Here are patches for SDL12 and SDL_mixer for 4 or 6 channel surround sound on Linux using the Alsa driver. To use them, naturally you need a sound card that will do 4 or 6 channels and probably also a recent version of the Alsa drivers and library. Since the only SDL output driver that knows about surround sound is the Alsa driver, you���ll want to choose it, using: export SDL_AUDIODRIVER=alsa There are no syntactic changes to the programming API. No new library calls, no differences in arguments. There are two semantic changes: (1) For library calls with number of channels as an argument, formerly you could use only 1 or 2 for the number of channels. Now you can also use 4 or 6. (2) The two "left" and "right" arguments to Mix_SetPanning, for the case of 4 or 6 channels, no longer simply control the volumes of the left and right channels. Now the "left" argument is converted to an angle and Mix_SetPosition is called, and the "right" argu- ment is ignored. With two exceptions, so far as I know, the modified SDL12 and SDL_mixer work the same way as the original versions, when opened for 1 or 2 channel output. The two exceptions are bugs which I fixed. Well, the first, anyway, is a bug for sure. When rate conversions up or down by a factor of two are applied (in src/audio/SDL_audiocvt.c), streams with different numbers of channels (that is, mono and stereo) are treated the same way: either each sample is copied or every other sample is omitted. This is ok for mono, but for stereo, it is frames that should be copied or omitted, where by "frame" I mean a portion of the stream containing one sample for each channel. (In the SDL source, confusingly, sometimes frames are called "samples".) So for these rate conversions, stereo streams have to be treated differently, and they are, in my modified version. The other problem that might be characterized as a bug arises when SDL_mixer is passed a multichannel chunk which does not have an integral number of frames. Due to the way the effect_position code loops over frames, when the chunk ends with a partial frame, memory outside the chunk buffer will be accessed. In the case of stereo, it���s possible that because malloc may give more memory than requested, this potential problem never actually causes a segment fault. I don���t know. For 6 channel chunks, I do know, and it does cause segment faults. If SDL_mixer is passed defective chunks and this causes a segment fault, arguably, that���s not a bug in SDL_mixer. Still, whether or not it counts as a bug, it���s easy to protect against, so why not? I added code in mixer.c to discard any partial frame at the end of a chunk. Then what about when SDL or SDL_mixer is opened for 4 or 6 chan- nel output? What happens with the parts of the current library designed for stereo? I don���t know whether I���ve covered all the bases, but I���ve tried: (1) For playing 2 channel waves, or other cases where SDL knows it has to match up a 2 channel source with a 4 or 6 channel output, I���ve added code in SDL_audiocvt.c to make the necessary conversions. (2) For playing midis using timidity, I���ve converted timidity to do 4 or 6 channel output, upon request. (3) For playing mods using mikmod, I put ad hoc code in music.c to convert the stereo output that mikmod produces to 4 or 6 chan- nels. Obviously it would be better to change the mikmod code to mix down into 4 or 6 channels, but I have a hard time following the code in mikmod, so I didn���t do that. (4) For playing mp3s, I put ad hoc code in smpeg to copy channels in the case when 4 or 6 channel output is needed. (5) There seems to be no problem with .ogg files - stereo .oggs can be up converted as .wavs are. (6) The effect_position code in SDL_mixer is now generalized to in- clude the cases of 4 and 6 channel streams. I���ve done a very limited amount of compatibility testing for some of the games using SDL I happen to have. For details, see the file TESTS. I���ve put into a separate archive, Surround-SDL-testfiles.tgz, a couple of 6 channel wave files for testing and a 6 channel ogg file. If you have the right hardware and version of Alsa, you should be able to play the wave files with the Alsa utility aplay (and hear all channels, except maybe lfe, for chan-id.wav, since it���s rather faint). Don���t expect aplay to give good sound, though. There���s something wrong with the current version of aplay. The canyon.ogg file is to test loading of 6 channel oggs. After patching and compiling, you can play it with playmus. (My version of ogg123 will not play it, and I had to patch mplayer to get it to play 6 channel oggs.) Greg Lee <greg@ling.lll.hawaii.edu> Thus, July 1, 2004

/*
    SDL - Simple DirectMedia Layer
    Copyright (C) 1997-2004 Sam Lantinga

    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Library General Public
    License as published by the Free Software Foundation; either
    version 2 of the License, or (at your option) any later version.

    This library 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
    Library General Public License for more details.

    You should have received a copy of the GNU Library General Public
    License along with this library; if not, write to the Free
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

    Sam Lantinga
    slouken@libsdl.org
*/

#ifdef SAVE_RCSID
static char rcsid =
 "@(#) $Id$";
#endif

#ifndef _SDL_xbios_h
#define _SDL_xbios_h

#include "SDL_types.h"
#include "SDL_sysvideo.h"

/* Hidden "this" pointer for the video functions */
#define _THIS	SDL_VideoDevice *this

/* TT video modes:	2
   Falcon RVB:		16 (could be *2 by adding PAL/NTSC modes)
   Falcon VGA:		6
   ST low:		1
*/
#define SDL_NUMMODES 16

typedef struct
{
	Uint16 number;		/* Video mode number */
	Uint16 width;		/* Size */	
	Uint16 height;
	Uint16 depth;		/* bits per plane */
	SDL_bool doubleline;	/* Double the lines ? */
} xbiosmode_t;

/* Private display data */
#define NUM_MODELISTS	2		/* 8 and 16 bits-per-pixel */

struct SDL_PrivateVideoData {
	long cookie_vdo;
	int old_video_mode;				/* Old video mode before entering SDL */
	void *old_video_base;			/* Old pointer to screen buffer */
	void *old_palette;				/* Old palette */
	Uint32 old_num_colors;			/* Nb of colors in saved palette */
	int num_modes;					/* Number of xbios video modes */
	xbiosmode_t	*mode_list;			/* List of xbios video modes */

	void *screens[2];		/* Pointers to aligned screen buffer */
	void *screensmem[2];	/* Pointers to screen buffer */
	void *shadowscreen;		/* Shadow screen for c2p conversion */
	int doubleline;			/* Double line mode ? */
	int frame_number;		/* Number of frame for double buffer */
	int pitch;				/* Destination line width for C2P */
	int width, height;		/* Screen size for centered C2P */

	SDL_Rect *SDL_modelist[NUM_MODELISTS][SDL_NUMMODES+1];
	xbiosmode_t *videomodes[NUM_MODELISTS][SDL_NUMMODES+1];
};

/* _VDO cookie values */
enum {
	VDO_ST=0,
	VDO_STE,
	VDO_TT,
	VDO_F30
};

/* Monitor types */
enum {
	MONITOR_MONO=0,
	MONITOR_TV,
	MONITOR_VGA,
	MONITOR_RGB
};

/* EgetShift masks */
#define ES_BANK		0x000f
#define ES_MODE		0x0700
#define ES_GRAY		0x1000
#define ES_SMEAR	0x8000

/* TT shifter modes */
#define ST_LOW	0x0000
#define ST_MED	0x0100
#define ST_HIGH	0x0200
#define TT_LOW	0x0700
#define TT_MED	0x0300
#define TT_HIGH	0x0600

/* Hidden structure -> variables names */
#define SDL_modelist		(this->hidden->SDL_modelist)
#define XBIOS_mutex		    (this->hidden->mutex)
#define XBIOS_cvdo		    (this->hidden->cookie_vdo)
#define XBIOS_oldpalette	(this->hidden->old_palette)
#define XBIOS_oldnumcol		(this->hidden->old_num_colors)
#define XBIOS_oldvbase		(this->hidden->old_video_base)
#define XBIOS_oldvmode		(this->hidden->old_video_mode)
#define XBIOS_nummodes		(this->hidden->num_modes)
#define XBIOS_modelist		(this->hidden->mode_list)
#define XBIOS_screens		(this->hidden->screens)
#define XBIOS_screensmem	(this->hidden->screensmem)
#define XBIOS_shadowscreen	(this->hidden->shadowscreen)
#define XBIOS_videomodes	(this->hidden->videomodes)
#define XBIOS_doubleline	(this->hidden->doubleline)
#define XBIOS_fbnum			(this->hidden->frame_number)
#define XBIOS_pitch			(this->hidden->pitch)
#define XBIOS_width			(this->hidden->width)
#define XBIOS_height		(this->hidden->height)

#endif /* _SDL_xbios_h */