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

[[email protected]]

I built a pink noise circuit using a shift register and the winfield
hill pink noise filter from 1997 but I need some way to verify the
output is pink. I have a digital storage scope (no FFT though), and I
do not have a spectrum analyzer. Is there some clever way I can test
the circuit to verify the output is pink? I have function generators
too.

 

[GregS]

I built a pink noise circuit using a shift register and the winfield
hill pink noise filter from 1997 but I need some way to verify the
output is pink. I have a digital storage scope (no FFT though), and I
do not have a spectrum analyzer. Is there some clever way I can test
the circuit to verify the output is pink? I have function generators
too.

Computer??

 

[Tim Wescott]

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 (http://www.scilab.org is
free). Ditto for reading into a sound card.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

 

[Tom Bruhns]

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

 

[Winfield Hill]

[email protected] wrote...

I built a pink noise circuit using a shift register and the winfield
hill pink noise filter from 1997 but I need some way to verify the
output is pink. I have a digital storage scope (no FFT though), and I
do not have a spectrum analyzer. Is there some clever way I can test
the circuit to verify the output is pink? I have function generators
too.

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!

 

[[email protected]]

Its not that I dont trust the filter, its the white noise generator
really, I mean, for all I know the whole circuit could be messed up, a
million things could have gone wrong and I would never have known the
output wasnt pink, you know how things can go.. I need some way to
check the whole thing. For all I know my PRBS isnt so pseudo and its
outputting black noise! Oh the humanity!

 

[Ken Smith]

Its not that I dont trust the filter, its the white noise generator
really, I mean, for all I know the whole circuit could be messed up, a
million things could have gone wrong and I would never have known the
output wasnt pink, you know how things can go.. I need some way to
check the whole thing. For all I know my PRBS isnt so pseudo and its
outputting black noise! Oh the humanity!

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.

 

[Rich Grise]

Its not that I dont trust the filter, its the white noise generator
really, I mean, for all I know the whole circuit could be messed up, a
million things could have gone wrong and I would never have known the
output wasnt pink, you know how things can go.. I need some way to check
the whole thing. For all I know my PRBS isnt so pseudo and its outputting
black noise! Oh the humanity!

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

Cheers!
Rich

 

[GregS]

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

 

[Tim Shoppa]

Its not that I dont trust the filter, its the white noise generator
really, I mean, for all I know the whole circuit could be messed up, a
million things could have gone wrong and I would never have known the
output wasnt pink, you know how things can go.. I need some way to
check the whole thing. For all I know my PRBS isnt so pseudo and its
outputting black noise! Oh the humanity!

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.

 

[Rich Grise]

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.

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

 

[John Fields]

The point was, a newborn baby can tell the difference between "Ah" and
"Ooh"

 

[Rich Grise]

You know that, how???

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.

- but getting a computer to do it is a different order of problem.

You know that, how???

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

Thanks!
Rich

 

[Rich Grise]

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!

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

 

[The Phantom]

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?

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.

 

[Keith]

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?)

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

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. ;-)

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

Oh, and by the way - it was nowheres near pink noise -

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

it was more like somebody playing the grinder. ;-)

Perhaps the frequency was too low?

 

[Ken Smith]

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 ....)

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.

 

[Tim Shoppa]

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.

 

[Rich Grise]

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

 

[Ken Smith]

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?

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.