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

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.

Author: jepler

ports/nrf/common-hal/audiopwmio/PWMAudioOut.c

@@ -97,27 +97,28 @@ STATIC void fill_buffers(audiopwmio_pwmaudioout_obj_t *self, int buf) {
         common_hal_audiopwmio_pwmaudioout_stop(self);
         return;
     }
-    uint32_t num_samples = buffer_length / self->bytes_per_sample / self->spacing;
+    uint32_t num_samples = buffer_length / self->bytes_per_sample / self->sample_channel_count;
+    uint16_t *end_dev_buffer = dev_buffer + 2 * num_samples;
 
     if (self->bytes_per_sample == 1) {
         uint8_t offset = self->signed_to_unsigned ? 0x80 : 0;
         uint16_t scale = self->scale;
-        for (uint32_t i=0; i<buffer_length/self->spacing; i++) {
+        while (dev_buffer < end_dev_buffer) {
             uint8_t rawval = (*buffer++ + offset);
             uint16_t val = (uint16_t)(((uint32_t)rawval * (uint32_t)scale) >> 8);
             *dev_buffer++ = val;
-            if (self->spacing == 1)
+            if (self->sample_channel_count == 1)
                 *dev_buffer++ = val;
         }
     } else {
         uint16_t offset = self->signed_to_unsigned ? 0x8000 : 0;
         uint16_t scale = self->scale;
         uint16_t *buffer16 = (uint16_t*)buffer;
-        for (uint32_t i=0; i<buffer_length/2/self->spacing; i++) {
+        while (dev_buffer < end_dev_buffer) {
             uint16_t rawval = (*buffer16++ + offset);
             uint16_t val = (uint16_t)((rawval * (uint32_t)scale) >> 16);
             *dev_buffer++ = val;
-            if (self->spacing == 1)
+            if (self->sample_channel_count == 1)
                 *dev_buffer++ = val;
         }
     }
@@ -231,9 +232,12 @@ void common_hal_audiopwmio_pwmaudioout_play(audiopwmio_pwmaudioout_obj_t* self,
     self->bytes_per_sample = audiosample_bits_per_sample(sample) / 8;
 
     uint32_t max_buffer_length;
+    uint8_t spacing;
     audiosample_get_buffer_structure(sample, /* single channel */ false,
         &self->single_buffer, &self->signed_to_unsigned, &max_buffer_length,
-        &self->spacing);
+        &spacing);
+    self->sample_channel_count = audiosample_channel_count(sample);
+
     if (max_buffer_length > UINT16_MAX) {
         mp_raise_ValueError_varg(translate("Buffer length %d too big. It must be less than %d"), max_buffer_length, UINT16_MAX);
     }

ports/nrf/common-hal/audiopwmio/PWMAudioOut.h

@@ -41,7 +41,7 @@ typedef struct {
 
     uint8_t left_channel_number;
     uint8_t right_channel_number;
-    uint8_t spacing;
+    uint8_t sample_channel_count;
     uint8_t bytes_per_sample;
 
     bool playing;