A small hack to enable libmikmod to play 2-channel PCM files

[dr_watson]

libmikmod (or mikmodlib) is pretty good but it only supports mono PCM (Wav) files at the moment. You'll get a NULL pointer if trying to load a Wav file with 2 channels. Here is a small hack to make it load and play 2-channel Wav files. Technically speaking, this is not REAL stereo because, libmikmod somehow mixes the 2 channels together. But still, it's better than just having the sound effect played in your left ear only

All we need to do is just to modify few lines in mwav.c as shown below (marked with @_@):

SAMPLE* Sample_LoadGeneric_internal(MREADER* reader)
{
	SAMPLE *si=NULL;
	WAV wh;
	BOOL have_fmt=0;

	/* read wav header */
	_mm_read_string(wh.rID,4,reader);
	wh.rLen = _mm_read_I_ULONG(reader);
	_mm_read_string(wh.wID,4,reader);

	/* check for correct header */
	if(_mm_eof(reader)|| memcmp(wh.rID,"RIFF",4) || memcmp(wh.wID,"WAVE",4)) {
		_mm_errno = MMERR_UNKNOWN_WAVE_TYPE;
		return NULL;
	}

	/* scan all RIFF blocks until we find the sample data */
	for(;;) {
		CHAR dID[4];
		ULONG len,start;

		_mm_read_string(dID,4,reader);
		len = _mm_read_I_ULONG(reader);
		/* truncated file ? */
		if (_mm_eof(reader)) {
			_mm_errno=MMERR_UNKNOWN_WAVE_TYPE;
			return NULL;
		}
		start = _mm_ftell(reader);

		/* sample format block
		   should be present only once and before a data block */
		if(!memcmp(dID,"fmt ",4)) {
			wh.wFormatTag      = _mm_read_I_UWORD(reader);
			wh.nChannels       = _mm_read_I_UWORD(reader);
			wh.nSamplesPerSec  = _mm_read_I_ULONG(reader);
			wh.nAvgBytesPerSec = _mm_read_I_ULONG(reader);
			wh.nBlockAlign     = _mm_read_I_UWORD(reader);
			wh.nFormatSpecific = _mm_read_I_UWORD(reader);

#ifdef MIKMOD_DEBUG
			fprintf(stderr,"\rwavloader : wFormatTag=%04x blockalign=%04x nFormatSpc=%04x\n",
			        wh.wFormatTag,wh.nBlockAlign,wh.nFormatSpecific);
#endif

			if((have_fmt)||(wh.nChannels>2)) {				// @_@: changed 1 to 2
				_mm_errno=MMERR_UNKNOWN_WAVE_TYPE;
				return NULL;
			}
			have_fmt=1;
		} else
		/* sample data block
		   should be present only once and after a format block */
		  if(!memcmp(dID,"data",4)) {

			  int samp_size, num_samp;						// @_@: newly added variables

			if(!have_fmt) {
				_mm_errno=MMERR_UNKNOWN_WAVE_TYPE;
				return NULL;
			}
			if(!(si=(SAMPLE*)_mm_malloc(sizeof(SAMPLE)))) return NULL;
			si->speed  = wh.nSamplesPerSec*wh.nChannels;	// @_@: changed from (/) to (*)
			si->volume = 64;
			si->length = len;

			samp_size = 1;
			if((wh.nBlockAlign/wh.nChannels) == 2) {			// @_@: take into account the channel as well
				si->flags    = SF_16BITS | SF_SIGNED;
				//si->length >>= 1;								// @_@: keep the size
				samp_size = 2;									// @_@: 16 bit sample
			}

			if (wh.nChannels == 2)								// @_@: stereo
				si->flags |= SF_STEREO;

			num_samp = si->length/samp_size/wh.nChannels;		// @_@: setting up proper values for si
			si->loopstart=0;
			si->length=num_samp;
			si->loopend=num_samp;
			si->panning = PAN_CENTER;							// @_@: this one is the most important one!
																//		was not initialized in the original code so
																//		it default to PAN_LEFT

			si->inflags = si->flags;
			SL_RegisterSample(si,MD_SNDFX,reader);
			SL_LoadSamples();
			
			/* skip any other remaining blocks - so in case of repeated sample
			   fragments, we'll return the first anyway instead of an error */
			break;
		}
		/* onto next block */
		_mm_fseek(reader,start+len,SEEK_SET);
		if (_mm_eof(reader))
			break;
	}

	return si;
}

I believe you can do the same hack to mikmodlib as well.

Cheers!

[FreePlay]

Very nice hack!

[Art]

How is that any different that using the pan variable to play a mono file
through both speakers?

int pan = 127;

voice = Sample_Play(sf,0,0);
Voice_SetVolume(voice, vol);
Voice_SetPanning(voice, pan);

-= Double Post =-
Personally, what I find interesting is it appears you loaded the sample from memory
rather than from a file,
care to share?

[FreePlay]

Originally Posted by Art

How is that any different that using the pan variable to play a mono file
through both speakers?

... Um... well, this is playing a stereo file through both speakers, in mono... while before it wouldn't even open a stereo file. Something like that, from what I gleaned.

[Art]

That's right, but it seems a long way round to reencoding the wav file as mono
for use in a PSP program, unless for some reason you are being forced to read stereo files.
... I'm not trying to be an ahole, I just wonder why that approach...

[dr_watson]

I'd just like to load the proper "stereo" wav files since I'm using the same wav files for both PSP and PC version of the same game. On the PC side I'm using fmod and of course it plays the wav files in real stereo.

Sure it'll be much better if someone can actually do a proper hack to enable libmikmod to play real stereo noises