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Squashing Shot Noise - help please

by MaximumPower
Tags: audio, filtering, hiss, noise, shot, shot noise, squashing
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jim hardy
#19
Dec30-12, 09:18 AM
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i may be wayyyy off track here

but had a thought...


Magnetic recording tape employs "Bias", a hf AC signal added to the signal to overcome tape's hysteresis and hiss. The technique was one of the War Prizes brought back from Germany at end of WW2.

Biasing is critical to this process of imposing the design of the symphony upon a medium which would tend to have random variations because of thermal energy and a kind of "inertia" in the form of hysteresis that resists the production of an undistorted image of the music
http://hyperphysics.phy-astr.gsu.edu...udio/bias.html

I'd tinker with the excitation current to the optocoupler to see if it helps. Perhaps add some ~40khz AC ? That you could do with existing circuit board....

Again - it's just a crazy idea at this point.
But i meant well.

old jim
sophiecentaur
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Dec30-12, 09:51 AM
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Jim
Afaik, the bias just reduces hysteresis distortion which, in itself gives a gritty, programme related noise like the quantising 'noise' in ADCs, which is a distortion rather than random noise, really. Using a high bias frequency is a bit like extreme oversampling, which spreads the quantising noise over a wide bandwidth, mostly outside the audible range. The Recording Audio pre and de emphasis curves do a lot to help the basic SNR too because of the spectrum of random tape noise.
Mind you, Tape = Ghastly reproduction. It's a wonder we got away with it for so long.

I thought some more about a capacitative pick up and realised that the dreaded spit could ruin it with the small clearances needed for getting any sensible values of C. But I guess that would also be a problem with any optics relying on clean reflections.
marcusl
#21
Dec30-12, 12:49 PM
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Just to reiterate sophiecentaur's insights: Your problem stems from having virtually no signal because of the very small angular motion of the reed and the nearly normal incidence angle that you chose for your light and sensor. It is no surprise that you see mostly noise.

It is always true in signal processing and instrumentation that 1 ounce of effort put into making the apparatus produce a better signal to noise ratio (SNR) is worth pounds of effort spent flailing about with modulations, filtration, adaptive noise cancellation, or whatever else you can come up with to put after your transducer.

Regarding some of the approaches currently under discussion: Dolby NR relies on the fact that tape noise has a colored spectrum. As a result, the audio signal can be predistorted, then signal+noise is filtered later. a) You think your noise is shot noise (a reasonable guess), which is white and thus not amenable to the Dolby approach, and b) you do not have the ability to predistort your signal, as already pointed out. As a result, this doesn't sound to me like a promising approach. Modulation (chopping) works to overcome 1/f noise, and is typically used in systems that operate at DC to maybe a few Hz where 1/f noise is problematic. You operate, instead, up to many kHz. Have you measured your noise power spectral density? Do you have reason to believe that your photo sensor 1/f noise predominates over the white noise? If it doesn't, then modulation won't help.

Before investing in partially thought-through efforts to "clean up" your almost nonexistent signal, I suggest that you spend some time now to rework your front end to boost that signal. At the very least, change the optical arrangement to be more sensitive to minute angular changes, as sophiecentaur suggested. Better yet, change to a capacitive pickup which, unlike your opto sensor, is directly sensitive to changes in distance between the pickup and reed. Changing to steel reeds and using magnetic guitar pickups might be another way to go. Or follow the example of Charlie Musselwhite and countless other blues musicians who hold a microphone to their harps. Just put a tiny microphone (electret or other type) into the harp case. You know this will work well!
In any case, maximize SNR first, and you're sure to have a better outcome.
jim hardy
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Dec30-12, 01:00 PM
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You're probably quite right, S C.

Been trying to figure out how to modulate a carrier to get around that noisy optocoupler.
But that seems like trying to correct a bad microphone by using Armstrong's FM technique, which corrects noise in the atmosphere not in the mike.
My thought was this: if the signal were made into amplitude modulated carrier and diode detected exactly as in AM radio - what would it sound like? Could implement AGC to hadle that nonlnear opto curve.
But - enough idle rambling from me.

I'll watch this one with interest.


old jim
sophiecentaur
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Dec30-12, 03:05 PM
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Quote Quote by marcusl View Post
Just to reiterate sophiecentaur's insights: Your problem stems from having virtually no signal because of the very small angular motion of the reed and the nearly normal incidence angle that you chose for your light and sensor. It is no surprise that you see mostly noise.

... .. .

. . . . .

Before investing in partially thought-through efforts to "clean up" your almost nonexistent signal, I suggest that you spend some time now to rework your front end to boost the minute angular changes, as sophiecentaur suggested. Better yet, change to a capacitive pickup which, unlike your opto sensor, is directly sensitive to changes in distance between the pickup and reed. Changing to steel reeds and using magnetic guitar pickups might be another way to go. Or follow the example of Charlie Musselwhite and countless other blues musicians who hold a microphone to their harps. Just put a tiny microphone (electret or other type) into the harp case. You know this will work well!
In any case, maximize SNR first, and you're sure to have a better outcome.
It's good that we've at last got some other contributors to the thread. This really does need to be kicked around by several minds if we are to come up with an optimum solution.

Yes- a serious handicap.

They would need to be Stainless to resist the dreaded spit factor and many SS alloys are non magnetic. Also I believe 'they' like bronze reeds because of the timbre of the note. But spit would also affect the capacitance and even short out the reed - sensor gap.

I do think optical is the way to go but that initial off the shelf solution is hardly ideal. How about a fibre optic supply from a central LED source and then light diodes placed to give a very oblique incidence on the reeds to magnify the effect of deflection? However it's done, there will be significant non linearity.

A good reason for not using a microphone could be the wind noise / turbulence inside the body of the harp.
sophiecentaur
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Dec30-12, 03:07 PM
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Quote Quote by jim hardy View Post
You're probably quite right, S C.

Been trying to figure out how to modulate a carrier to get around that noisy optocoupler.
But that seems like trying to correct a bad microphone by using Armstrong's FM technique, which corrects noise in the atmosphere not in the mike.
My thought was this: if the signal were made into amplitude modulated carrier and diode detected exactly as in AM radio - what would it sound like? Could implement AGC to hadle that nonlnear opto curve.
But - enough idle rambling from me.

I'll watch this one with interest.


old jim
Jim
I think synchronous detection would be better than a diode detector as the SNR would be better. You would already have your local oscillator at the right frequency.
jim hardy
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Dec30-12, 04:17 PM
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Thanks Sophie for the kind word. I am just interested, never did this so can't add much.

here's a scholarly paper by some folks who did measurements on harmonica reeds using proximity sensors in a lab setting.
Around pages 6-7 might be some data on displacement and frequency measurements that could be of interest to OP. Apparently there's more interaction between blow and draw reeds than one would expect.
http://www.www.harpinanawhinin.com/h...es_article.pdf

Their sensor was a mite big - i think 4mm,, ~KD-2400, Kaman Instruments Corp
http://www.intertechnology.com/Kaman/pdfs/KD2440.pdf

old jim
marcusl
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Dec30-12, 10:52 PM
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Quote Quote by sophiecentaur View Post

They would need to be Stainless to resist the dreaded spit factor and many SS alloys are non magnetic.
400 series stainless steels (among others) are magnetic.
sophiecentaur
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Dec31-12, 03:34 AM
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Quote Quote by marcusl View Post
400 series stainless steels (among others) are magnetic.
True but I'm not sure the reeds would be massive enough to produce a big enough signal. Guitar pickups have only a low output power with strings which are hundreds of times more massive. In any case, would you want to produce a replacement set of reeds with DIY tools?
sophiecentaur
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Jan2-13, 04:40 AM
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Quote Quote by jim hardy View Post
Thanks Sophie for the kind word. I am just interested, never did this so can't add much.

here's a scholarly paper by some folks who did measurements on harmonica reeds using proximity sensors in a lab setting.
Around pages 6-7 might be some data on displacement and frequency measurements that could be of interest to OP. Apparently there's more interaction between blow and draw reeds than one would expect.
http://www.www.harpinanawhinin.com/h...es_article.pdf

Their sensor was a mite big - i think 4mm,, ~KD-2400, Kaman Instruments Corp
http://www.intertechnology.com/Kaman/pdfs/KD2440.pdf

old jim
Hey Jim. That article was really interesting. I can manage to bend harmonica notes a bit and always wondered how it worked. I totally ignored the presence of the other reed in my attempted explanation. It always seems to be much easier on the 'suck' than the blow and the angle of the harp needs to be just right. I'll dig my harp out and see if the explanation helps with my technique (last practiced seriously when my 28 year old daughter was about 10).

btw, not only is that sensor a bit big but to fit two octave's worth of reeds out with them would be a bit pricy!
johnbbahm
#29
Jan2-13, 02:54 PM
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I am not sure about the reflective characteristic of brass, but the 960 NM would
reflect great with aluminum.
Maybe you could sputter the reeds with a few microns of aluminum to improve the
IR reflectivity.
Also, what is the frequency nature of your hiss. can you use a scope to isolate the source?
Based on the size, I am assuming this is battery powered(cool Idea by the way).
The battery rules out power supply hiss.
jim hardy
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Jan2-13, 03:47 PM
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I'm as bad as Detective Columbo - 'just one more thing....'


MikeinPlano brought up the subject of amp induced noise.
The LM833 looks like a quiet amp but i wonder about the resistors surrounding it? Carbon is notoriously noisy i am told....

Mike - Just how noisy are phototransistors? Are diodes quieter?
This fellow observed noise differences due to manufacturing technique. He used a low noise preamp, unfortunately it was of vacuum tube type.
http://ufdcimages.uflib.ufl.edu/UF/0...00delarich.pdf

if irrelevant - please advise and i'll delete.

PS Sophie - glad the harmonica article was of intrest . Thanks !

old jim
AlephZero
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Jan2-13, 07:45 PM
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See http://www.harmsol.co.uk/ for how somebody else solved the problem (i.e. using a mcrophone!)

If you change the material of the reeds you are effectively making a new instrument from scratch. Even if you could get stainless steel reeds ready made, you would have to tune the instrument after you rebuilt it, and that isn't a trivial job!

Capacitance probes would probably work well. They have a frequency response up to 15 kHz which isn't quite "hi-fi audio" but might be near enough. The only problem is they would probably be too big to fit inside the case (but a line of cylindrical probes stickng out of the case might look "interesting"). The output can be as high as 10v/mm of movement, which should give you plenty of signal.

I would have thought you could fix any "wind noise" problems with a microphone easily enough by using a bit of acoustic wadding (the stuff that is used to fill loudspeaker cabinets to damp out resonances) as a windshield.

If you really want an optical solution, you could look at how optical computer mice work. The cost of the electronics in the cheapest ones must only be a dollar or two.
johnbbahm
#32
Jan3-13, 06:49 AM
P: 143
I was looking at your picture again, maybe your noise is splash from the other
sources in the case.
How about a coat of black ink (flat) on all the brass surfaces except the reeds.
This would make the only reflective part, and the only moving part the same.
It also might help to look at this with a IR viewer (most digital cameras
can see some of the IR), and see how much IR is bouncing around inside your case.
sophiecentaur
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Jan3-13, 07:16 AM
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Quote Quote by AlephZero View Post
See http://www.harmsol.co.uk/ for how somebody else solved the problem (i.e. using a mcrophone!)

If you change the material of the reeds you are effectively making a new instrument from scratch. Even if you could get stainless steel reeds ready made, you would have to tune the instrument after you rebuilt it, and that isn't a trivial job!

Capacitance probes would probably work well. They have a frequency response up to 15 kHz which isn't quite "hi-fi audio" but might be near enough. The only problem is they would probably be too big to fit inside the case (but a line of cylindrical probes stickng out of the case might look "interesting"). The output can be as high as 10v/mm of movement, which should give you plenty of signal.

I would have thought you could fix any "wind noise" problems with a microphone easily enough by using a bit of acoustic wadding (the stuff that is used to fill loudspeaker cabinets to damp out resonances) as a windshield.

If you really want an optical solution, you could look at how optical computer mice work. The cost of the electronics in the cheapest ones must only be a dollar or two.
The Capacitance Probes I can find all seem very bulky and they are supplied in their own case. I don't see why the frequency response is fundamentally limited - they just look for changes in RF signal level as the value of capacitance changes with movement. The commercial units are probably LP filtered for optimum noise performance. The problem with the capacitance method is that the value of capacitance of a reed against an earth plane would be less than 0.01pF and the variation due to vibration would be even less. Also, no one has mentioned the linearity of any of these transducers which would affect the timbre of each note.

Your reference to optical mouse technology is interesting and it made me think about interferometry as a possible way into the problem.

@johnbbahm: I do like the idea of blacking the insides and dying the reed surface to improve wanted signal levels.
MaximumPower
#34
Jan11-13, 02:12 PM
P: 10
EVERYONE,

Sincere THANKS for your continued discussion on this topic. I am embarrassed to say that I didnt notice the thread continued beyond Page 1. So I apologize for the radio silence.

Just to catch up, please allow me to respond en masse to your questions and comments:

====


Boosting SNR at the Source
Marcus: It is always true in signal processing and instrumentation that 1 ounce of effort put into making the apparatus produce a better signal to noise ratio (SNR) is worth pounds of effort spent flailing about with modulations, filtration, adaptive noise cancellation, or whatever else you can come up with to put after your transducer.

Jim: Truer words have not been spoken. Oh how I WISh I could eliminate the noise at the source! Hopeful that the illumination was the culprit, I’ve tried a variety of power sources and filters in the LED driver circuit. I also learned that LED’s are sometimes used explicitly for the purpose of *generating* white noise. But alas, I have found that even a dc-powered incandescent lamp produces the same amount of SNR.

Improvement of IR Reflectivity:
JohnBBahm: How about a coat of black ink (flat) on all the brass surfaces except the reeds. This would make the only reflective part, and the only moving part the same.
….I am not sure about the reflective characteristic of brass, but the 960 NM would reflect great with aluminum. Maybe you could sputter the reeds with a few microns of aluminum to improve the IR reflectivity.

SC: I do like the idea of blacking the insides and dying the reed surface to improve wanted signal levels.

Jim: I’ve experimented with coloring the reed with both a black sharpee marker, and white-out. (The sensor datasheet uses both a reflective aluminum target and a white piece of paper.) The effect is negligible. Yet the signal intensity itself is surprisingly good: on the order of 50mV without any amplification – with just a pull up resistor.

IR Viewer:
JohnBBahm: It also might help to look at this with a IR viewer (most digital cameras
can see some of the IR), and see how much IR is bouncing around inside your case.

Jim: I just discovered this handy tip recently. Its actually really cool. (Or hot, as the case may be.)

Dolby NR… not promising.
Jim: My original reference to “Dolby” was for loss of a better word. Now that I know what Dolby actually does, I think the term I’m searching for is dynamic noise reduction (DNR.) That said, I found several Dolby IC’s that do not require the recording to be encoded. I’ve been tempted to try them. But for the time being, I’m tinkering with the LM1894 which is a kind-of signal-intensity-controlled high-order-low-pass-filter.

Chopping
SC: Modulation (chopping) works to overcome 1/f noise, and is typically used in systems that operate at DC to maybe a few Hz where 1/f noise is problematic. You operate, instead, up to many kHz.

Excitation (related to above, I think)
OldJim: I'd tinker with the excitation current to the optocoupler to see if it helps. Perhaps add some ~40khz AC ? That you could do with existing circuit board....

Inductance Pickup
SC: Changing to steel reeds and using magnetic guitar pickups might be another way to go.

Jim: It probably is worthwhile. I tried building a miniature version of the Kaman eddy current sensor, with little ferrite rods wound with hair-thin copper. Couldn’t get a good signal from the high pitched reeds. Gave up.

Capacitive Probes:
SC: Did you consider some sort of capacitative pick up, for instance?

AlephZero: Capacitance probes would probably work well. They have a frequency response up to 15 kHz which isn't quite "hi-fi audio" but might be near enough. The only problem is they would probably be too big to fit inside the case (but a line of cylindrical probes stickng out of the case might look "interesting").

SC: The problem with the capacitance method is that the value of capacitance of a reed against an earth plane would be less than 0.01pF and the variation due to vibration would be even less. Also, no one has mentioned the linearity of any of these transducers which would affect the timbre of each note.

Jim: I actually experimented with a type of poor-man’s capacitive pickup. Using nothing but the trace on a circuit board in proximity of the reed, I was able to frequency-modulate a kind-of broadband oscillator (made by Schmitt trigger, I think). The signal could then by any nearby FM radio receiver (tuned to virtually any station . I cannot remember why I dropped this idea. Perhaps fear of FCC.

Regarding Microphone:
Old Jim: Just put a tiny microphone (electret or other type) into the harp case. You know this will work well!

SC: Or follow the example of Charlie Musselwhite and countless other blues musicians who hold a microphone to their harps.

SC reply: A good reason for not using a microphone could be the wind noise / turbulence inside the body of the harp.

Jim: there are at least two reasons for avoiding a microphone. One is feedback. Harp players are constantly competing with electric guitar players. The second is somewhat philosophical. I consider this to be an entirely new instrument. Just as an electric guitar .NE. acoustic guitar + microphone, the electric harp allows far vaster range of effects. From Les Paul to Jimi Hendrix.

ps: A harmonica with built-in electret microphone was also previously marketed by a colleague. I think the name of the company is Harmonic Solutions.

Computer Mice
AlephZero: If you really want an optical solution, you could look at how optical computer mice work. The cost of the electronics in the cheapest ones must only be a dollar or two.

Jim: I also wondered, “how do the optocoupler and fiber optic people contend with this problem?” I think the answer is digitization. Alas, I’m an analog guy.

Armstrong
Old Jim: If the signal were made into amplitude modulated carrier and diode detected exactly as in AM radio - what would it sound like?

Jim: Although my formal training is in biomedical engineering, I got my start as a young boy exploring antique vacuum tube radios. I actually know much more about triodes and tetrodes than semiconductors. Which is both a handicap and a source of “out of the box” inspiration.

Noise Source, and Peripheral Noise (Amplifier, carbon, etc.)
Old Jim: The LM833 looks like a quiet amp but i wonder about the resistors surrounding it? Carbon is notoriously noisy i am told....

Jim: Although there are very few things about this problem that I understand for sure, I have unequivocally concluded that the source is the phototransistor. The definitive test was to simply attach one isolated sensor to a 9V battery through a dropping (pull up) resistor. I illuminated the transistor with an old fashioned incandescent (grain of wheat) lamp, again powered by a different battery. Result is HISSSSSS, loud as day. Light off: silence. Brighter the light: greater hiss. Up to a point of saturation (at least that’s what I call it) whereupon the signal cuts out completely.

Regarding illumination
SC: How about a fibre optic supply from a central LED source and then light diodes placed to give a very oblique incidence on the reeds to magnify the effect of deflection? However it's done, there will be significant non linearity.


Frequency Spectrum of the Hiss:
JohnBahm: Also, what is the frequency nature of your hiss. can you use a scope to isolate the source?

SC: Have you measured your noise power spectral density? Do you have reason to believe that your photo sensor 1/f noise predominates over the white noise? If it doesn't, then modulation won't help.

Jim: I do not have access to a spectrum analyzer, but as a next-best thing, I used a graphic equalizer to systematically try notching out the noise. Turns out that it is broad band. But from what I’ve read about audio hiss, the human ear finds certain frequencies more “irritating” than others. (namely 800-8kHz). The noise literally “sounds” like magnetic tape hiss, hence my initial inspiration for something “Dolby-like.”

Harmonica’s for Everyone
SC wrote: I'll dig my harp out and see if the explanation helps with my technique (last practiced seriously when my 28 year old daughter was about 10).
Jim: As a small token of my appreciateion, I would be pleased to send you a new harmonica. You can choose anyone you like from my website: www.turboharp.com.

Last but not least
the “Scholarly Article” discovered by Big Jim
Guess what. That’s MY article! Did you really think there would be two people in the world crazy enough to build a harmonica reed transducer??

I’ve actually been struggling with this project for many many years. Over 20 in fact. Attached is a little travelogue of the incarnations of the harmonica, beginning with the bulky Kaman eddy-current sensors. Progress has been slow, and in fits and starts. Since it is a total diversion from my day job, I’ve had to steal time from arguably more noble pursuits to work on this. Its been a Sisyphysian challenge.



Thanks again for sharing your experience, time, and creativity to help me with this challenge!

Jim
MaximumPower
#35
Jan11-13, 02:21 PM
P: 10
Attachment - A brief travelogue of the electric harmonica."
Attached Files
File Type: pdf History of Electric Harmonica.pdf (561.4 KB, 4 views)
jim hardy
#36
Jan11-13, 02:59 PM
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Wow thanks for all that feedback !

And that article was YOU ! I am honored by the association.

I got my start as a young boy exploring antique vacuum tube radios.
i too started on vacuum AM radio.

You wrote:
Brighter the light: greater hiss.
aha - does the hiss vary with reflected light as well? If so , perhaps your "hiss" is the desired carrier.

Highpass to block audio out of hiss, amplify, halfwave rectify exactly as with old fashioned AM radio diode detector, lowpass hiss out of detected signal..

Trouble is you need one detector per reed.

But doubtless you're well ahead of me.


old jim


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