The Effect-Box is an audio effect device that takes an audio input, modifies it by applying effects and plays the modified audio on a DAC. The audio input and output is provided by two 3.5 mm jacks. To select the desired audio effects and configure these the PCB is equipped with an LCD screen and buttons.
For an overview of the final setup with physical connections, see images.
- Connect an audio source to the audio input jack. You probably need an adapter (see [Audio connection](#Audio connection).
- Connect the audio output jack to a line level input, such as a sound card or an active speaker.
- Connect power cable
There are three effects which can be enabled simultaneously. They are chained after each other, meaning the original audio signal is the input to the first effect, and each effect's output is the input to the next effect in the chain. The signal chain is ADC -> delay -> tremolo -> bit crusher -> DAC. Each effect can be bypassed or enabled, but the order is fixed and not configurable.
This effect mixes the direct signal with a delayed copy of itself. The mixed signal is fed back into the delay line.
- Delay: the delay time in milliseconds.
- Feedback: the percentage of feedback of the delay back into itself.
- Mix: the ratio of the delayed to the original signal that is output by the module.
This effect modulates the amplitude of the signal based on an LFO, producing an oscillating volume envelope.
- Rate: the frequency of the LFO.
- Depth: the factor by which the amplitude is modulated.
- Wave Form: the wave that is used 0: sine, 1: square, 2: triangle, 3: sawtooth
This effect reduced either the effective sample rate, the effective bit depth, or both, of the signal.
- Bits: the amount of bit depth reduction
- Rate: the number of samples that are dropped before a sample is passed through.
The audio effects are implemented in Chisel and run on a Xilinx Artix 7 FPGA. The buttons and LCD are connected to an EFM32GG MCU which in turn sends control signals to the FPGA to enable and configure the effects as the user specifies in the interface. Communication with the LCD happens over SPI using Silicon Labs libraries. MCU-FPGA communication is also over SPI, using a custom protocol.
The LCD menu displays the available audio effects and their configurable settings. The settings will be different for each effect, but every effect has an enable setting.
The button group to the left is for navigation, and the buttons to the right increment and decrement the selected setting value. When a setting value is changed the MCU sends a corresponding SPI command to the FPGA and the display is updated.
The Effect Box is a mono balanced input, stereo unbalanced output device, although the outputs are currently mirrored and not true stereo. However, this allows you to connect the output directly to a typical 3.5 mm unbalanced stereo input on a computer sound card or active speaker. When outputting to a balanced mono input (typically a 6.35 mm jack or XLR), a split cable should be used to separate one channel (if not, the balanced input will treat the identical channels as a differential pair).
The input is capable of receiving either balanced or unbalanced mono signals. These should be somewhere between consumer and professional line level (maximum 1 V peak or 0 dBV). The input impedance is around 1 MΩ, so the device is also capable of receiving instrument level signals like guitars, although this is not optimal and they will need to be turned up to the maximum possible level.
To connect a sound source to the Effect Box, you probably need to use a 6.35 to 3.5 mm jack adapter, for example if you want to hook up a synthesizer or a line level output from a mixer or sound card (these typically have 6.35 mm jack outputs). You could also use a split cable to connect only one channel of a phone or laptop if you want to send music through the Effect Box.
Connect to the FPGA with a Xilinx platform cable connector with the "tab" on the connector pointing down with respect to the PCB. To be able to build and upload the hardware description code, follow the instructions of this FPGA wiki.
Connect to the MCU with an ARM 20-pin connector with the "tab" on the connector pointing downwards with respect to the PCB.
