Pitch Shifting

Pitch shifting is the ability to modify the pitch of an audio signal to either increase or decrease the pitch. For example, when a fast car passes you in the street, the pitch of the sound from the car increases as the vehicle approaches you and decreases as the vehicle moves away from you. As the source of the audio moves closer or further away from the receiver, the successive signals reach the receiver at either smaller or larger intervals of time. This time difference causes change in the frequency of the audio being heard by the receiver. You can perform a similar pitch-shifting operation by splitting an audio signal into two components, delaying the signals using varying time periods, and then adding them back together. This process simulates the effect of either increasing or decreasing the pitch depending on how the two signals are delayed or overlapped. To ensure uniform power levels, the individual gains for the split signals must be modulated.

Required Products

Required Hardware

Step 1: Connect Raspberry Pi Hardware for Pitch Shift

Before you start this example, we recommend you complete the Get Started with Simulink Support Package for Raspberry Pi Hardware example.

1. Connect the micro-USB end of the USB cable to the Raspberry Pi and the regular USB end of the USB cable to the computer. Wait until the PWR LED on the hardware starts blinking.

2. Connect the webcam or the microphone to one of the USB ports on the hardware. Note that some webcams draw too much power and may require a powered USB hub for proper operation.

3. Connect a pair of headphones to the 3.5 mm headphone jack of the hardware.

4. Using the Hardware Setup screen, configure the Raspberry Pi network.

Step 2: Change Audio Output Mode to Headphone Jack

1. In the hardware terminal, execute this command to open the Software Configuration Tool of the hardware.

sudo raspi-config

2. In the Software Configuration Tool Window, select Advanced Options and press Enter, then select Audio and press Enter.

3. Select the Force 3.5mm ('headphone') jack option and press Enter.

4. Press OK.

Step 3: Open Pitch Shift Simulink Model

Open the raspberrypi_pitchshift Simulink model.

The model is divided into these areas based on the functionality of the blocks in each area.

Audio Input

The ALSA Audio Capture block captures live audio from the microphone connected to the hardware. Double-click the block and specify the name of the device to capture the audio data in the Device name parameter. If you do not know the name of the device, use the listAudioDevices function as described in List Available ALSA Audio Input Devices.

If you want to capture audio from an audio file saved in your computer, replace the Audio Capture block by an Audio File Read block and then specify the name of the audio file in the File Name parameter of the block.

Pitch Shift Algorithm

This section includes a MATLAB Function block that implements the delay-based pitch shift algorithm on the acquired audio data.

The inputs to the function block are:

Audio Output

The ALSA Audio Playback block sends the processed audio to the headphones connected to the hardware. Double-click the block and specify the name of the audio output device in the Device name parameter. If you do not know the name of the device, use the listAudioDevices function as described in List Available ALSA Audio Output Devices.

Step 4: Configure Pitch Shift Simulink Model

1. To open the Model Configuration Parameters dialog box, click the gear icon on the Simulink model toolbar.

2. In the Configuration Parameters dialog box, select Hardware Implementation.

3. Set the Hardware board parameter to Raspberry Pi. This selection automatically populates the parameters in the Hardware board settings with the default values for Raspberry Pi.

4. In the Hardware board settings pane, expand Target hardware resources and select Board Parameters. Specify these parameter values:

5. Click Apply. Click OK to save your changes.

Step 5: Deploy Pitch Shift Simulink Model on Raspberry Pi Hardware

On the Hardware tab of the Simulink model, in the Mode section, select Run on board and then click Build, Deploy & Start. The lower-left corner of the model window displays status while the support package generates the code. After successfully generating the code, the support package loads and runs the code on the hardware. The executable starts acquiring live audio from the audio device, shifts the pitch of the acquired audio, and then plays the processed audio through the audio jack of the hardware. You can now hear the pitch-shifted audio through the pair of headphones connected to the hardware.

You can also run this model in external mode to tune the pitch and overlap parameter values in the model and observe the changes in the characteristics of the output audio during simulation.

Other Things to Try

See Also

Spatialize Monaural Audio into 5.1 Channel Surround Sound Using Raspberry Pi