pcsound_sdl.c

//
// Copyright(C) 2005-2014 Simon Howard
//
// This program is free software; you can redistribute it and/or
// modify it under the terms of the GNU General Public License
// as published by the Free Software Foundation; either version 2
// of the License, or (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// DESCRIPTION:
//    PC speaker interface.
//

#include <stdio.h>
#include <stdlib.h>

#include "SDL.h"
#include "SDL_mixer.h"

#include "pcsound.h"
#include "pcsound_internal.h"

#define MAX_SOUND_SLICE_TIME 70 /* ms */
#define SQUARE_WAVE_AMP 0x2000

// If true, we initialized SDL and have the responsibility to shut it 
// down

static int sdl_was_initialized = 0;

// Callback function to invoke when we want new sound data

static pcsound_callback_func callback;

// Output sound format

static int mixing_freq;
static Uint16 mixing_format;
static int mixing_channels;

// Currently playing sound
// current_remaining is the number of remaining samples that must be played
// before we invoke the callback to get the next frequency.

static int current_remaining;
static int current_freq;

static int phase_offset = 0;

// Mixer function that does the PC speaker emulation

static void PCSound_Mix_Callback(void *udata, Uint8 *stream, int len)
{
    Sint16 *leftptr;
    Sint16 *rightptr;
    Sint16 this_value;
    int oldfreq;
    int i;
    int nsamples;

    // Number of samples is quadrupled, because of 16-bit and stereo

    nsamples = len / 4;

    leftptr = (Sint16 *) stream;
    rightptr = ((Sint16 *) stream) + 1;
    
    // Fill the output buffer

    for (i=0; i<nsamples; ++i)
    {
        // Has this sound expired? If so, invoke the callback to get 
        // the next frequency.

        while (current_remaining == 0) 
        {
            oldfreq = current_freq;

            // Get the next frequency to play

            callback(&current_remaining, &current_freq);

            if (current_freq != 0)
            {
                // Adjust phase to match to the new frequency.
                // This gives us a smooth transition between different tones,
                // with no impulse changes.

                phase_offset = (phase_offset * oldfreq) / current_freq;
            }

            current_remaining = (current_remaining * mixing_freq) / 1000;
        }

        // Set the value for this sample.
        
        if (current_freq == 0)
        {
            // Silence

            this_value = 0;
        }
        else 
        {
            int frac;

            // Determine whether we are at a peak or trough in the current
            // sound.  Multiply by 2 so that frac % 2 will give 0 or 1 
            // depending on whether we are at a peak or trough.

            frac = (phase_offset * current_freq * 2) / mixing_freq;

            if ((frac % 2) == 0) 
            {
                this_value = SQUARE_WAVE_AMP;
            }
            else
            {
                this_value = -SQUARE_WAVE_AMP;
            }

            ++phase_offset;
        }

        --current_remaining;

        // Use the same value for the left and right channels.

        *leftptr += this_value;
        *rightptr += this_value;

        leftptr += 2;
        rightptr += 2;
    }
}

static int SDLIsInitialized(void)
{
    int freq, channels;
    Uint16 format;

    return Mix_QuerySpec(&freq, &format, &channels);
}

static void PCSound_SDL_Shutdown(void)
{
    if (sdl_was_initialized)
    {
        Mix_CloseAudio();
        SDL_QuitSubSystem(SDL_INIT_AUDIO);
        sdl_was_initialized = 0;
    }
}

// Calculate slice size, based on MAX_SOUND_SLICE_TIME.
// The result must be a power of two.

static int GetSliceSize(void)
{
    int limit;
    int n;

    limit = (pcsound_sample_rate * MAX_SOUND_SLICE_TIME) / 1000;

    // Try all powers of two, not exceeding the limit.

    for (n=0;; ++n)
    {
        // 2^n <= limit < 2^n+1 ?

        if ((1 << (n + 1)) > limit)
        {
            return (1 << n);
        }
    }

    // Should never happen?

    return 1024;
}

static int PCSound_SDL_Init(pcsound_callback_func callback_func)
{
    int slicesize;

    // Check if SDL_mixer has been opened already
    // If not, we must initialize it now

    if (!SDLIsInitialized())
    {
        if (SDL_Init(SDL_INIT_AUDIO) < 0)
        {
            fprintf(stderr, "Unable to set up sound.\n");
            return 0;
        }

        slicesize = GetSliceSize();

        if (Mix_OpenAudio(pcsound_sample_rate, AUDIO_S16SYS, 2, slicesize) < 0)
        {
            fprintf(stderr, "Error initializing SDL_mixer: %s\n", Mix_GetError());

            SDL_QuitSubSystem(SDL_INIT_AUDIO);
            return 0;
        }

        SDL_PauseAudio(0);

        // When this module shuts down, it has the responsibility to 
        // shut down SDL.

        sdl_was_initialized = 1;
    }

    // Get the mixer frequency, format and number of channels.

    Mix_QuerySpec(&mixing_freq, &mixing_format, &mixing_channels);

    // Only supports AUDIO_S16SYS

    if (mixing_format != AUDIO_S16SYS || mixing_channels != 2)
    {
        fprintf(stderr, 
                "PCSound_SDL only supports native signed 16-bit LSB, "
                "stereo format!\n");

        PCSound_SDL_Shutdown();
        return 0;
    }

    callback = callback_func;
    current_freq = 0;
    current_remaining = 0;

    Mix_SetPostMix(PCSound_Mix_Callback, NULL);

    return 1;
}

pcsound_driver_t pcsound_sdl_driver = 
{
    "SDL",
    PCSound_SDL_Init,
    PCSound_SDL_Shutdown,
};