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// Emacs style mode select -*- C++ -*-
//-----------------------------------------------------------------------------
//
// Copyright(C) 2007 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.
//
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
// 02111-1307, USA.
//
// DESCRIPTION:
// PC speaker interface.
//
//-----------------------------------------------------------------------------
#include <stdio.h>
#include <stdlib.h>
#include "SDL.h"
#include "SDL_mixer.h"
#include "pcsound.h"
#define SQUARE_WAVE_AMP 0x2000
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(¤t_remaining, ¤t_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 PCSound_SDL_Init(pcsound_callback_func callback_func)
{
// Check that SDL_mixer has been opened already
// If not, fail
if (!Mix_QuerySpec(&mixing_freq, &mixing_format, &mixing_channels))
{
return 0;
}
// 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");
return 0;
}
callback = callback_func;
current_freq = 0;
current_remaining = 0;
Mix_SetPostMix(PCSound_Mix_Callback, NULL);
return 1;
}
static void PCSound_SDL_Shutdown(void)
{
}
pcsound_driver_t pcsound_sdl_driver =
{
"SDL",
PCSound_SDL_Init,
PCSound_SDL_Shutdown,
};
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