Lesson 4: Data Processing & micro:bit Integration

Learn how to process sensor data and connect the micro:bit to Pure Data for interactive sound control.

Goals for This Lesson

• Understand threshold processing and branching logic
• Send sensor data from micro:bit via serial communication
• Convert serial data to OSC for use in Pd
• Apply mapping, clipping, and smoothing to sensor values

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Threshold Processing and Branching

What Is Threshold Processing?

When working with sensor data, you often need to trigger an action only when a value crosses a certain boundary. This boundary is called a threshold.

For example:

• Play a sound only when the light level drops below 50
• Trigger a drum hit only when acceleration exceeds 800

Branching with moses

The moses object in Pd splits incoming values into two streams based on a threshold.

[moses 50]
 |      |
left   right
(<50)  (>=50)

• Values less than the threshold exit the left outlet
• Values greater than or equal to the threshold exit the right outlet

Multi-Level Branching

You can chain multiple moses objects for multi-level branching:

[moses 30]
 |       |
low    [moses 70]
        |       |
       mid     high

This creates three zones: low (below 30), mid (30--69), and high (70 and above).

Using select for Exact Values

If you need to match exact values (e.g., button states 0 and 1), use the select (or sel) object:

[sel 0 1]
 |    |
off  on

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Serial Communication with micro:bit

Sending Sensor Data from MakeCode

In the MakeCode editor, use the serial communication blocks to send sensor values to your PC.

Basic program structure:

forever:
  serial write value "light" = light level
  serial write value "acc_x" = acceleration (mg) x
  serial write value "acc_y" = acceleration (mg) y
  pause 50ms

Each serial write value sends a labeled number over USB to the PC.

Sending Interval

Setting the pause to 50ms sends approximately 20 readings per second, which is sufficient for musical applications. Setting it too low may cause data overflow.

Understanding the Data Format

The serial output from MakeCode uses a simple label-value format:

light:128
acc_x:-450
acc_y:230

This data needs to be converted to OSC (Open Sound Control) messages for Pd to receive.

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OSC (Open Sound Control)

What Is OSC?

OSC (Open Sound Control) is a protocol for sending structured messages over a network. It uses a URL-like address format:

/light 128
/acc_x -450
/acc_y 230

SerialOSCConverter

To bridge between micro:bit's serial output and Pd's OSC input, use SerialOSCConverter:

1. Launch SerialOSCConverter
2. Select the serial port where micro:bit is connected
3. Set the baud rate to 115200
4. Confirm the OSC port (default: 8000)
5. Click "Start" to begin conversion

Receiving OSC in Pd

Use these objects to receive and parse OSC messages in Pd:

[netreceive -u -b 8000]   ← Receive on UDP port 8000
       |
[oscparse]                 ← Parse OSC messages
       |
[route /light /acc_x /acc_y]  ← Route by address
  |        |         |
(light)  (acc_x)   (acc_y)

Alternatively, with the else library:

[else/osc.receive 8000]
       |
[else/osc.route /light /acc_x /acc_y]
  |        |         |

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Mapping Sensor Values

Why Map Values?

Raw sensor values are rarely in the right range for controlling sound parameters. Mapping converts values from one range to another.

For example:

• Light sensor (0--255) → frequency (200--2000 Hz)
• Acceleration (-1024 to 1024) → volume (0 to 1)

Using scale (cyclone library)

The cyclone/scale object maps values from one range to another:

[cyclone/scale 0 255 200 2000]

This converts input values in the range 0--255 to output values in the range 200--2000.

Manual Mapping with Math

You can also map values using basic arithmetic:

value_normalized = (input - input_min) / (input_max - input_min)
output = output_min + value_normalized * (output_max - output_min)

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Clipping Values

What Is Clipping?

Clipping restricts values to stay within a defined range. This prevents unexpected extreme values from causing problems.

Using clip

[clip 0 1]

Any input value below 0 becomes 0, and any value above 1 becomes 1. Values between 0 and 1 pass through unchanged.

Why Clipping Matters

Sensor values can sometimes spike or produce outliers. Without clipping, these extreme values might cause distortion (if used for volume) or unpleasant sounds (if used for frequency). Always clip your values to a safe range.

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Smoothing Values

Why Smooth?

Raw sensor data is often noisy and jumpy. Feeding these values directly into audio parameters causes clicks, pops, and unpleasant artifacts. Smoothing (also called low-pass filtering on the control signal) makes the transitions gradual.

Using else/lowpass

The else/lowpass object smooths value changes:

[else/lowpass 5]

The argument controls the smoothing amount (higher = smoother but slower response).

Using line for Smooth Transitions

You can also use line to interpolate between values:

[pack f 50]    ← Pack value with ramp time (50ms)
    |
[line]         ← Smooth transition over 50ms

Choosing the Right Amount of Smoothing

• Too little: Sound still has clicks and jumps
• Too much: Response feels sluggish and unresponsive
• Start with a value around 5--20 and adjust by ear

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Normalizing Sensor Values

Different sensors output different ranges. It is good practice to normalize values to a standard range (typically 0 to 1, or -1 to 1).

Sensor	Min Value	Max Value	Normalization
Acceleration X/Y/Z	-2048	2048	value / 2048 for -1 to 1
Acceleration strength	0	2048	value / 2048 for 0 to 1
Light level	0	255	value / 255 for 0 to 1
Microphone level	0	255	value / 255 for 0 to 1
Compass heading	0	360	value / 360 for 0 to 1

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Practice Exercises

Exercise 1: Light-Controlled Sine Wave

Receive the micro:bit light sensor value in Pd and use it to control the frequency of a sine wave (mapping 0--255 to 200--2000 Hz). Apply smoothing so the pitch changes gradually.

Hint

Use cyclone/scale 0 255 200 2000 to map the value, then else/lowpass 10 to smooth it, and connect to osc~.

Exercise 2: Acceleration Threshold Trigger

Receive the accelerometer's strength value and play a drum sample only when the value exceeds a threshold (e.g., 600). Use moses for the threshold detection.

Hint

Connect the acceleration strength value to moses 600. The right outlet triggers a bang that plays a sound with else/play.file~.

Exercise 3: Multi-Sensor Instrument

Create a patch that uses at least two different sensor values simultaneously: one for pitch and one for volume (or filter cutoff). Normalize, clip, and smooth all values.

Hint

For example, use light level for pitch (mapped to frequency) and acceleration Y for volume (mapped to 0--1). Use clip 0 1 on the volume value and else/lowpass on both values.