summaryrefslogtreecommitdiff
path: root/pcsound/pcsound_sdl.c
blob: 546e6a369918aebe445c99847ca860cd968dc903 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
// 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"
#include "pcsound_internal.h"

#define SOUND_SLICE_TIME 100 /* ms */
#define SQUARE_WAVE_AMP 0x2000

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

static int sdl_was_initialised = 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 SDLIsInitialised(void)
{
    int freq, channels;
    Uint16 format;

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

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

static int PCSound_SDL_Init(pcsound_callback_func callback_func)
{
    int slicesize;

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

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

        slicesize = (SOUND_SLICE_TIME * pcsound_sample_rate) / 1000;

        if (Mix_OpenAudio(pcsound_sample_rate, AUDIO_S16SYS, 2, slicesize) < 0)
        {
            fprintf(stderr, "Error initialising 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_initialised = 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,
};