After you soldered your headers and mics, it's time to try and get some sound in and output from your teensy devices! Here is a short how-to, partially summarizing what we did in the last online session.

  1. Installl Arduino IDE[[wikipedia:Integrated development environment]] and Teensyduino:
  2. Open and program the example patch
    opening the example test patch
  3. Connect some headphones on the audio shield's headphone out and listen! You should be able to hear a sine wave switching amplitudes.

    If you are having problems connecting your teensies :( read through the following sections:
  4. Now we want to listen to the actual piezo disc sounds. Check out the teensy web ide patches:
    • add the following nodes from the gui:
      1. from "input" add "i2s"
      2. from "output" add "i2s"
      3. from "control" add "sgtl5000"
        Audio System Design Tool "Audio through patch"
  5. Click the export button to generate the setup code for your teensy and paste it in your Arduino IDE[[wikipedia:Integrated development environment]] patch, just before the setup routine! You should get output similar to the following block, paste it at the beginning of your patch.
    #include <Audio.h>
    #include <Wire.h>
    #include <SPI.h>
    #include <SD.h>
    #include <SerialFlash.h>
    // GUItool: begin automatically generated code
    AudioInputI2S            i2s1;           //xy=200,69
    AudioOutputI2S           i2s2;           //xy=365,94
    AudioConnection          patchCord1(i2s1, 0, i2s2, 0);
    AudioConnection          patchCord2(i2s1, 1, i2s2, 1);
    AudioControlSGTL5000     sgtl5000_1;     //xy=302,184
    // GUItool: end automatically generated code
  6. In the setup routine of the arduino IDE[[wikipedia:Integrated development environment]] you need code to choose the microphone port and audio levels. It should look more or less like this:
    void setup() {
      // Audio connections require memory to work.  For more
      // detailed information, see the MemoryAndCpuUsage example
      // Enable the audio shield and set the output volume.
      sgtl5000_1.micGain(36); //from 0dB 40dB
      sgtl5000_1.volume(0.8); // from 0.0 to 1.0 // 0.8 standard
    Note that you really need to declare the loop() function, otherwise your code doesn't compile!
    void loop(){
    // nothing to be done here
    // it just needs to be declared to compile correctly :)
  7. Fingers crossed, press upload and compile the patch!

Now put some headphones on the teensy headphone connector and hopefully listen to some interesting piezo disc microphonic sounds (this is where the word micro-phone really comes from, it's analogous to microscope, actually).

Homework: Coding an Envelope follower

If you were successful with setting up the teensy and piezo microphone, you can make a step further and learn how to wrestle with audio data directly on the teensy!

We want to code a so called envelope follower, it tracks the peaks of our audio signal and gets the rough volume shape. With the help of the volume (~loudness) of our signal we can detect wether there is vibration on our piezo or not. In rough terms we get the volume envelope in two steps:

  1. Get the maximum (peak) of the signal samples seen so far. This is also called a peak hold or peak detector circuit Peak hold.png
  2. Multiply this maximum with a decay factor (<1) for every new sample, or sample block (envelope follower) Envelope follower.png

This is a code snippet that illustrates how to search for the peak in a block of audio samples:

float getPeakOfBlock(float* block, int n_smps){
    float peak=0;
    // imagine that the audio block and its length (n_smps) is given to you
    for (int i=0; i<n_smps; i++) {
        if (block[i]>peak) {
            // we remember every peak that 
            // is greater then we seen so far
            peak = block[i]; 
    return peak;

For our envelope follower, we just decay the peak signal with every new sample by a decay factor that is slightly smaller then 1 (e.g. 0.95). [To get a bit more accuracy you might want to get the absolute value of our signal before tracking the maximum.]

With the help of the teensy documentation and my empty template file you should be able to grasp how to write your own code for the teensy audio library. Basically you write your own teensy audio object by including a C++ class that is structured as documented in the link above. This class will receive a pointer to the audio input stream coming from the piezo and can do some work on it. Look at my blank example file to see how to access the audio samples from the piezo. If you have questions about the template code please ask on our signal group!

As a last step, print out the calculated envelope on your serial plotter to get a picture like this: piezo envelope signal in serial plotter Send me your code and a picture like this (hopefully) until Sunday!

Happy Coding!!!