how to tell if source is pink noise, no spectrum or fft

Computer??

Like, if your DSO will save to floppy or let you read stuff of of ethernet you could do an FFT in a math package

Same suggestion as others: hose it into your 'puter's sound card and record some of it. There's freeware and trialware that will analyze it for you with a nice GUI, and you can import it into the free Scilab program to do an FFT on it. Suggest you first check the white to make sure it's flat. By ratioing the pink filter output to its input you can get the shape of the pink filter independent of the non-flatness of your source+sound card, if the sound card channels are matched...and you can check their match by looking at the same signal on both.

Cheers, Tom

What, you don't trust me??? OK, there is a simple thing you can do with your scope and function generator: measure the plain old frequency response of the filter and plot it!

Actually, it is more likely that it is outputting blue or green noise. If the odds of the bit toggling are much greater than 50%, there won't be enough low frequency component to the noise.

You really want to bring in a few complete cycles of the psudo-random sequence for your FFT process. You want to do FFTs of various lengths of time. If you are using a digital scope, run it at different sample rates. If you get desparate for a method to FFT, let me know.

I'm also suggest that you listen to the noise. There are many things that are hard to find in processing that really jump out at you if you listen to it.

On Fri, 24 Mar 2006 00:27:58 -0800, acannell top-posted:

Run it through a speaker and see if it _sounds_ pink. ;-)

Cheers! Rich

Right. If one has another generator, do an ABX. By simple listening, the amount of difference should be easily noticed, allthough comming up with a spec would be difficult. O course you could use the computer as the ear.

greg

Since you built the pseudorandom shift/XOR chain, so you know the period. Verify that it has that period and no shorter period. (Details will vary as to whether you did it in software or hardware or ....)

Now if you built a generator with a chain so long that the sun will turn into a cold ball of lead before it repeats, then you've got some testability issues!

Tim.

I'd argue with this a little - one time I built a little active filter thingie - 8 active filters spaced "geometrically" from about 300 Hz up to about 3 KHz, to pick up the timbre of voice sounds - it's amazing to see the spectrum of, for example, the long 'A' sound, like fate, or late; the 'ah' sound, like in father; the 'O' sound, like in boat - and it was as I suspected, the pattern is the same - the instantaneous spectrum, when you filter out the fundamental, is identical from person to person, but different from phoneme to phoneme. Even people who have very small nasal cavities and sinuses and pharynxes show the same pattern in the spectral content of the harmonics. I had not, and still haven't, learned enough about pattern recognition to go any further with it - the intent was a real-time speech input device. FWIW, I lifted the 8 filters directly from Don's Active Filter Cookbook. :-)

The point was, a newborn baby can tell the difference between "Ah" and "Ooh" - but getting a computer to do it is a different order of problem. :-)

Cheers! Rich

Well, you asked.

I saw a thing on Edjamacational TeeVee where they trained newborns, much like Pavlov's dog, to respond to certain vowel sounds. They had this baby strapped in some kind of chair, with the camera recording its responses, and they'd play recordings of various people making various sounds, like "Ah" or "Ee" or "Oh" or "OOh", and on certain sounds, they'd light up a toy - actually, one of those animated drumming monkey things - which would catch the infant's interest. So, it got trained, such that when it heard certain phonemes, it would anticipate the entertaining toy, just like Pavlov's dogs would salivate. (this was also a verrrrrry interesting demo of how easy it is to train humans), and for a certain phoneme, or vowel, it didn't matter who spoke the sound. Man, Woman, Child, Bass, Baritone, Tenor, Contralto, Alto, Mezzo-Soprano, Soprano - it didn't matter. They responded to the vowel (or phoneme) itself.

Of course, this was merely on edjamacational teevee, so it could have been as real as the Roswell Alien Autopsies, for all I know.

By trying it - how do _you_ learn stuff? ;-)

Thanks! Rich

Speaking of pseudorandom sequences - sorry for the thread drift - I've used some of Don's designs, and wonder how they come up with "which bits to XOR and shift in" for "maximal length". Trial and error? Is there any kind of "formula?" Does anybody know? (in Don's book, he had "maximal length" circuits up to, I think, 17 bits, but I can't help but wonder, is there some kind of algorithm for, say, 32 bits, or 64, or, well, N?)

I did a 16- or 24-bit one, and played it through a speaker, and I actually heard its periodicity. And even slapping a DAC on any given 8 bits, and playing the resulting analog, I still heard repeating patterns. Maybe I just have really sensitive ears. ;-)

Oh, and by the way - it was nowheres near pink noise - it was more like somebody playing the grinder. ;-)

Thanks! Rich

Go get the book out of the library, "Shift Register Sequences", Solomon W. Golomb, and read chapter V, "On the Factorization of Trinomials Over GF(2)", and you will understand all.

Try searching the Xilinx application notes for xapp052.pdf. The app note lists the taps needed for maximal length LFSRs up to 168bits.

Note that the bits are not (pseudo) random WRT each other. They're simply delayed.

An LFSR should be more white over its spectrum (it's supposed to be pseudo-random), not pink.

Perhaps the frequency was too low?

I don't think that that proves that the chain is making white noise does it. Think about a real bad 3 bit "psudo random" sequence like this:

A B C 0 0 0 1 0 0 0 1 0 0 0 1 1 0 1 0 1 1 1 1 1

If you took the A as the output, it would be sort of white. B would be less so and C would be very bad.

True, but I wasn't suggesting that he do a seat-of-the-pants pseudorandom sequence. I thought he had already chosen one with known good properties and that he wanted to verify that he had implemented it correctly.

For the traditional shift/XOR chain (feedback into one end, not back into the middle), isn't it true that it doesn't matter which bit in the chain you look at, as they are all the same sequence just delayed in phase slightly?

Tim.

I once did a 16-bit one in TTL (or probably LSTTL - I'm not _THAT_ old!), with 16 LEDs, clocked at a human-perceptible rate [ ;-) ], and it was like watching a shift register, with new bits, yes, but watching them march down that line of LEDs made me think, "There's something not very random about this...."

In another post I had mentioned that I had hung a speaker on one of the bits, and I could hear the periodicity. But it _was_ only 16 bits. :-)

Thanks! Rich

In article , Tim Shoppa wrote: [....]

Yes all the ones in the body of the shift register have exactly the same pattern on them. There may be one extra flip-flop in the circuit that jumps it out of the "lock up" state.