Creating Procedural Music with MetaSounds

MetaSound is a high-performance audio system providing audio designers with complete control over a Digital Signal Processing (DSP) graph to generate sound sources.

This guide teaches you how to create a gameplay-driven procedural music system using a MetaSound Source and Blueprint.

Prerequisites

This guide requires a project with player character movement support, such as the First Person Template project.

1 - Create the Music MetaSound

Click image for full size.

Start by creating a MetaSound that plays procedural music. Follow each substep below to build the graph above, section by section.

This process exposes several input parameters for access from Blueprint in future steps.

1.1 - Initial Setup

Create a MetaSound Source that supports persistent stereo audio.

1. Create a MetaSound Source.
   1. In the Content Browser, click the Add button.
   2. Select Audio > MetaSound Source.
   3. Give the newly created MetaSound a name, such as MSS_Music.
2. Double-click the MetaSound to open the MetaSound Editor.
3. In the Interfaces panel, click the Remove (Trash Bin) button next to the UE.Source.OneShot Interface entry. This removes the On Finished Output node, which isn't used on persistent sounds such as ambience or music.
4. Click the MetaSound button on the MetaSound Editor Toolbar.
5. In the Details panel, click the MetaSound > Output Format dropdown and select Stereo. This replaces the Out Mono Output node with an Out Left and an Out Right Output node.

1.2 - Tempo and Rhythm

Build the Tempo and Rhythm section to control the timing of the music with Trigger nodes.

1. Find the On Play Input node and drag off of the pin into an empty space. Enter "Trigger Repeat" into the node search to create a connected node. You can move the node around the graph by dragging it.
2. On the Trigger Repeat node:
   1. Drag off the Period pin and create a BPM To Seconds node.
   2. Drag off the RepeatOut pin and create a Trigger Counter node.
3. On the BPM to Seconds node:
   1. Set Divisions of Whole Note to 16.
   2. Drag off the BPM pin and select Promote to Graph Input. This creates a Float Input node named BPM.
4. Select the BPM node.
5. In the Details panel, set Default Value > Range to 60.0, 180.0.
6. On the Trigger Counter node, set Reset Count to 8.
7. Select all of the nodes except the On Play Input node, right-click one of the selected nodes, and select Create Comment From Selection.
8. Name the comment box "Tempo and Rhythm."

You can change the color of a selected comment box in the Details panel.

1.3 - Melody Generation

Build the Melody Generation section to produce a random melody in a specified scale.

1. On the Trigger Counter node in the Tempo and Rhythm section:
   1. Drag off the On Trigger pin and create a Random Get (Float:Array) node.
   2. Drag off the On Reset pin and connect it to Reset on the Random Get (Float:Array) node.
2. On the Random Get (Float:Array) node:
   1. Drag off the In Array pin and create a Scale to Note Array node.
   2. Drag off the Seed pin and create a Random (Int) node.
   3. Drag off the Value pin and create an Add (Float) node.
3. On the Scale to Note Array node, drag off the Scale Degrees pin and select Promote to Graph Input. This creates an Enum Input node named Scale Degrees.
4. Select the Scale Degrees node.
5. In the Details panel:
   1. Set General > Input to "Scale" to rename the input.
   2. (Optional) Set Default Value > Default to the music scale you want to use. This defaults to Major Scale.
6. On the Random (Int) node:
   1. Set Max to 100000.
   2. Drag off the Next pin and select Promote to Graph Input. This creates a Trigger Input node named Next.
7. Select the Next node.
8. In the Details panel, set General > Input to "NewMelody" to rename the input.
9. On the Add (Float) node, set the bottom addend to 48.0. This is an offset of four octaves.
10. Enclose the new nodes in a comment box named "Melody Generation."

1.4 - Synthesis (Sine)

Build the Synthesis (Sine) section to produce a sine wave at the frequency of the input note.

1. On the Add (Float) node in the Melody Generation section:
   1. Drag off the output pin and create a MIDI To Frequency (Float) node.
   2. Drag off the output pin again and create another Add (Float) node.
2. On the new Add (Float) node:
   1. Drag off the output pin and create another MIDI To Frequency (Float) node.
   2. Drag off the bottom addend pin and select Promote to Graph Input. This creates a Float Input node named AdditionalOperands.
3. Select the AdditionalOperands node.
4. In the Details panel:
   1. Set General > Input to "Detune" to rename the input.
   2. Set Default Value > Default to 12.0.
   3. Set Default Value > Range to 0.0, 12.0.
5. On each of the MIDI To Frequency (Float) nodes, drag off the Out Frequency pin and create a Sine node.
6. On the first Sine node, drag off the Audio pin and create an Add (Audio) node.
7. On the second Sine node, connect the Audio pin to the bottom addend of the Add (Audio) node.
8. Enclose the new nodes in a comment box named "Synthesis (Sine)."

1.5 - Synthesis (Saw)

Build the Synthesis (Saw) section to produce a saw wave at the frequency of the input note.

1. Select all of the nodes in the Synthesis (Sine) section, right-click one of the selected nodes, and select Duplicate.
2. Ensure that the input pins of the Add (Float) and MIDI to Frequency (Float) nodes are connected to the output pin of the Add (Float) node from the Melody Generation section.
3. Delete both of the Sine nodes and replace them with Saw nodes.
4. Enclose the new nodes in a comment box named "Synthesis (Saw)."

1.6 - Crossfade

Build the Crossfade section to control the ratio of the sine and saw synths.

1. On the Add (Audio) in the Synthesis (Sine) section, drag off the output pin and create a Crossfade (Audio, 2) node.
2. On the Crossfade (Audio, 2) node:
   1. Connect the In 1 pin to the output pin on the Add (Audio) node in the Synthesis (Saw) section.
   2. Drag off the Crossfade Value pin and select Promote to Graph Input. This creates a Float Input node named Crossfade Value.
3. Select Crossfade Value.
4. In the Details panel, set General > Input to "Crossfade" to rename the input.
5. Enclose the new nodes in a comment box named "Synthesis (Crossfade)."

1.7 - Filtering

Build the Filtering section to smooth out the sound.

1. On the Crossfade (Audio, 2) node in the Crossfade section, drag off the Out pin and create a Ladder Filter node.
2. On the Ladder Filter node:
   1. Set Resonance to 6.0.
   2. Drag off the Cutoff Frequency pin and create an LFO node.
3. On the LFO node:
   1. Set Frequency to 0.5.
   2. Set Min Value to 500.0.
   3. Set Max Value to 5000.0.
4. Enclose the new nodes in a comment box named "Filtering."

1.8 - Enveloping

Build the Enveloping section with an attack-decay envelope to remove the sustain of the notes in the melody.

1. On the Random Get (Float:Array) node in the Melody Generation section, drag off the On Next pin and create an AD Envelope (Audio) node.
2. On the AD Envelope (Audio) node, set Decay Time to 0.1.
3. Enclose the new node in a comment box named "Enveloping."

1.9 - Effects Processing

Build the Effects Processing section to produce a stereo widening effect. The delay creates an illusion that the mono signal is a stereo signal.

1. On the AD Envelope (Audio) node in the Enveloping section, drag off the Out Envelope pin and create a Multiply (Audio) node.
2. On the Multiply (Audio) node:
   1. Connect the bottom multiplicand pin to the Out pin of the Ladder Filter node in the Filtering section.
   2. Drag off the output pin and create a Delay node.
3. On the Delay node:
   1. Set Delay Time to 0.02.
   2. Drag off the Out pin and create a Stereo Delay node.
4. On the Stereo Delay node:
   1. Connect the In Right pin to the output pin of the Multiply (Audio) node.
   2. Set Delay Mode to Ping Pong.
   3. Set Delay Time to 0.2.
   4. Set Delay Ratio to 0.2.
   5. Set Dry Level to 0.7.
   6. Set Wet Level to 0.2.
   7. Set Feedback to 0.4.
   8. Connect the Out Left pin to the Out Left Output node.
   9. Connect the Out Right pin to the Out Right Output node.
5. Enclose the Delay and Stereo Delay nodes in a comment box named "Effects Processing."

1.11 - Play the MetaSound

The MetaSound is now ready to play.

1. Save the MetaSound.
2. Click the Play button on the MetaSound Editor Toolbar to play the MetaSound.
3. Use the widgets or the Details panel to adjust input values and listen to the changes in real-time.

2 - Build the Blueprint Actor

Create a Blueprint Actor with Box Colliders that triggers changes in the MetaSound during runtime.

2.1 - Create the Blueprint Class

1. In the Content Browser, click the Add button.
2. Select Blueprint Class.
3. From the Pick Parent Class window, select Actor.
4. Give the newly created Blueprint Actor a name, such as BP_MusicPlayer.

2.2 - Add Components

Add Components to the Actor to create three separate trigger zones that respond when the Player moves in and out of them.

1. Double-click the Blueprint Actor to open the Blueprint Editor.
2. Add a Box Collision Component.
   1. In the Components panel:
      1. Click the Add button.
      2. Type "Box Collision" into the search bar and press Enter.
   2. In the Details panel:
      1. Set Transform > Scale to 5.0, 5.0, 1.0.
      2. Disable Rendering > Hidden In Game.
3. Add a Text Render Component to the Box Collision Component.
   1. In the Components panel:
      1. Select the Box Collision Component.
      2. Click the Add button.
      3. Type "Text Render" into the search bar and press Enter.
   2. In the Details panel:
      1. Set Transform > Location to 0.0, 0.0, 150.0.
      2. Set Text > Horizontal Alignment to Center.
      3. Set Text > World Size to 64.0.
4. Create two additional copies of the Components. Do the following twice:
   1. In the Components panel, Ctrl + click to select both the Box Collision Component and Text Render Component.
   2. Right-click and select Duplicate.
5. Customize the BPM Trigger.
   1. In the Components panel, right-click the first Box Collision Component, select Rename, and name it Trigger_BPM.
   2. In the Details panel, set Transform > Location to 0.0, 350.0, 0.0.
   3. In the Components panel, select the child Text Render Component.
   4. In the Details panel, set Text > Text to BPM.
6. Customize the Crossfade Trigger.
   1. In the Components panel, right-click the second Box Collision Component, select Rename, and name it Trigger_Crossfade.
   2. Select the child Text Render Component.
   3. In the Details panel, set Text > Text to Crossfade.
7. Customize the Melody Trigger.
   1. In the Components panel, right-click the third Box Collision Component, select Rename, and name it Trigger_Melody.
   2. In the Details panel, set Transform > Location to 0.0, -350.0, 0.0.
   3. In the Components panel, select the child Text Render Component.
   4. In the Details panel, set Text > Text to Melody.
8. Add an Audio Component for the MetaSound.
   1. In the Components panel:
      1. Click the Add button.
      2. Type "Audio" into the search bar and press Enter.
      3. In the Details panel, set Sound > Sound to the Music MetaSound.

2.3 - Add Trigger Events

Add collision response events for each of the zones.

1. In the Components panel, select Trigger_BPM.
2. In the Details panel:
   1. Select the Add (+) button next to Events > On Component Begin Overlap.
   2. Select the Add (+) button next to Events > On Component End Overlap.
3. In the Components panel, select Trigger_Crossfade.
4. In the Details panel:
   1. Select the Add (+) button next to Events > On Component Begin Overlap.
   2. Select the Add (+) button next to Events > On Component End Overlap.
5. In the Components panel, select Trigger_Melody.
6. In the Details panel, select the Add (+) button next to Events > On Component Begin Overlap.

2.4 - Build Trigger Events

Click image for full size.

Attach nodes to each event to control the music.

You might need to turn off the Context Sensitive filter located at the top-right of the search context menu to find the Set Float Parameter (Audio) and Execute Trigger Parameter nodes used below.

1. On the On Component Begin Overlap (Trigger_BPM) node, drag off the Exec Output (>) pin, and create a Set Float Parameter (Audio) node.
2. On the Set Float Parameter node:
   1. Set In Name to BPM.
   2. Set In Float to 120.0.
3. On the On Component End Overlap (Trigger_BPM) node, drag off the Exec Output (>) pin, and create another Set Float Parameter (Audio) node.
4. On the new Set Float Parameter node:
   1. Set In Name to BPM.
   2. Set In Float to 90.0.
5. On the On Component Begin Overlap (Trigger_Crossfade) node, drag off the Exec Output (>) pin, and create another Set Float Parameter (Audio) node.
6. On the new Set Float Parameter node:
   1. Set In Name to Crossfade.
   2. Set In Float to 1.0.
7. On the On Component End Overlap (Trigger_Crossfade) node, drag off the Exec Output (>) pin, and create another Set Float Parameter (Audio) node.
8. On the new Set Float Parameter node:
   1. Set In Name to Crossfade.
   2. Set In Float to 0.0.
9. On the On Component Begin Overlap (Trigger_Melody) node, drag off the Exec Output (>) pin, and create an Execute Trigger Parameter node.
10. On the Execute Trigger Parameter node:
    1. Set In Name to NewMelody.
    2. In the Components panel, drag the Audio Component onto the Target pin.
11. Compile and save the Blueprint.

3 - Test the Level

The Blueprint is ready to test.

1. Place the BP_MusicPlayer by dragging it from the Content Browser into the Level.
2. Using the transformation widget, place the Actor so your character can move in and out of it.
3. Click the Play button on the Level Editor Toolbar.
4. Move in and out of the triggers and observe their effects on the music.