Hardware
This page provides the necessary information to build a simple working prototype.
The following diagram gives an overview of the device components and their interactions.
The bzzzbz prototype (v1.0) uses a Raspberry Pi 3B+ as the core of the system to exploit the capabilities of a Linux system with direct access to hardware. Custom electronics circuits are connected to the Pi's GPIOs in order to interface the Pi with external analogue inputs: audio input and control surface. The following sections will outline each subsystem in detail.
The audio interface uses the WM8731 audio CODEC to perform analogue to digital conversion of the incoming audio signal (line input). The chip also provides headphones/line outputs that allow our prototype to have an audio output together with the generated visuals. Furthermore, the Linux kernel (v4.19) in the Raspbian OS comes with an integrated driver for the WM8731 so that the chip can be configured and used directly from the Raspberry Pi.
The chip is mounted on a breakout board in order to be connected to the Raspberry Pi header pins. The schematic for the breakout board is shown in the figure below:
The board uses decoupling capacitors to protect the analogue inputs and the power lines, as advised in the codec datasheet. A 12MHz crystal was connected to the XTO/MCLK inputs of the chip enabling it as a master clocking source (with the Raspberry Pi consequently acting as slave). Optionally, these inputs can be left disconnected so that the chip receives the clock from the Raspberry Pi, with the two configurations being interchangeable.
Standard PCB 2.54mm headers are used for power and wired connection to the Raspberry Pi. The line and headphones connections use 3.5mm TRS jack sockets. The final breakout board is shown below.
The schematic and PCB files for KiCad can be found in the hardware folder in the main repository under wm8731-breakout.
The control interface consists of 3 potentiometers connected to the MCP3008 multi-channel ADC. The chip is then connected to the Raspberry Pi via jumper cables, where the values of the incoming potentiometers are read through SPI communication. A schematic of the control breakout board is shown below.
Due to the current emergency lockdown, the breakout board could not be completed before the release date. The PCB files are therefore missing. However, the same result can be achieved with a breadboard! The picture below shows a reproduction of the schematic on a breadboard.
The schematic file for KiCad can be found in the hardware folder in the main repository under mcp3008-breakout.
When all the subsystems are assembled, the connectors on both boards can be connected to the Raspberry Pi header according to the following table using jumper cables:
| WM8731 pin # | MCP3008 pin # | Raspberry Pi pin |
|---|---|---|
| 2 | - | GND |
| 3 | - | 12 |
| 4 | - | 40 |
| 5 | - | 35 |
| 6 | - | 38 |
| 7 | - | 35 |
| 21 | - | GND |
| 22 | - | GND |
| 23 | - | 3 |
| 24 | - | 5 |
| - | 10 | 24 |
| - | 11 | 19 |
| - | 12 | 21 |
| - | 13 | 23 |
Then connect the VDD/VDDA from the audio breakout to 3.3V, 5V to the power lines in the control breakout breadboard, grounds accordingly. The hardware prototype is now ready! The following section briefly describes how to configure the Raspberry Pi to communicate with the hardware.
Flash the SD card with the latest Raspbian distribution (choose the version without the GUI as it would introduce several issues and latencies in the software).
Log in and edit the /boot/config.txt with replacing the following line:
#dtparam-audio=on <-- comment this out
dtoverlay=audioinjector-wm8731-audio
The audio codec is now configured. Reboot the system and run sudo alsamixer. If the hardware was assembled correctly you should see the mixer interface displaying the WM8731 configuration options. Enable the Line In input, Output Mixer Line Bypass and Capture - Line in. The system is now fully configured for the software, visit the Getting started page to start making some bzzzbz visuals.
The current prototype can be used mainly for demonstration and testing purposes due to the emergency lockdown. It is however very fragile and introduces some latencies and faults in the system (such as SPI/I2S long cables). Our objective is to develop a finished enclosure and compact surface-mount pi-hat for the next release. Meanwhile you can still start playing and experimenting with this easy-to-assemble prototype!