RP2040: fix 8-bit WAV "audio buffer too small" bug

Also subtly fixed 8- to 16-bit scaling math

Author: PaintYourDragon

ports/raspberrypi/audio_dma.c

@@ -64,20 +64,23 @@ STATIC size_t audio_dma_convert_samples(audio_dma_t *dma, uint8_t *input, uint32
     if (dma->sample_resolution <= 8 && dma->output_resolution > 8) {
         // reading bytes, writing 16-bit words, so output buffer will be bigger.
 
-        output_length_used = output_length * 2;
-        if (output_length_used > output_length) {
-            mp_raise_RuntimeError(translate("Internal audio buffer too small"));
-        }
+        output_length_used *= 2;
 
-        size_t shift = dma->output_resolution - dma->sample_resolution;
+        // Correct "rail-to-rail" scaling of arbitrary-depth input to output
+        // requires more operations than this, but at least the vital 8- to
+        // 16-bit cases are correctly scaled now. Prior code was only
+        // considering 8-to-16 anyway, but had a slight DC offset in the
+        // result, so this is no worse off WRT supported resolutions.
+        uint16_t mul = ((1 << dma->output_resolution) - 1) / ((1 << dma->sample_resolution) - 1);
+        uint16_t offset = (1 << dma->output_resolution) / 2;
 
         for (uint32_t i = 0; i < input_length; i += dma->sample_spacing) {
             if (dma->signed_to_unsigned) {
-                ((uint16_t *)output)[out_i] = ((uint16_t)((int8_t *)input)[i] + 0x80) << shift;
+                ((uint16_t *)output)[out_i] = (uint16_t)((((int8_t *)input)[i] + 0x80) * mul);
             } else if (dma->unsigned_to_signed) {
-                ((int16_t *)output)[out_i] = ((int16_t)((uint8_t *)input)[i] - 0x80) << shift;
+                ((int16_t *)output)[out_i] = (int16_t)(((uint8_t *)input)[i] * mul - offset);
             } else {
-                ((uint16_t *)output)[out_i] = ((uint16_t)((uint8_t *)input)[i]) << shift;
+                ((uint16_t *)output)[out_i] = (uint16_t)(((uint8_t *)input)[i] * mul);
             }
             out_i += 1;
         }