ftp.nice.ch/pub/next/unix/audio/sms.N.bs.tar.gz#/sms/library/filters.c

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#include "../sms.h"

/* pre-emphasis filter function, it returns the filtered value   
 *
 * float fInput;   sound sample
 */
float PreEmphasis (float fInput)
{
	static float fLastValue = 0;
	float fOutput = 0;
  
	fOutput = fInput - EMPHASIS_COEFF * fLastValue;
	fLastValue = fOutput;
  
	return (fOutput);
}

/* de-emphasis filter function, it returns the filtered value 
 *
 * float fInput;   sound input
 */
float DeEmphasis (float fInput)
{
	static float fLastValue = 0;
	float fOutput = 0;
  
	fOutput = fInput + EMPHASIS_COEFF * fLastValue;
	fLastValue = fInput;
  
	return(fOutput);
}

These are the contents of the former NiCE NeXT User Group NeXTSTEP/OpenStep software archive, currently hosted by Netfuture.ch.