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Allow SDL2 to change the audio frequency

The mixer can handle arbitrary frequencies, so there's no point in
forcing SDL2 to use 44100Hz.

--- a/src/Backends/Audio/SDL2.cpp

+++ b/src/Backends/Audio/SDL2.cpp

@@ -10,8 +10,6 @@

 #include "../../Organya.h"

 #include "../../WindowsWrapper.h"

-#define OUTPUT_FREQUENCY 44100

-

 #define MIN(a, b) ((a) < (b) ? (a) : (b))

 #define MAX(a, b) ((a) > (b) ? (a) : (b))

 #define clamp(x, y, z) MIN(MAX((x), (y)), (z))

@@ -37,6 +35,8 @@

 static AudioBackend_Sound *sound_list_head;

 static SDL_AudioDeviceID device_id;

+static unsigned long output_frequency;

+

 static double MillibelToScale(long volume)

 {

 	// Volume is in hundredths of decibels, from 0 to -10000

@@ -47,7 +47,7 @@

 static void SetSoundFrequency(AudioBackend_Sound *sound, unsigned int frequency)

 {

 	sound->frequency = frequency;

-	sound->advance_delta = (double)frequency / (double)OUTPUT_FREQUENCY;

+	sound->advance_delta = (double)frequency / (double)output_frequency;

 }

 static void SetSoundVolume(AudioBackend_Sound *sound, long volume)

@@ -150,7 +150,7 @@

 			if (organya_countdown == 0)

 			{

-				organya_countdown = (organya_timer * OUTPUT_FREQUENCY) / 1000;	// organya_timer is in milliseconds, so convert it to audio frames

+				organya_countdown = (organya_timer * output_frequency) / 1000;	// organya_timer is in milliseconds, so convert it to audio frames

 				UpdateOrganya();

 			}

@@ -170,7 +170,7 @@

 		return FALSE;

 	SDL_AudioSpec specification;

-	specification.freq = OUTPUT_FREQUENCY;

+	specification.freq = 44100;

 	specification.format = AUDIO_F32;

 	specification.channels = 2;

 	specification.samples = 0x400;	// Roughly 10 milliseconds for 44100Hz

@@ -177,7 +177,9 @@

 	specification.callback = Callback;

 	specification.userdata = NULL;

-	device_id = SDL_OpenAudioDevice(NULL, 0, &specification, NULL, 0);

+	SDL_AudioSpec obtained_specification;

+	device_id = SDL_OpenAudioDevice(NULL, 0, &specification, &obtained_specification, SDL_AUDIO_ALLOW_FREQUENCY_CHANGE);

+	output_frequency = obtained_specification.freq;

 	if (device_id == 0)

 		return FALSE;

-- 

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