This makes all the samples from Dan's collection register as 440Hz
when playing on pwmio or i2sout, using https://webaudiodemos.appspot.com/pitchdetect/index.html
to detect the frequency played (all should show as A 440Hz; an error
of up to 20 "cents" should be treated as OK)
There's an audible carrier with PWM output and the 8kHz samples. This is
probably a limitation of the peripheral which is documented as being for
input signals of 44 kHz or 48 kHz; the carrier frequency is a fixed
multiple of the sample frequency.
Closes#7800
.. via a peripheral known as the "MQS" (medium quality sound). It uses an
~192kHz PWM signal to generate audio. It sounds OK on a small speaker with
no amplifier. There's a small pop when starting/stopping audio, as is
typical.
tested on metro m7 (green prototype version) with max98357a i2s amplifier and the following test code:
```py
import board
import time
import digitalio
from audiobusio import I2SOut
from audiocore import RawSample
from microcontroller import pin
from ulab import numpy as np
n = np.array(np.sin(np.linspace(0, np.pi*2, 218, endpoint=False)) * 200, dtype=np.int16)
print(n)
r = RawSample(n, sample_rate=8000, channel_count=2)
def main():
with digitalio.DigitalInOut(board.LED) as l:
l.switch_to_output(True)
value = False
while True:
with I2SOut(pin.GPIO_06, pin.GPIO_07, pin.GPIO_04) as i:
time.sleep(.01)
l.value = value = not value
i.play(r, loop=True)
print(i.playing)
time.sleep(.5)
i.stop()
print("STOPPED")
print(i.playing)
time.sleep(.5)
i.play(r, loop=True)
print(i.playing)
print("PLAY AGAIN")
time.sleep(.5)
time.sleep(1)
```
Only stereo, 16-bit, raw samples were tested; the sample rate is actually fixed
at 48kHz in the core right now. There is more to do, but the basics work.
# Conflicts:
# ports/mimxrt10xx/Makefile
# ports/mimxrt10xx/mpconfigport.mk
