audio_processor

Audio Processor project

This project runs on the STM32MP157 Discovery board, using the board's codec and mic/headphone jack.

Note that the OSD32BRK board is not supported in this project since it lacks a codec. (although you could connect it to a breakout board with a codec).

Six different synths can be selected by pressing the User1 button.

To use, first make sure U-Boot is installed on an SD card, as usual (see README in the project root directory).

Then, in this directory run:

make

In the build/ dir, you should see the a7-main.uimg file. Copy it to the 4th partition of the SD-card, as usual (again, see README in the project root dir). Power up and play!

Press the buttons to change effect. You'll see a message like:

Starting Audio Processor

Using Synth: Harmonic Osc
Current load: 12%

Using Synth: Reverb Osc
Current load: 9%

Harmonic Osc: Example project from DaisySP library. AD Envelope sweeps the harmonics of a 16-osc harmonic oscillator, as well as a VCA to make notes. A simple sequencer of pitches plays, too.

Reverb Osc: Example project from DaisySP library. AD envelope + VCA + triangle oscillator playing a simple sequencer. This is fed into a stereo reverb effect.

Djembes: Plays eight physical modeled djembes hand-drums, panned in stereo. Algorithm taken from Faust library.

Passthrough: Just copies the inputs to the outputs. Unfortunately the Disco board has a TRRS jack and I haven't yet found a mic that works well with it. I'm still looking... We may need to adjust the codec's biasing and boost settings to get it to work. With a scope you can verify it's passing the input signal to the output, it's getting noise from the mic.

DualFMOsc: Ramp Osc frequency modulatates (FM's) a triangle oscillator which is heard on the left output. The triangle oscillator FM's a sine oscillator, which is heard on the right output.

Sine and Tri Osc: Left output has a 400Hz triangle wave. Right output has a 600Hz sine wave.

What's under the hood

DaisySP

DaisySP is "A Powerful, Open Source DSP Library in C++" that works with the Daisy embedded platform. I've taken a couple of the example projects and converted them to run on the Cortex-A7.

Faust

Faust is "a functional programming language for sound synthesis and audio processing..." I'm using the Djembe physical model algorithm.

lib/mdrivlib

mdrivlib is an eMbedded DRIVer LIBrary written by me. I intend to release it fully after cleaning up a few more things, but for now it's here as part of this project under the MIT license. Here we're using a few portions of mdrivlib:

• DebouncePin: debounce a GPIO pin and provide high-level state polling like is_just_pressed(). Uses mdrivlib::Debouncer and mdrivlib::Toggler

• CodecCS42L51: driver for the codec chip found on the STM32MP1 Discovery board. Uses mdrivlib::SaiTdmPeriph (multi-channel SAI DMA driver) and mdrivlib::I2CPeriph (basic wrapper over STM32-HAL's I2C driver). Also handles the DMA callbacks.

• CycleCounter: cross-architecture processor cycle counter. The MP1-specific implementation file is compiled into this project, which uses the Cortex-A PL1 Physical Timer (see the CMSIS file core_ca.h). The timer runs at 24MHZ, so our resolution is something like 27 cycles. Good enough for measuring load percentage.

lib/util

util are some general-purpose utilities I've picked up over the years (not specific to ARM or anything). Most have unit tests. I need a better name before I can release it.

• zip: Iterate over two or more containers. No overhead: compiles to the same assembly that you get from the normal way for (int i=0; i<BUFFER_SIZE; i++) { out[i] = in[i]... Also has a cousin countzip which is like Python's enumerate

• AudioFrame: multi-channel audio frame struct. Has some nice conversion utilities (24=>32 bit sign extension)

• oscs: Simple phase accum ramp and triangle oscillators.

• math: various math stuff

• math\_tables: Trig and expo tables. Uses InterpArray, which provides a interpolation for both periodic and non-periodic tables.

Program flow and potential optimizations

There is some degree of indirection in the audio stream method used here, in order to provide a flexible interface for setting and changing the audio processing function on the fly. It could easily be made more efficient if your application required it. Here's what happens in this project:

1. The SAI DMA transfer/half-transfer complete interrupt is triggered, which calls the IRQ_Handler asm code in lib/mdrivlib/target/stm32mp1_ca7/drivers/interrupt_handler.cc

2. The IRQ_Handler calls a lambda from Interrupt::ISRs[]. This lambda was created by SaiTdmPeriph

3. The SaiTdmPeriph lambda checks and clears the DMA flags and calls one of two callback functions, depending on if it was a DMA TX-Complete IRQ or DMA TX-Half-Complete IRQ

4. This callback function (which is a lambda created by the AudioStream object) calls the function that you set using audio.set_process_function(). It also measures the processor load that the processing function takes.