EMRFD Message Archive 5844
Message Date From Subject 5844 2011-02-16 14:35:39 Bob An Audio Noise Generator A noise generator design effort is described in the files section folder entitled Audio Noise Generator. The purpose of this HB instrument is to evaluate filters via Spectrogram software. The folder contains circuit details and data for several zener diode and EB diode noise sources - which ultimately were not used in favor of an alternative approach. All parts are readily available and low cost.
5845 2011-02-17 04:50:19 longjohn119 Re: An Audio Noise Generator This is probably the one audio application where the crossover distortion and noise of the LM358/324 devices actually works to your advantage. The limited bandwidth probably helps 'pink' the noise a bit also. You might even find the cruddier the op amp, the better this circuit works.
You might also try something like this in the last op amp which makes it a 'pinking' (low pass) filter although since you aren't using a diode/transistor junction the noise you are generating might not be all that 'white' to begin with, probably amplified flicker noise which is 'pinkish'. You could probably enhance that circuit by using a pair of carbon resistors, the older and higher the value the better. Use a couple of 1 meg 1/2 W carbons and really generate some noise
5846 2011-02-17 06:17:15 drmail377 Re: An Audio Noise Generator Have you tried LED's, different colors?
73's David, WB4ONA
5847 2011-02-17 07:50:07 Tim Re: An Audio Noise Generator An alternative approach:
Pseudorandom sequences in a shift register, fed through resistors, are an excellent way to homebrew a broadband noise spectrum of known power density and distribution at audio frequencies.
What's very nice about this is that the noise level is not dependent on the noisiness of an analog device, but is instead predicted entirely from the pseudorandom mathematics.
It ends up being a little more complicated than your LM358 example but if for some reas
5848 2011-02-17 08:28:55 Chris Howard Re: An Audio Noise Generator Interesting that noise generation comes up on this list.
I just read an article by wa0spm in this month's "Electric Radio" magazine
about generating noise using a Magnetron tube which
he says, "When properly warmed up it passes the
(NIST) test suite every time with excellent scores."
5849 2011-02-17 10:51:36 davidpnewkirk Re: An Audio Noise Generator 5854 2011-02-18 11:54:23 Gary, WB9JPS Re: An Audio Noise Generator Or just play white noise thru your computer's audio output. There are free mp3 and wav files all over the web containing white, pink, and other noise types. Or use one of the various audio virtual instruments that generate it.
5857 2011-02-19 05:29:11 Bob Re: An Audio Noise Generator Hi Gary and Everyone -
Thanks to all for the inputs and comments. I can see where I was a bit short on some aspects in my original posting so here is some add-in and response.
What I had in mind was a uniform distribution over frequency - a "flat" spectral power for test purposes - in my mind sort of like sweeping a leveled generator over the range of interest. My understanding was that "pink" noise was not "flat" but "white" was what I needed and the application in mind was not that of a "sound system", as below. I used Spectrogram to view the various iterations of circuits / devices / output to get a "flat-looking" source.
My intended immediate application was that of looking at various CW and SSB audio filters so I felt I needed a "source" that at least covered enough bandwidth that I could see the whole top and upper skirt pretty well down. So far I've not applied my new source to any filters - that is yet to be seen.
I was aware of the psuedorandom sequence approach but I wanted to use stuff I had in the junk box - I normally don't have much digital stuff on hand.
BTW - I don't see any evidence of crossover distortion for the circuit as presented i.e. I could see no benefit to adding a source or sink DC resistive load on the last op amp output.
Sorry - I didn't think of trying LEDS - I suppose that would have been an interesting pursuit.
Since I was thinking "Spectrogram" I also briefly thought of pc generated signal but the analog approach is something I'm very comfortable doing - it's something I can actually get my hands on - and is not another tie-in to the pc with it's hum, ground loops, 'n stuff! A shielded battery operated "floating" signal source is what I wanted - and I think - achieved.
Thanks again for all the reaction and I'm still interested if you are. Hopefully later I'll have some application examples to post as well - although right now I'm not so sure the Spectrogram thing is the best way to look at filter response in detail given the not-so-good resoluti
5873 2011-02-23 02:18:33 longjohn119 Re: An Audio Noise Generator Velleman sells a pseudo-random noise generator kit for around 20 bucks
I'm sure it's the same basic circuit as the one I built 30 years ago using a CD4001, CD 4006, and a CD4070 and then run through a pinking filter. Actually mine uses two circuits clocked at slightly different frequencies and then mixed and filtered. (The circuit has an inherent glitch when the sequence restarts so using with two they cover up each other's glitch) I used it with a spectrum analyzer that was basically a gutted 1/3 octave equalizer with each channel fixed at 0 db gain and instead of summing the outputs to all the bandpass filters each one ran to 10 LED indicator circuit mounted where all the sliders used to be. It was used mainly to set club sound systems and equalize out the feedback resonances and generally flatten the room response. At that time it was around 2 grand to buy one (And that's 2 grand in early 80's recession dollars) so my options were making one myself or doing without.
Now days I do the same thing with a netbook using an MLS noise burst instead of pumping several hundred watts of noise through the system which can get nerve racking with long exposure. Still 31 rows of ten LED's each puts