[SDL] Audio Output

[Paul Newton]

I have  had a look at SDL_sound.  I do need the decoded waveform.  I am
still lost though so I would really appreciate some pointers.  In the code
that I had instead of outputting the pcm data to std out using

        	fwrite(convbuffer,2*vi.channels,bout,outputFile);

I want to output this pcm data to the soundcard.

My understanding is that the pcm data is contained in convbuffer.  This is
declared as an array of 4096 elements.  I have created a 2D array to act as
a buffer.  This is declared as 

		ogg_int16_t audioBuffer[BUFFERSIZE][4096];

BUFFERSIZE is #define BUFFERSIZE 20

I have setup a SDL_AudioSpec structure

		SDL_AudioSpec fmt;    //SDL structure definition

For the moment I have set this up as follows

/************************SDL Play**********************************/
    /* Set 16-bit stereo audio at 44Khz */
    fmt.freq = 44100;
    fmt.format = AUDIO_S16SYS;
    fmt.channels = 2;
    fmt.samples = 4096;        
    fmt.callback = fill_audio;
    fmt.userdata = NULL;


	//audio_chunk = &convbuffer;
    /* Open the audio device and start playing sound! */
    if ( SDL_OpenAudio(&fmt, NULL) < 0 ) {
        fprintf(stderr, "Unable to open audio: %s\n", SDL_GetError());
        //exit(1);
    }
        
/************************SDL Play**********************************/

I have two variable acting as pointers on my buffer these are

		int audioPointer=0;

		int dataPointer=BUFFERSIZE/2;

In the decode loop I have th following condition to see whether or not to
decode the next part of the file

		if(
               (dataPointer >= BUFFERSIZE/2 && audioPointer < BUFFERSIZE/2)
||
		   (dataPointer < BUFFERSIZE/2 && audioPointer >=
BUFFERSIZE/2)
      	  )

I put this in because I found that the buffer was being filled with data too
quickly and that by the time that the audio got read the data had been
overwritten a few times.  This caused the file to produce high frequency
noise which didn't last anywhere near the time of the ogg file.

Instead of writing the pcm data to stdout I do this

/****************Play Audio*************************************/


 memcpy(audioBuffer[dataPointer], convbuffer, 1024/*sizeof(convbuffer)*/);


 dataPointer = (dataPointer+1)%BUFFERSIZE;
 fprintf(stderr, "Data\n");
 fprintf(stderr, "Frame %ld\n", (long)(vd.sequence));
 //SDL_PauseAudio(0);

/****************Play Audio*************************************/

I have the SDL_PauseAudio(0); at the end of my if condition which contains
the whole of the looped decode part.


My callback function is declared as

/*******************callback function*******************************/

/* The audio function callback takes the following parameters:
    stream:  A pointer to the audio buffer to be filled
    len:     The length (in bytes) of the audio buffer
*/


void fill_audio(void *udata, Uint8 *stream, int len)
{

  memcpy(stream, audioBuffer[audioPointer], len);
  fprintf(stderr, "Audio\n");
  audioPointer = (audioPointer+1)%BUFFERSIZE;
}
/*******************callback function*******************************/


The sound output now appaers to go too slowly and is not recognisable as the
ogg file.

Any help would be really appreciated.

I have included the code as an attachment

The ogg file I am using is 841kb in size so if anybody wants that I can send
that to them

Thanks in advance,
Paul






-----Original Message-----
From: Ryan C. Gordon [mailto:icculus at clutteredmind.org]
Sent: 22 August 2002 10:29
To: sdl at libsdl.org
Subject: Re: [SDL] Audio Output



> The goal is simply to get sound output.  The project is basically to
develop
> a GUI for this Vorbis.  One of my colleagues is trasferring part of the
> Vorbis algorithm to hardwaer and we intend to run that with the hardware
as
> well.  First of all though I need to get th e GUI working with the
existing
> software.  My plan was just to use the decoder_example code when the user
> hit play.  If there is a simper way to get audio output then I would like
to
> know about it.  If not then I need a solution as to why what I had was not
> correctly outputting sound.

If you need the decoded audio waveform, look at SDL_sound:

     http://icculus.org/SDL_sound/

And the included playsound program that dumps a decoded Ogg file to an
SDL audio callback.

If you don't need the decoded waveform, and only want to play one Ogg at a
time, SDL_mixer makes this a ten-line-or-less program.  :)

--ryan.



_______________________________________________
SDL mailing list
SDL at libsdl.org
http://www.libsdl.org/mailman/listinfo/sdl

-------------- next part --------------
A non-text attachment was scrubbed...
Name: decoder_example.c
Type: application/octet-stream
Size: 13133 bytes
Desc: not available
URL: <http://lists.libsdl.org/pipermail/sdl-libsdl.org/attachments/20020822/2199b138/attachment.obj>