source: rtos_arduino/trunk/arduino_lib/libraries/Audio/src/Audio.cpp@ 136

/*
 * Copyright (c) 2012 by Cristian Maglie <c.maglie@arduino.cc>
 * Audio library for Arduino Due.
 *
 * This file is free software; you can redistribute it and/or modify
 * it under the terms of either the GNU General Public License version 2
 * or the GNU Lesser General Public License version 2.1, both as
 * published by the Free Software Foundation.
 */

#include "Audio.h"

void AudioClass::begin(uint32_t sampleRate, uint32_t msPreBuffer) {
        // Allocate a buffer to keep msPreBuffer milliseconds of audio
        bufferSize = msPreBuffer * sampleRate / 1000;
        if (bufferSize < 1024)
                bufferSize = 1024;
        buffer = (uint32_t *) malloc(bufferSize * sizeof(uint32_t));
        half = buffer + bufferSize / 2;
        last = buffer + bufferSize;

        // Buffering starts from the beginning
        running = buffer;
        next = buffer;

        // Start DAC
        dac->begin(VARIANT_MCK / sampleRate);
        dac->setOnTransmitEnd_CB(onTransmitEnd, this);
}

void AudioClass::end() {
        dac->end();
        free( buffer);
}

void AudioClass::prepare(int16_t *buffer, int S, int volume){
    uint16_t *ubuffer = (uint16_t*) buffer;
    for (int i=0; i<S; i++) {
        // set volume amplitude (signed multiply)
        buffer[i] = buffer[i] * volume / 1024;
        // convert from signed 16 bit to unsigned 12 bit for DAC.
        ubuffer[i] += 0x8000;
        ubuffer[i] >>= 4;
    }
}

size_t AudioClass::write(const uint32_t *data, size_t size) {
        const uint32_t TAG = 0x10000000;
        int i;
        for (i = 0; i < size; i++) {
                *next = data[i] | TAG;
                next++;

                if (next == half || next == last) {
                        enqueue();
                        while (next == running)
                                ;
                }
        }

        return i;
}

void AudioClass::enqueue() {
        if (!dac->canQueue()) {
                // DMA queue full
                return;
        }

        if (next == half) {
                // Enqueue the first half
                dac->queueBuffer(buffer, bufferSize / 2);
        } else {
                // Enqueue the second half
                dac->queueBuffer(half, bufferSize / 2);
                next = buffer; // wrap around
        }
}

void AudioClass::onTransmitEnd(void *_me) {
        AudioClass *me = reinterpret_cast<AudioClass *> (_me);
        if (me->running == me->buffer)
                me->running = me->half;
        else
                me->running = me->buffer;
}

AudioClass Audio(DAC);