Re: Noise source processing

[David Honig <honig@sprynet.com>]

At 09:12 PM 8/5/98 -0400, mgraffam@mhv.net wrote:
>Hello all..
>
>After a little bit of work I have put together a white noise source 
>(hacked FM reciever, heavily shielded to reduce local EMR bias) and
>have it jacked into a mic port on one of my machines. I would like
>to develop of bit of code (oh, horror :) to take bits from the sound
>card, and bits from /dev/random, and mix them together to get a
>random number stream.
>
>The noise coming off of the sound card is more beige than white though..
>
>Does anyone know of any papers, articles or whatever on good techniques to
>remove bias from the audio source? 

Let me save you the trouble.  First, RFC 1750 gives some background
including the info-theory behind bit 'distillation'.

I have digitized FM radio hiss using a $10 transistor radio feeding
into the LINE in of my soundcard.  The spectrum looks poisson
with prominant, periodically spaced noise spikes.  It does not pass Diehard.

But if you take the PARITY of 8 bits to get one bit, then assemble bytes
out of these bits, the results pass Diehard.  

This works with 44Khz, 16 bit sampling, with full-volume hiss feeding the
soundcard.
I have yet to try this with a video adc sampling snow.

1. There are some nice spectrogramming shareware programs out there.
2. Diehard is invaluable.
3. Language is so vague.  Don't export this filter:


/*
Alemb.c
Alembic bit distiller
Takes 8 bytes, computes the parity of each, composes a byte from the bits.

2 Aug Ported from Alembic.java which I had prototyped earlier
3 Aug modified to be faster
*/

#include <stdio.h>
/* return parity of byte input */
int parity(int c)
{
	int i,p=0;
	for (i=0; i<8; i++) {
		p ^= c & 1;
		c >>= 1;
		}
	return p;
}



main(int argc, char **argv) {
	int c=0, p, n, i;
	int count=0, accum=0;
	int ones=0, zeroes=0;
	unsigned char parlut[256];
	unsigned char buff[8];
	FILE *fptr, *of;
	char *infname="test.wav";
	char *outfname="test.bin";
	int hist[256], max;

	if (argc>1) infname=argv[1];
	if (argc>2) outfname=argv[2];

    fprintf(stderr,"Alembic C 4 in=%s out=%s\n", infname, outfname);


	fptr = fopen(infname,  "rb");
       of   = fopen(outfname, "wb");

 	for (p=0; p<256; p++) { parlut[p]=parity(p); /*  initialize parity table */
							hist[p]=0;	}

	n=8;
	while (n==8) {

		n=fread(buff, 1,8, fptr);
		count += n;

		if (n==8) {
			for (i=0; i<8; i++) {
				p =  parlut[ buff[i] ] ;
				accum <<= 1;
				accum |= p;

				/* measure */
				if (p==1) ones++; else zeroes++;
				}
			 fwrite(&accum,1,1,of);
			 hist[accum]++;
			accum=0;

		}


	}

	printf("In: %0d bytes\n", count);
	printf("Out: %0d bytes\n", count/8);
	printf("output: 1's=%0d %0f% \n", ones, 100.0*ones/(ones+zeroes));
	printf("output: 0's=%0d %0f% \n", zeroes, 100.0*zeroes/(ones+zeroes));
	max=0;
	for (i=0; i<256; i++) {if (max<hist[i]) max=hist[i];}
	for (i=0; i<256; i++) {
		printf("%0d %0d %lf\n", i, hist[i], 100.0*hist[i]/max);
	}

}





honig@alum.mit.edu