ref: 2053768a9251cbd4c731681a3e1e04c800ad21f4
dir: /pcsound/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" #ifndef DISABLE_SDL2MIXER #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(int chan, void *stream, int len, void *udata) { Sint16 *leftptr; Sint16 *rightptr; Sint16 this_value; int frequency; 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) { // Get the next frequency to play callback(¤t_remaining, &frequency); if (current_freq != frequency) { current_freq = frequency; phase_offset = 0; } 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_OpenAudioDevice(pcsound_sample_rate, AUDIO_S16SYS, 2, slicesize, NULL, SDL_AUDIO_ALLOW_FREQUENCY_CHANGE) < 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_RegisterEffect(MIX_CHANNEL_POST, PCSound_Mix_Callback, NULL, NULL); return 1; } pcsound_driver_t pcsound_sdl_driver = { "SDL", PCSound_SDL_Init, PCSound_SDL_Shutdown, }; #endif // DISABLE_SDL2MIXER