PWM audio on pico with 8KHz up to 44.1KHz stereo wave file example of audio output using PWM 10 bits and DMA The wave file needs to be stereo and 16 bits.
GPIO pin 14 and 15 are the output you need to use head phone with a 1K resistor in series on left and right speaker
The myPWM subclass set the maximum count to 255(8 bits) or 1023(10bits) at a frequency around 122.5KHz.
**the PWM is now on 10 bits (0..1023)
The myDMA class allows to use direct memory access to transfer each frame at the current sample rate
You need to install the wave.py and chunk.py from https://github.com/joeky888/awesome-micropython-lib/tree/master/Audio
Don't forget to increase the SPI clock up to 3Mhz in order to get SD card work properly.
from machine import SPI,Pin
sd = SDCard.SDCard(SPI(1),Pin(13))
sd.init_spi(50_000_000)
See example in wavePlayer.py
1 - We set the PWM to a range of 255 at 122Khz 2 - We read the wave file using the class wave which will set the sample rate and read the audio data by chunk 3 - Each chunk are converted to 16 bit signed to unsigned char with the middle at 128, (512 for 10 bits) 4 - Wait for the DMA to be completed. On first it will be anyway. 5 - The converted chunk is then pass to the DMA to be transfer at the sample rate using one of build in timer 6 - Go on step 2 until is done.
P.S. to transfer wave file use rshell.
2K
PIO2 -/\/\/-----+----- headphone left
|
=== 0.1uF
|
PIO4 -----------+----- headphone ground
|
=== 0.1uF
2k |
PIO3 -/\/\/-----+----- headphone right
For amplifier don't use PIO4 and the capacitor should be 2200pF and connected to GND, this should drain high frequency noise to the ground.
This formula cam be used to determine capacitance for desired frequency cut off C = 1/(2*π*R*f), with 2K and 2200pF the cutoff frequency would be 36171Hz, results may vary depending on actual components selection.