cutoff: 70
    synth :prophet, note: :e1 + 4, release: 8, cutoff: 80

Now, let’s pipe it through the `:slicer` FX:

    with_fx :slicer do
      synth :prophet, note: :e1, release: 8, cutoff: 70
      synth :prophet, note: :e1 + 4, release: 8, cutoff: 80
    end

Hear how the slicer acts like it’s muting and unmuting the audio with a regular beat. Also, notice how the `:slicer` affects all the audio generated between the `do`/`end` blocks. You can control the speed at which it turns the audio on and off with the `phase:` opt which is short for phase duration. Its default value is `0.25` which means 4 times a second at the default BPM of 60. Let’s make it faster:

    with_fx :slicer, phase: 0.125 do
      synth :prophet, note: :e1, release: 8, cutoff: 70
      synth :prophet, note: :e1 + 4, release: 8, cutoff: 80
    end

Now, play with different `phase:` durations yourself. Try longer and shorter values. See what happens when you choose a really short value. Also, try different synths such as `:beep` or `:dsaw` and different notes. Take a look at the following diagram to see how different `phase:` values change the number of amplitude changes per beat.

Phase duration is the length of time for one on/off cycle. Therefore smaller values will make the FX switch on and off much faster than larger values. Good values to start playing with are `0.125`, `0.25`, `0.5` and `1`.

## Control Waves

By default, the `:slicer` FX uses a square wave to manipulate the amplitude through time. This is why we hear the amplitude on for a period, then immediately off for a period, then back on again. It turns out that the square wave is just one of 4 different control waves that are supported by `:slicer`. The others are saw, triangle and (co)sine. Take a look at the diagram below to see what these look like. We can also hear what they sound like. For example, the following code uses (co)sine as the control wave. Hear how the sound doesn’t turn on and off abruptly but instead smoothly fades in and out:

    with_fx :slicer, phase: 0.5, wave: 3 do
      synth :dsaw, note: :e3, release: 8, cutoff: 120
      synth :dsaw, note: :e2, release: 8, cutoff: 100
    end

Have a play with the different wave forms by changing the `wave:` opt to `0` for saw, `1` for square, `2` for triangle and `3` for sine. See how different waves sound with different `phase:` opts too.

Each of these waves can be inverted with the `invert_wave:` opt which flips it on the y axis. For example, in a single phase the saw wave typically starts high, and slowly goes down before jumping back to the top. With `invert_wave: 1` it will start low and slowly go up before jumping back down again. Additionally, the control wave can be started at different points with the `phase_offset:` opt which should be a value between `0` and `1`. By playing around with `phase:`, `wave:`, `invert_wave:` and `phase_offset` opts you can dramatically change how the amplitude is modified through time.

## Setting your levels

By default, `:slicer` switches between amplitude values `1` (fully loud) and `0` (silent). This can be changed with the `amp_min:` and `amp_max:` opts. You can use this alongside the sine wave setting to create a simple tremolo effect:

    with_fx :slicer, amp_min: 0.25, amp_max: 0.75, wave: 3, phase: 0.25 do
      synth :saw, release: 8
    end

This is just like grabbing the volume knob on your hi-fi and moving it up and down just a little so the sound ‘wobbles’ in and out.

## Probabilities

One of `:slicer`’s powerful features is its ability to use probability to choose whether or not to turn the slicer on or off. Before the `:slicer` FX starts a new phase it rolls a dice and based on the result either uses the selected control wave or keeps the amplitude off. Let’s take a listen:

    with_fx :slicer, phase: 0.125, probability: 0.6  do
      synth :tb303, note: :e1, cutoff_attack: 8, release: 8
      synth :tb303, note: :e2, cutoff_attack: 4, release: 8
      synth :tb303, note: :e3, cutoff_attack: 2, release: 8
    end

Hear how we now have an interesting rhythm of pulses. Try changing the `probability:` opt to a different value between `0` and `1`. Values closer to `0` will have more space between each sound due to the likelihood of the sound being triggered being much lower.

Another thing to notice is that the probability system in the FX is just like the randomisation system accessible via fns such as `rand` and `shuffle`. They are both completely deterministic. This means that each time you hit Run you’ll hear exactly the same rhythm of pulses for a given probability. If you would like to change things around you can use the `seed:` opt to select a different starting seed. This works exactly the same as `use_random_seed` but only affects that particular FX.

Finally, you can change the ‘resting’ position of the control wave when the probability test fails from `0` to any other position with the `prob_pos:` opt:

    with_fx :slicer, phase: 0.125, probability: 0.6, prob_pos: 1  do
      synth :tb303, note: :e1, cutoff_attack: 8, release: 8
      synth :tb303, note: :e2, cutoff_attack: 4, release: 8
      synth :tb303, note: :e3, cutoff_attack: 2, release: 8
    end

## Slicing Beats

One really fun thing to do is to use `:slicer` to chop a drum beat in and out:

    with_fx :slicer, phase: 0.125 do
      sample :loop_mika
    end

This allows us to take any sample and create new rhythmical possibilites which is a lot of fun. However, one thing to be careful about is to make sure that the tempo of the sample matches the current BPM in Sonic Pi otherwise the slicing will sound totally off. For example, try swapping `:loop_mika` with the `loop_amen` sample to hear how bad this can sound when the tempos don’t align.

## Changing tempo

As we have already seen, changing the default BPM with `use_bpm` will make all the sleep times and synth envelope durations grow or shrink to match the beat. The `:slicer` FX honours this too, as the `phase:` opt is actually measured in beats not seconds. We can therefore fix the issue with `loop_amen` above by changing the BPM to match the sample:

    use_sample_bpm :loop_amen

    with_fx :slicer, phase: 0.125 do
      sample :loop_amen
    end

## Bringing it all together

Let’s apply all these ideas into a final example that only uses the `:slicer` FX to create an interesting combination. Go ahead, start changing it and make it into your own piece!

    live_loop :dark_mist do
      co = (line 70, 130, steps: 8).tick
      with_fx :slicer, probability: 0.7, prob_pos: 1 do
        synth :prophet, note: :e1, release: 8, cutoff: co
      end

      with_fx :slicer, phase: [0.125, 0.25].choose do
        sample :guit_em9, rate: 0.5
      end
      sleep 8
    end

    live_loop :crashing_waves do
      with_fx :slicer, wave: 0, phase: 0.25 do
        sample :loop_mika, rate: 0.5
      end
      sleep 16
    end