Lesson 5: Signal Processing & Audio File Handling

Goals for This Lesson

Learn how to work with audio files and control them using micro:bit sensors.

Specifically, this lesson covers:

• Playing, stopping, and looping audio files
• Pitch shifting and time stretching
• Fade-in, fade-out, and crossfade
• Recording audio

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Playing, Stopping, and Looping Audio Files

In Pd, you can play audio files using the else/play.file~ object. It supports WAV, AIFF, and other audio formats, and lets you control playback, stopping, and looping.

How to Use

1. Create an else/play.file~ object
2. Place the audio file in the same directory as your Pd patch
3. Use message objects to send the following commands

Basic Commands

Message	Action
open filename	Open an audio file
start	Start playback
stop	Stop playback
loop 1	Enable loop playback
loop 0	Disable loop playback

Important

Audio files must be in the same folder as the Pd patch. If they are in a different folder, Pd will not find them and will produce an error.

Patch Example

Send the start message to begin playback and stop to stop it. Send loop 1 before start to loop the audio continuously.

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Pitch Shifting and Time Stretching

The else/stretch.shift~ object lets you change the pitch or tempo of an audio file in real time.

Inlet Description

else/stretch.shift~ has multiple inlets, each controlling a different parameter.

Inlet	Parameter	Description
1st inlet	Buffer size	Size of the processing buffer
2nd inlet	Pitch	Pitch change amount (in cents)
3rd inlet	Tempo	Playback speed (in percent)

Pitch Settings

Pitch is specified in cents.

• 0: Original pitch (no change)
• 1200: One octave up
• -1200: One octave down
• 700: A perfect fifth up

Pitch Reference

100 cents = 1 semitone. 1200 cents = 12 semitones = 1 octave.

Tempo Settings

Tempo is specified as a percentage.

• 100: Original speed (no change)
• 200: Double speed
• 50: Half speed

Patch Example

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Fade-In, Fade-Out, and Crossfade

To smoothly switch between audio sources or gradually change volume, use fade-related objects.

Objects Used

Object	Function
else/xselect~	Select one from multiple inputs for output
else/xfade~	Crossfade between two audio sources
else/xgate~	Gate that passes or blocks audio

How Crossfade Works

A crossfade gradually lowers the volume of one source while raising the volume of another, creating a smooth transition.

With else/xfade~, a value from 0 to 1 controls the mix ratio between two sources.

• 0: Only input 1 is heard
• 0.5: Both inputs are mixed equally
• 1: Only input 2 is heard

Patch Example

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Recording

Pd can also record audio from a microphone input and save it to a file.

Objects Used

Object	Function
adc~	Receive audio signal from the microphone input
else/rec.file~	Record an audio signal to a file

Steps

1. Use adc~ to capture the microphone signal
2. Connect it to else/rec.file~
3. Send an open filename message to specify the file name
4. Send start to begin recording
5. Send stop to stop recording

File Save Location

Recorded files are saved in the same directory as the Pd patch. If the patch has not been saved, the file location will be undefined.

Patch Example

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Practice Exercises

Try the following exercises. Use what you have learned about micro:bit integration from previous lessons.

Exercise 1: Drum Loop Play/Stop

Create a patch that uses the micro:bit's A and B buttons to control playback and stopping of a drum loop.

Hint

• Use else/play.file~ to load a drum loop audio file
• Send start when the A button is pressed
• Send stop when the B button is pressed
• Set loop 1 for continuous looping
• Use the select object to route button values to the appropriate messages

Exercise 2: Tilt-Controlled Tempo

Add a feature to the Exercise 1 patch where tilting the micro:bit (accelerometer) changes the playback tempo.

Hint

• Use else/stretch.shift~ to control playback speed
• Map the micro:bit's tilt value to a tempo percentage (e.g., 50--200) using scale or similar
• Check the tilt value range first, then convert it to an appropriate range

Exercise 3: Light Sensor Crossfade

Create a patch that switches audio sources based on the light sensor value:

Light Sensor Value	Behavior
70 or above	Silence (no sound)
30--70	Play source 1
Below 30	Play source 2

Hint

• Use else/xfade~ or else/xselect~ for crossfading
• Use moses to branch based on the light sensor value
• Map the value range to the crossfade parameter (0--1)
• You can also use else/xgate~ to mute the sound when the value is above 70

Exercise 4: Light-Controlled Recording, A Button Playback

Create a patch that starts/stops recording based on the light sensor value and plays back the recording when the A button is pressed.

Hint

• Use adc~ and else/rec.file~ for the recording function
• Start recording when the light value drops below a threshold, stop when it goes above
• Use else/play.file~ to play back the recorded file
• When the A button is pressed, send open filename followed by start
• Use the same file name for both recording and playback