10000 AudiogeneratorMOD: use full 16bit with slightly better accuracy (#479) · murarduino/ESP8266Audio@7a82ba2 · GitHub
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AudiogeneratorMOD: use full 16bit with slightly better accuracy (earlephilhower#479)
* Fix a tricky bug that causes delays in playback ... this bug was already present in stellaplayer. Obviously "effectNumber" must be checked, because "effectParameterX" just holds the "upper nibble" of effectParameter * fix for issue earlephilhower#474 - do channel mixing in 32bit, to avoid noise from over/underflow - write audio output as "signed int" (not unsigned) - add int16 saturation check ("clipping") * avoid crash when MOD file has to many CHANNELS - avoid guru meditation, by logging an error and refusing playback in case MOD.numberOfChannels > CHANNELS - correct small typo * internal DAC: avoid possible overflow in conversion int16 -> uint16 * update constants .. we can do 16bit and 8 channels, so let's do it ;-) This adds some debugging code to better understand what is going on inside the mixer code (AudioGeneratorMOD::GetSample). And it also prints out some usefull information from the MOD file. Only active when do_MIXER_DEBUG is defined. * MOD generator: 12 bits of "real" resolution (instead of 10) With this change, we gain 2 additional bits of "real resolution" from the sample interpolation step. Still need some testing to be sure that nothing get "lost" at the same time. * limit to 4 channels on ESP8266 As discussed in earlephilhower#479 (comment)
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‎src/AudioGeneratorMOD.cpp

Lines changed: 17 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -69,6 +69,11 @@ bool AudioGeneratorMOD::stop()
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free(FatBuffer.channels[i]);
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FatBuffer.channels[i] = NULL;
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}
72+
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if(running && (file != NULL) && (file->isOpen() == true)) {
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output->flush(); //flush I2S output buffer, if the player was actually running before.
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}
76+
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if (file) file->close();
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running = false;
7479
output->stop();
@@ -803,14 +808,20 @@ void AudioGeneratorMOD::GetSample(int16_t sample[2])
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804809
current = FatBuffer.channels[channel][(samplePointer - FatBuffer.samplePointer[channel]) /*& (FATBUFFERSIZE - 1)*/];
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next = FatBuffer.channels[channel][(samplePointer + 1 - FatBuffer.samplePointer[channel]) /*& (FATBUFFERSIZE - 1)*/];
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807-
out = current;
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812+
// preserve a few more bits from sample interpolation, by upscaling input values.
813+
// This does (slightly) reduce quantization noise in higher frequencies, typically above 8kHz.
814+
// Actually we could could even gain more bits, I was just not sure if more bits would cause overflows in other conputations.
815+
int16_t current16 = (int16_t) current << 2;
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int16_t next16 = (int16_t) next << 2;
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out = current16;
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// Integer linear interpolation - only works correctly in 16bit
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out += (next - current) * (Mixer.channelSampleOffset[channel] & ((1 << FIXED_DIVIDER) - 1)) >> FIXED_DIVIDER;
821+
out += (next16 - current16) * (Mixer.channelSampleOffset[channel] & ((1 << FIXED_DIVIDER) - 1)) >> FIXED_DIVIDER;
811822

812-
// Upscale to BITDEPTH
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out32 = (int32_t)out << (BITDEPTH - 8);
823+
// Upscale to BITDEPTH, considering the we already gained two bits in the previous step
824+
out32 = (int32_t)out << (BITDEPTH - 10);
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815826
// Channel volume
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out32 = out32 * Mixer.channelVolume[channel] >> 6;
@@ -819,7 +830,7 @@ void AudioGeneratorMOD::GetSample(int16_t sample[2])
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sumL += out32 * min(128 - Mixer.channelPanning[channel], 64) >> 6;
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sumR += out32 * min(Mixer.channelPanning[channel], 64) >> 6;
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}
822-
833+
823834
// Downscale to BITDEPTH - a bit faster because the compiler can replaced division by constants with proper "right shift" + correct handling of sign bit
824835
if (Mod.numberOfChannels <= 4) {
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// up to 4 channels

‎src/AudioGeneratorMOD.h

Lines changed: 8 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -53,7 +53,7 @@ class AudioGeneratorMOD : public AudioGenerator
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protected:
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int mixerTick;
56-
enum {BITDEPTH = 15};
56+
enum {BITDEPTH = 16};
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int sampleRate;
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int fatBufferSize; //(6*1024) // File system buffers per-CHANNEL (i.e. total mem required is 4 * FATBUFFERSIZE)
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enum {FIXED_DIVIDER = 10}; // Fixed-point mantissa used for integer arithmetic
@@ -64,8 +64,14 @@ class AudioGeneratorMOD : public AudioGenerator
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// Hz = 7159091 / (amigaPeriod * 2) for NTSC
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int AMIGA;
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void UpdateAmiga() { AMIGA = ((usePAL?7159091:7093789) / 2 / sampleRate << FIXED_DIVIDER); }
67-
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#ifdef ESP8266 // Not sure if C3/C2 have RAM constraints, maybe add them here?
69+
// support max 4 channels
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enum {ROWS = 64, SAMPLES = 31, CHANNELS = 4, NONOTE = 0xFFFF, NONOTE8 = 0xff };
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#else
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// support max 8 channels
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enum {ROWS = 64, SAMPLES = 31, CHANNELS = 8, NONOTE = 0xFFFF, NONOTE8 = 0xff };
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#endif
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typedef struct Sample {
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uint16_t length;

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