Music Maker IV

Review — a song in two lists 5 min

• Two lists in step keep a note and its length on the same row.
• One loop with i plays every row.
• Two sprites reading the same lists each need their own i.

Quick-fire

A list called pattern holds 1 0 0 1. The script runs replace item (2) of [pattern v] with ((1) - (item (2) of [pattern v])). What does the list hold now?

Today’s Topic 3 min

• A grid of 32 pads, all clones of one sprite
• A list of 32 slots — 1 for on, 0 for off
• A playhead that broadcasts each step
• play drum (item (row) of [drums v]) for (0.25) beats

Learning outcome

By the end of this lesson you will be able to:

1. Store a grid in one list and find any square in it.
2. Make clones that each know their own row and step.
3. Build a beat machine that plays whatever pattern you click.

Learnt — a beat machine 28 min

1 · A grid is a list

The machine has 4 rows, one per drum, and 8 steps from left to right. That is 32 pads. We keep them in one list of 32, row after row.

To find the slot for any pad:

((((row) - (1)) * (8)) + (step))
            

A second list, drums, says which drum each row plays: 6 4 1 2 — hi-hat, cymbal, snare, kick.

2 · 32 pads from one sprite

One hidden Pad sprite makes all 32. Each clone remembers its own row and step, so those variables are for this sprite only. A clone keeps the values the original had when the clone was made.

define build grid
set [row v] to (1)
repeat (4)
  set [step v] to (1)
  repeat (8)
    create clone of [myself v]
    change [step v] by (1)
  end
  change [row v] by (1)
end

when I start as a clone
set [is clone v] to (1)
go to x: (((step) * (44)) - (198)) y: ((100) - ((row) * (44)))
if <(item ((((row) - (1)) * (8)) + (step)) of [pattern v]) = (1)> then
  switch costume to [on v]
else
  switch costume to [off v]
end
show
            

3 · Click to switch a pad

A clone can be clicked like any sprite. The click flips its slot in the list.

when this sprite clicked
if <(is clone) = (1)> then
  replace item ((((row) - (1)) * (8)) + (step)) of [pattern v] with ((1) - (item ((((row) - (1)) * (8)) + (step)) of [pattern v]))
  if <(item ((((row) - (1)) * (8)) + (step)) of [pattern v]) = (1)> then
    switch costume to [on v]
  else
    switch costume to [off v]
  end
end
            

4 · The playhead

The Playhead sprite walks across the 8 steps. At each step it broadcasts tick and rests for half a beat.

when flag clicked
set [bpm v] to (120)
set tempo to (bpm)
forever
  set [current v] to (1)
  repeat (8)
    set x to (((current) * (44)) - (198))
    broadcast [tick v]
    rest for (0.5) beats
    change [current v] by (1)
  end
end

when I receive [tick v]
if <<(is clone) = (1)> and <(step) = (current)>> then
  if <(item ((((row) - (1)) * (8)) + (step)) of [pattern v]) = (1)> then
    switch costume to [hit v]
    play drum (item (row) of [drums v]) for (0.25) beats
    switch costume to [on v]
  end
end
            

Look at the drum block. Instead of picking a drum from the dropdown, we dropped a round (item (row) of [drums v]) block into it. Row 1 plays drum 6, row 4 plays drum 2.

Basic — the beat sequencer 24 min

Build the whole machine from the Learnt, then play with it.

Step 1 — the lists and variables

1. Lists for all sprites: pattern and drums. Type 6 4 1 2 into drums.
2. Fill pattern with 32 zeros using the little script below. Click it once — you do not need it after that.
3. Variables for all sprites: bpm, current and pads.
4. Hide the two list boxes. Keep bpm showing.

delete all of [pattern v]
repeat (32)
  add (0) to [pattern v]
end
            

Step 2 — the Pad sprite

1. Paint a Pad sprite with three costumes: off (dark), on (bright) and hit (yellow). A rounded square about 36 pixels wide.
2. Give it three variables for this sprite only: row, step and is clone.
3. Under the green flag: hide, set [is clone v] to (0), then build grid :: custom.
4. Add the build grid block, the clone script, the click script and the tick script.

Step 3 — the Playhead

1. Paint a tall, see-through yellow bar as the Playhead.
2. Give it the forever loop from the Learnt.

Step 4 — label the rows

1. Add small Drum-highhat, Drum-cymbal, Drum-snare and Drum sprites down the left, one beside each row.
2. Click pads to make a beat. Try a kick on steps 1 and 5, a snare on 3 and 7.

A tempo knob

Let the player speed the machine up and slow it down.

• ↑ adds 10 to bpm and ↓ takes 10 away, and the tempo follows.
• bpm never goes below 60 or above 200.
• The beat never stops while you change it.

when [up arrow v] key pressed
if <(bpm) < (200)> then
  change [bpm v] by (10)
  set tempo to (bpm)
end
            

Clear and random

Two quick tools for a beat maker.

• Make a variable n to count through the 32 slots.
• C switches every pad off.
• R switches each pad on or off at random.
• The pads change colour straight away, and the green flag starts on a random beat.

when [r v] key pressed
set [n v] to (1)
repeat (32)
  replace item (n) of [pattern v] with (pick random (0) to (1))
  change [n v] by (1)
end
broadcast [redraw v]

when I receive [redraw v]
if <(is clone) = (1)> then
  if <(item ((((row) - (1)) * (8)) + (step)) of [pattern v]) = (1)> then
    switch costume to [on v]
  else
    switch costume to [off v]
  end
end
            

Saved beats

Keep two beats ready, and switch between them with one key.

• Two more lists, rock and dance, each holding 32 slots.
• 1 copies rock into pattern; 2 copies dance.
• The pads change as soon as the beat loads, and it keeps playing.

when [2 v] key pressed
set [n v] to (1)
repeat (32)
  replace item (n) of [pattern v] with (item (n) of [dance v])
  change [n v] by (1)
end
broadcast [redraw v]
            

A bass row

Join Lesson 11’s notes to today’s grid: a fifth row that plays a tune, not a drum.

• 8 more pads along the floor, made by a second sprite, Bass Pad.
• A list bass says which of them are on. A list bass notes says which note each step plays.
• A switched-on bass pad plays its note on the Bass instrument when the playhead reaches it.

when I receive [tick v]
if <<(is clone) = (1)> and <(step) = (current)>> then
  if <(item (step) of [bass v]) = (1)> then
    switch costume to [hit v]
    play note (item (step) of [bass notes v]) for (0.5) beats
    switch costume to [on v]
  end
end
            

Summary 5 min

• A grid can live in one list: slot = (row − 1) × 8 + step.
• Clones keep their own row and step when they are for this sprite only.
• A playhead broadcasts each step; the pads decide for themselves whether to play.
• A round block can go into a dropdown: play drum (item (row) of [drums v]) for (0.25) beats.
• The Music Maker arc is finished: drums, notes, lists and a beat machine.

Sequencer

A machine that plays a pattern of steps, over and over.

Step

One column of the grid — half a beat of time.

Playhead

The marker that shows which step is playing now.

Slot

One item of the pattern list, holding 1 for on or 0 for off.

Ship it

1. Give the project a title and add instructions in the project notes: click pads, ↑ ↓ for speed.
2. Save it, close it for ten minutes, then play it cold, reading only your own notes.
3. Share it on scratch.mit.edu, so you have a link to your beat machine.