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audio: Offer a hint for libsamplerate quality/speed tradeoff.

This defaults to the internal SDL resampler, since that's the likely default
without a system-wide install of libsamplerate, but those that need more can
tweak this.
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icculus committed Jan 24, 2017
1 parent 5ea0c89 commit c7f9dcb6fc32a8801fed3b10b3ea7d04571bd649
Showing with 57 additions and 4 deletions.
  1. +36 −0 include/SDL_hints.h
  2. +19 −3 src/audio/SDL_audio.c
  3. +1 −0 src/audio/SDL_audio_c.h
  4. +1 −1 src/audio/SDL_audiocvt.c
@@ -775,6 +775,42 @@ extern "C" {
*/
#define SDL_HINT_OPENGL_ES_DRIVER "SDL_OPENGL_ES_DRIVER"

/**
* \brief A variable controlling speed/quality tradeoff of audio resampling.
*
* If available, SDL can use libsamplerate ( http://www.mega-nerd.com/SRC/ )
* to handle audio resampling. There are different resampling modes available
* that produce different levels of quality, possibly using more CPU.
*
* If this hint isn't specified to a valid setting, or libsamplerate isn't
* available, SDL will act as if this hint was set to "fast".
*
* Note that this is currently only applicable to resampling audio that is
* being written to a device for playback or audio being read from a device
* for capture. SDL_AudioCVT always uses the "fast" resampler (although this
* might change for SDL 2.1).
*
* Most things can probably live with the "fast" resampler, but if quality
* is important or you can spare some CPU cycles, the other options are
* worth exploring!
*
* libsamplerate's interpolators, that these hints map to, are explained here:
* http://www.mega-nerd.com/SRC/api_misc.html#Converters
*
* This hint is only checked at audio subsystem init time and changes to it
* at other times are ignored.
*
* This variable can be set to the following values:
*
* "default" - Use SDL's internal, resampler. (Default when not set. low quality, fast.)
* "linear" - Use libsamplerate's Linear interpolator (low quality, fast).
* "zero_order_hold" - Use libsamplerate's Zero Order Hold interpolator (low quality, fast).
* "sinc_fastest" - Use libsamplerate's fastest (lowest quality) sinc interpolator.
* "sinc_medium" - Use libsamplerate's medium quality sinc interpolator.
* "sinc_best" - Use libsamplerate's best quality sinc interpolator.
*/
#define SDL_HINT_AUDIO_RESAMPLER_MODE "SDL_AUDIO_RESAMPLER_MODE"

/**
* \brief An enumeration of hint priorities
*/
@@ -113,6 +113,7 @@ static const AudioBootStrap *const bootstrap[] = {
static void *SRC_lib = NULL;
#endif
SDL_bool SRC_available = SDL_FALSE;
int SRC_converter = 0;
SRC_STATE* (*SRC_src_new)(int converter_type, int channels, int *error) = NULL;
int (*SRC_src_process)(SRC_STATE *state, SRC_DATA *data) = NULL;
int (*SRC_src_reset)(SRC_STATE *state) = NULL;
@@ -122,10 +123,25 @@ const char* (*SRC_src_strerror)(int error) = NULL;
static SDL_bool
LoadLibSampleRate(void)
{
SRC_available = SDL_FALSE;
const char *hint = SDL_GetHint(SDL_HINT_AUDIO_RESAMPLER_MODE);

if (!SDL_GetHintBoolean("SDL_AUDIO_ALLOW_LIBRESAMPLE", SDL_TRUE)) {
return SDL_FALSE;
SRC_available = SDL_FALSE;
SRC_converter = 0;

if (!hint || (SDL_strcasecmp(hint, "default") == 0)) {
return SDL_FALSE; /* don't load anything. */
} else if (SDL_strcasecmp(hint, "linear") == 0) {
SRC_converter = SRC_LINEAR;
} else if (SDL_strcasecmp(hint, "zero_order_hold") == 0) {
SRC_converter = SRC_ZERO_ORDER_HOLD;
} else if (SDL_strcasecmp(hint, "sinc_fastest") == 0) {
SRC_converter = SRC_SINC_FASTEST;
} else if (SDL_strcasecmp(hint, "sinc_medium") == 0) {
SRC_converter = SRC_SINC_MEDIUM_QUALITY;
} else if (SDL_strcasecmp(hint, "sinc_best") == 0) {
SRC_converter = SRC_SINC_BEST_QUALITY;
} else {
return SDL_FALSE; /* treat it like "default", don't load anything. */
}

#ifdef SDL_LIBSAMPLERATE_DYNAMIC
@@ -39,6 +39,7 @@
#ifdef HAVE_LIBSAMPLERATE_H
#include "samplerate.h"
extern SDL_bool SRC_available;
extern int SRC_converter;
extern SRC_STATE* (*SRC_src_new)(int converter_type, int channels, int *error);
extern int (*SRC_src_process)(SRC_STATE *state, SRC_DATA *data);
extern int (*SRC_src_reset)(SRC_STATE *state);
@@ -947,7 +947,7 @@ SetupLibSampleRateResampling(SDL_AudioStream *stream)
SRC_STATE *state = NULL;

if (SRC_available) {
state = SRC_src_new(SRC_SINC_FASTEST, stream->pre_resample_channels, &result);
state = SRC_src_new(SRC_converter, stream->pre_resample_channels, &result);
if (!state) {
SDL_SetError("src_new() failed: %s", SRC_src_strerror(result));
}

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