How to Use the Game-Music-Emu Library to Play NSF Files in Mono?

[Thysbelon]

I admire your project. I wanted to use your code to embed a small "play NSF chiptune" button on my blog. I have successfully compiled your fork of game_music_emu with emscripten,
emcmake cmake ../ -DUSE_GME_AY=0 -DUSE_GME_GBS=0 -DUSE_GME_GYM=0 -DUSE_GME_HES=0 -DUSE_GME_KSS=0 -DUSE_GME_SAP=0 -DUSE_GME_SPC=0 -DUSE_GME_VGM=0 -DENABLE_UBSAN=OFF -DBUILD_SHARED_LIBS=OFF (After editing your CMakeList to include ym2413, and editing gme.cpp to comment out all emu types except NSF and NSFe)
and have written a few c and javascript helper functions.
I want to be able to pass true or false into my javascript helper function to render the song in stereo or mono. I have no issue playing back NSF music in stereo, but I cannot get it to switch to mono. The gme_set_effects function described in gme.h looks like it should play the song in mono if I pass NULL into it, but it has no effect.

Here is my c code:

#include <stdlib.h>
#include <stdio.h>

#include <gme.h>
#include <emscripten/webaudio.h>

#include <math.h> //for sin

//uint8_t audioThreadStack[4096];
gme_t* emu;
void* nsfdata;
long nsfdataSize=0;
int track;
EM_BOOL stereopref;
int limitlogs=0;
int buf_size;

short* buf;

void handle_error( const char* str );

static void play_siren( long count, short* out )
	{
		static double a, a2;
		while ( count-- )
			*out++ = 0x2000 * sin( a += .1 + .05*sin( a2+=.00005 ) );
	}

EMSCRIPTEN_KEEPALIVE
int setupNSF(void* inputdata, long inputdataSize, int inputBufSize, int tracknum, EM_BOOL stereoInput)
{
	printf("setupNSF\n");
	
	nsfdata = inputdata;
	
	nsfdataSize = inputdataSize;
	
	track = tracknum;
	
	stereopref = stereoInput;
	
	buf_size = inputBufSize;
	
	int sample_rate = 44100;
	
	/* Open music file in new emulator */
	handle_error( gme_open_data( nsfdata, nsfdataSize, &emu, sample_rate ) );
	if (stereopref == EM_TRUE) {
		printf("STEREOPREF IS TRUE\n");
		struct gme_effects_t effects;
		effects.enabled = 1;
		effects.stereo = 0.5;
		effects.echo = 0.0;
		gme_set_effects( emu, &effects );
	} else {
		printf("STEREOPREF IS FALSE\n");
		gme_set_effects( emu, NULL ); // this does not work
		//struct gme_effects_t effects;
		//effects.enabled = 0;
		//effects.stereo = 0.0;
		//gme_set_effects( emu, &effects ); //these also do not work
	}

	// I have tried putting open_data here, it made no difference.
	
	printf("nsfdataSize: %li \n", nsfdataSize);
	
	/* Start track */
	handle_error( gme_start_track( emu, track ) );
	
	buf = malloc(sizeof(*buf) * buf_size); // https://stackoverflow.com/questions/4240331/c-initializing-a-global-array-in-a-function
	printf("array buf entry number 3: %d\n", buf[3]);
	
	printf("setupNSF done\n");
	return 0;
}

EMSCRIPTEN_KEEPALIVE
short* playNSF() { // run in script processor node's on audio process event
	handle_error( gme_play( emu, buf_size*2, buf ) );
	//play_siren(buf_size, buf);
	
	// the c code cannot return an array to javascript, use runtime method getValue to access the buf array.
	return buf;
}

void handle_error( const char* str )
{
	if ( str )
	{
		printf( "Error: %s\n", str ); //getchar();
		//exit( EXIT_FAILURE );
	}
}

Here is my Javascript:

document.getElementById("nsfplaybut").addEventListener("click",myclick,{once:true})

function myclick(){
	nsfplay("Akumajou Densetsu (VRC6).nsfe",0,false);
}

const INT16_MAX = 65535;
async function nsfplay(url, tracknum, stereopref) {
	var response = await fetch(url)
	
	const audioCtx=new AudioContext({latencyHint:"playback",sampleRate:44100});
	
	const bufferSize = Math.max( // Make sure script node bufferSize is at least baseLatency
		Math.pow(2, Math.ceil(Math.log2((audioCtx.baseLatency || 0.001) * audioCtx.sampleRate))), 2048);
	console.log(bufferSize);
	
	var audioNode=audioCtx.createScriptProcessor(bufferSize,2,2)
	audioNode.connect(audioCtx.destination)
	
	response = await response.arrayBuffer()
	var data=new Uint8Array(response)
	Module.ccall(
		"setupNSF",
		"number",
		["array", "number", "number", "number", "boolean"],
		[data, data.length, bufferSize, tracknum, stereopref]
	)
	
	var playNSF=Module.cwrap("playNSF", "number", null)
	audioNode.onaudioprocess=function(e){
		let bufPtr=playNSF()
		let outputData=[];
		for(let channel=0; channel<e.outputBuffer.numberOfChannels; channel++){
			outputData[channel]=e.outputBuffer.getChannelData(channel); /*nested loop*/
			for(let i=0; i<bufferSize; i++){
				outputData[channel][i] = Module.getValue(bufPtr+i  * 2  * 2 + /* frame offset * bytes per sample * num channels + */ channel  * 2  /* channel offset * bytes per sample */, 'i16') / INT16_MAX /* convert int16 to float */
			}
		} 
	}
}

Thank you.

[mmontag]

Good question; before digging into your code, I'll just mention that I did a quick and dirty modification to game-music-emu to output the NES sound in stereo: 839b9c2

Simply put: the two 2A03 square channels are hard panned Left and Right.

That should be easy to undo, but if you want to make the stereo/mono switchable at runtime, you would have to do some further modifications.

You could always do a mixdown in Javascript to keep things simple. (Add both NSF channels to both output channels. Or create a mono Audio Node?)

[Thysbelon]

Thank you, that helps a lot!