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List:       nas
Subject:    Playing only parts of a file
From:       Eric Wehr <eswehr () csti ! nosc ! mil>
Date:       1996-04-03 17:21:08
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> And even if I would, I
>couldn't figure out, how the for the starting to the ending time are
>computed if you have some time values for start and end.

Since I've only worked with the low level functions, that's the only place I 
can help.  My application has called for reading out file data and sending 
it to a remote D/A.  I simply search into the file so many bytes to get to 
the point I want.  The following two functions do all of my conversion. 
 Minor modification will be needed to make them more generic.

/* Using 16 bit signed linear at 8kHz */
#define SAMPLE_RATE 8000  /* 8k samples per second */
#define SAMPLE_SIZE 2  /* 2 bytes per sample */

/********************************************************************/
/* Function:     long int SecondsToBytes(int seconds);
/* Description:  Converts a time given in seconds to a number of bytes
/*         of audio data based on #define values
/********************************************************************/
long int SecondsToBytes(int seconds)
{
  long int RetVal;

  RetVal = (long int) seconds * (long int) SAMPLE_RATE * (long int) 
SAMPLE_SIZE;
  return( RetVal );
}

/********************************************************************/
/* Function:     int BytesToSeconds(long int bytes);
/* Description:  Converts a number of bytes of audio data to a time
/*         in seconds based on #define values
/********************************************************************/
int BytesToSeconds(long int bytes)
{
  long int RetVal;

  RetVal = bytes / (long int) SAMPLE_RATE / (long int) SAMPLE_SIZE;
  return( (int) RetVal );
}

Hope this helps if you need to go to the low level API calls.

Eric

Eric S. Wehr                       | Hey folks, this is
Communication Systems Technology, Inc.       | me, not CSTI, not
Charleston, SC Office:                  | the Navy, not
     Naval Weapons Station South Annex  | _anyone_ else!

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