nRF: PWMAudioOut: fix half-speed playback of stereo samples

[jepler]

The "spacing" of "buffer structure" is confusing, use the "channel count" instead.

Testing performed on nrf52840 feather:

Play stereo and mono, 8- and 16-bit, 8kHz RawSamples representing 333.33Hz square waves.

Use both mono and stereo PWMAudioOut instances.

Scope the RC-filtered signal and use the scope's frequency measurement function, verify the frequency is 333 or 334Hz in all tested cases.

In the "stereo output" cases, verify both the L and R channels. Verify the output amplitude is the same in both channels.

In the "stereo output" cases, run a second test where the L channel's amplitude is attenuated 50%. Verify the output amplitude is correct in each channel.

import array
try:
    import audiobusio
except ImportError:
    pass
try:
    import audiopwmio
except ImportError:
    pass
import audiocore
import board
import time

values = {
    'b': (-128, 127),
    'B': (0, 255),
    'h': (-32768, 32767),
    'H': (0, 65535),
}

# Should play an audio sample at 333Hz (=8000/24)
def go(audio, at = 'h', channel_count = 1, attenuate_left=False):
    wave = array.array(at, [values[at][0]]) * channel_count * 24
    for i in range(len(wave)//2, len(wave)):
        wave[i] = values[at][1]
    if channel_count > 1 and attenuate_left:
        for i in range(0, len(wave), channel_count):
            wave[i] //= 2
    print(wave)

    wave_sample = audiocore.RawSample(wave, channel_count = channel_count)

    audio.play(wave_sample, loop=True)
    time.sleep(1)
    audio.stop()

def go_i2s(at='h', channel_count=1, attenuate_left=False):
    with audiobusio.I2SOut(bit_clock=board.D6, word_select=board.D9, data=board.D10) as audio:
        go(audio, at, channel_count, attenuate_left)

def go_pwm(at='h', channel_count=1, attenuate_left=False):
    with audiopwmio.PWMAudioOut(board.D2) as audio:
        go(audio, at, channel_count, attenuate_left)

def go_pwm_stereo(at='h', channel_count=1, attenuate_left=False):
    with audiopwmio.PWMAudioOut(board.D2, right_channel=board.D5) as audio:
        go(audio, at, channel_count, attenuate_left)

[tannewt]

Sweet! Thank you!