even if there are solid commercial solutions for the plausible use cases, the aim here is to create a working low-budget basis that can be customized for special needs. (also, the cheaper commercial solutions seem to have a delay/loop time or channel count that is not satisfactory)
also consider this a proof of concept for using stm32 for simple audio effect units..
also see file sample.wav: it somewhat works, but is rather noisy.
testsetup (and interfacing circuits, which obviously could be better):
- computer audio jack connected directly to mixer (A&H PA12) line in
- bal (aux) line to ADC / controller (neg and ground connected)
- DAC / controller to mixer line in (neg and ground connected)
- mixer PFL to headphones
An interrupt handler at sampling frequency performs the following operations:
- get new sample from ADC
- pass new sample to storage logic
- get old sample from storage logic (using given delay)
- pass a half-wet mix to DAC
The storage logic stores or retrieves single blocks (in polling mode) once the buffers as used by above interrupt handler are full/empty. Alternative/possible operation/optimization:
- Use bigger buffers to allow for writes/reads of more than one block at a time -> singular delays exceeding the timing restrictions may thus be spread over several blocks.
- Use non-blocking sdio operation.
- Combine sdio read/write operation instead of separating logic -> if delay is align blockwise block buffers are full/empty at the same time
for sampling rate of 48khz and 8bit sample resolution needs throughput of at least 48kHz x 1 bytes = 48kByte / sec / channel; or given storage blocks of 512 bytes: 48k x 1 / 512 = 93.75 combined average block read+writes / sec, or a max time limit of 10.666.. ms per read/write operation.
furthermore 48kB / sec / channel means a required storage space of 48kB * 60 = 2.81... MB / minute / channel or ~164.79... MB / hour / channel.
microcontrollers have quite the memory restrictions thus making (longer) storage of audio data impossible. SD cards offer themselves as a cheap and accessible way to provide the memory capacity needed (and could even be used as recorders).
the basic SPI interface of SD cards requires 7-10ms on average; as tested on a nucleo- f303k8.
spi test code: https://gist.github.com/tschiemer/c77ccae718c401fa7604f56b68034944
the 4bit SDIO mode (on nucleo-l476rg) seems to typically require roughly 3200 usec for one combined read/write operation occassionally significantly more (10+ ms) as tested on a stm32l476rg with suboptimal code.
Note: a recorder or player only requires write or read operations (obviously) and might have better sdio timing conditions
For basic throughput limitations given due to chosen testboards: stm32l476rg: "Data transfer up to 50 MHz for the 8 bit mode" (-> 25 MByte/s for 4 bit mode) stm32h476zi2: "Data transfer up to 208 Mbyte/s for the 8-bit mode." (104 MByte/s for 4 bit mode)
(some) stm32s provide 12bit ADCs and DACs, possibly opamps, so along with the SDIO interface they offer themselves for a budget friendly basis. nucleo-l476rg was chosen for the moment being.
Basic line in circuit that works for a mixer line level out signal:
3V3
|
R1 100k
|
LineIn -----C1---x---- ADC
0.1u |
R2 100k
|
GND
It is really suboptimal, especially if the 3V3 source is taken directly from the nucleo board and other components (like the sdio) are connected to it aswell: noise on the power line is clearly heard, also any noise from the power source (power adapter or computer) is passed on. But hey, it's a proof of concept.
Interesting: STM32 ADCs allow for differential input.
Also see: https://forum.arduino.cc/index.php?topic=567581.0
ADC optimziations as explained by stm: https://www.st.com/resource/en/application_note/cd00004444-understanding-and-minimising-adc-conversion-errors-stmicroelectronics.pdf
Differential biasing techniques: http://ww1.microchip.com/downloads/en/appnotes/00842a.pdf
A line out circuit isn't necessarily needed (no guarantees are given for any attached equipment) but are suggested. Essentially the DC bias should be removed with an inline capacitor, a post-capacitor resistor connecting line to ground has been suggested to avoid backflow/charging of capacitor (as I understand). Audio equipment often comes with an input DC block already - but relying on this might risk your equipment.
STM32s often have two (buffered) DACs if they have any, so outputting a differential signal would be possible aswell.
For more: https://forum.allaboutcircuits.com/threads/removing-dc-offset-from-a-signal.65288/
NUCLEO-L476RG
| connector | pin | function |
|---|---|---|
| CN7 | 1 / PC10 | D2 (sdio) |
| 2 / PC11 | D3 (sdio) | |
| 3 / PC12 | CLK (sdio) | |
| 4 / PD2 | CMD (sdio) | |
| CN10 | 1 / PC9 | D1 (sdio) |
| 2 / PC8 | D2 (sdio) | |
| CN8 | 1 / PA0 | ADC |
| 3 / PA4 | DAC |
Ground and 3v3 are self speaking.
SD card:
- https://www.sparkfun.com/products/12941
- https://support.keith-koep.com/service/doku.php/service/hardware/appnote/connectsdcard
AD/DA: see above