Raspberry-Pi based Audio Player (TAPAS as Class D amplifier)

About Tapas

The Siemens SDI TAPAS is a 48V GaN (Gallium Nitride) based three phase inverter board with onboard filters, expected to change the way people approach power electronics. The design choice allows for a high switching frequency, while producing a smooth output waveform. The unique combination of high bandwidth and smooth output makes TAPAS to some degree universal, giving rise to applications ranging from robotics to power conversion, making the inverter truly software configurable. The TAPAS board is designed as an educational platform that addresses ethusiasts as well as experts at universities and in the industry. The development of the TAPAS board is community driven. Together with open-source code examples and extensive documentation we made TAPAS as accesible to the community as possible.

Why an Audio Player?

One of the big benefits of the GaN technology is its capability to switch on and off really fast and that this yields an ideal power amplifier in the end. To support this thesis, we decided to implement a TAPAS Audio Player. It demonstrates the bandwidth of TAPAS as it can play music although it is designed as a three phase inverter optimized for operating electrical drives. In the end, TAPAS now combines the function of a digital-to-analog converter and a class D power amplifier which shows the preformance and capabilities of the board.

TAPAS Audio Player

Highlights

• Operates at any DC Power supply from 8 to 16V
• TAPAS is unmodified, only an external filter is connected ´, made of standard-components
• Standalone operation with two push buttons as user interface to select a song and play or stop it
• TAPAS works as a class D audio amplifier and digital-to-analog converter in one unit
• any kind of speaker can be used
• Raspberry Pi attached to the TAPAS feeds in the audio stream and handles the user interface
• The DSP-Code-Project is based on the controlSuite package from Texas Instruments

By implementing this use case, we found out the limits of TAPAS in terms of maximum output frequency. When going into audio frequency range, there were some resonant effects on the board that will be further examined and eliminated with the next version of the board. For this reason, we decided to add an external filter, which yields a huge improvement of the hearable audio playback quality.

Technical Implementation & Details

• TAPAS baord is class D audio amplifier
• Raspberry Pi with dietpi-image as pcm-audio-playback-platform
• Audio data streaming from Raspberry Pi to DSP via buffered SPI communication sample by sample in realtime, playback sampling using CPU timer
• External filter circuitry for enhanced audio perfromance, using all three power outputs of TAPAS in parallel
• Audio-Sampling-Rate of 22, 05kHz achieved
• Playback of 16Bit pcm files (mono)
• PWM-frequency 175, 78126kHz
• 9Bit Standard-PWM-resolution and high resolution PWM
• Standalone mode with two push buttons as user interface conncected to Raspberry Pi GPIOs enabled

Happy listening! :)