The New Hotness

A reader (you know who you are!) proposed an interesting project that will involve measuring audio passbands and suggested using white noise to show the entire shape on a spectrum analyzer. He pointed me at the NOISE 1B Noise Generator based on a PIC microcontroller, which led to trying out the same idea on an Arduino.

The first pass used the low bit from the Arduino runtime’s built-in random() function:

Well, that’s a tad pokey for audio: 54 μs/bit = 18.5 kHz. Turns out they use an algorithm based on multiplication and division to produce nice-looking numbers, but doing that to 32 bit quantities takes quite a while on an 8 bit microcontroller teleported from the mid 1990s.

The general idea is to send a bit from the end of a linear feedback shift register to an output to produce a randomly switching binary signal. Because successive values involve only shifts and XORs, it should trundle along at a pretty good clip and, indeed, it does:

I used the Galois optimization, rather than a traditional LFSR, because I only need one random bit and don’t care about the actual sequence of values. In round numbers, it spits out bits an order of magnitude faster at 6 μs/bit = 160 kHz.

For lack of anything smarter, I picked the first set of coefficients from the list of 32 bit maximal-length values at https://users.ece.cmu.edu/~koopman/lfsr/index.html:
0x80000057.

The spectrum looks pretty good, particularly if you’re only interested in the audio range way over on the left side:

It’s down 3 dB at 76 kHz, about half the 160 kHz bit flipping pace.

If you were fussy, you’d turn off the 1 ms timer interrupt to remove a slight jitter in the output.

It’s built with an old Arduino Pro Mini wired up to a counterfeit FTDI USB converter. Maybe this is the best thing I can do with it: put it in a box with a few audio filters for various noise colors and be done with it.

It occurs to me I could fire it into the 60 kHz preamp’s snout to measure the response over a fairly broad range while I’m waiting for better RF reception across the continent.

The Arduino source code as a GitHub Gist:

	// Quick test for random bit generation timing
	// Ed Nisley KE4ZNU – 2017-10-25
	// Observe output bit on an oscilloscope

	// LFSR info https://en.wikipedia.org/wiki/Linear-feedback_shift_register
	// This code uses the Galois implementation

	// Coefficients from https://users.ece.cmu.edu/~koopman/lfsr/index.html

	#define PIN_RND 13

	#include <Entropy.h>

	uint32_t Rnd;
	byte LowBit;

	void setup() {

	Serial.begin(57600);
	Serial.println("Random bit timing");
	Serial.println("Ed Nisley KE4ZNU – 2017-10-25");

	Entropy.initialize();

	pinMode(PIN_RND,OUTPUT);

	uint32_t Seed = Entropy.random();

	Serial.print("Seed: ");
	Serial.println(Seed,HEX);

	randomSeed(Seed);

	do {
	Rnd = random();
	} while (!Rnd); // get nonzero initial value

	}

	void loop() {

	// digitalWrite(PIN_RND,Rnd & 1); // about 55 us/bit
	// Rnd = random();

	LowBit = Rnd & 1;
	digitalWrite(PIN_RND,LowBit); // about 6 us/bit

	Rnd >>= 1;
	Rnd ^= LowBit ? 0x80000057ul : 0ul;

	}

view raw Random_Time.ino hosted with ❤ by GitHub

The New Hotness

Arduino Pseudo-Random White Noise Source