How Can I Reduce Shot Noise in a Phototransistor Audio Application?

[AlephZero]

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]

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]

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]

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]

Attachment - A brief travelogue of the electric harmonica."

 

[jim hardy]

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

 

[humblepie]

I play harmonica, I tuned my own accordians, I can sympathise with the task at hand here and the seemingly strange behaviour of the noise from photodiodes.
Having worked on exactly these shot noise issues in research over the last 30 years, I hope my belated humble contribution offers some useful information.
When the incandescent light bulb experiment gave a strong hiss, there was a good (ie high) reading on the DMM.
But there seems something amiss here, and it's the absence of any modulation to generate an audio signal.
A comb or somesuch whipped in front of the light filament.
What your reed pick-up needs to see is modulation of the reflected light, not just lots of light per se.
Polish the ends of the reeds, shouldn't alter tuning much but take care by wedging with a slip of brass shim bridging between block sides and under reed. Then take the reflection tween LED and photodiode from that.
A narrow slit on LED and Sensor. Google Optical Film Soundtrack for the more advanced optical train, replacing the Variable Width soundtrack with your vibrating reeds. Yes, first experiments will be strange sounding. Maybe square waves, but the modulation will be there to get you into the acceptable S/N ratio region.

Useful link here - http://www.jensign.com//noise/noisecalculator.html

Dolby was originally invented to help Optical Sound on films where Pre-emphasis on quiet passages could be reduced on playback with a compander/expander process. This noise issue became important when amps and speakers became powerful enough to raise the overall theatre volume to high enough levels. Your reeds are already heavily pre-emphasised in this context.
I have no trouble picking up the reflection of a weak red pointer laser from a brass piezo sounder fed from the headphone socket of an iPod and getting good hifi sound. The movement of the reflecting surface must be in the micro-metre range.
Your reeds should be a cinch! You potentially have plenty of Contrast, Close Proximity tween LED/Sensor, Plenty of Modulation. The metal reed shield on a harp is a bonus too for avoiding mains hum issues.

By the way, I still haven't found ideal solutions to many problems where shot noise is an issue. The 'pumping' or 'breathing' effect of the noise following the amplitude of the light modulation is more obvious because it is 'single-sided'. It is correlated with the positive side of the audio wave. Well, yes, like a synthesiser special effect! I have been frustrated with how intrusive this can sound when there is clearly on the test bench a good S/N ratio! It needs a clear understanding of the physics of audio and perceived loudness at different frequencies to make sense of how to use photodiodes as audio sensors.

One issue to look forward to is Microphony, as in the harp acting as a mike. At that point you should be pleased to have the project successfully in hand. Knowing how reeds work, the limited dynamic range for example, should make for an easier project to me than it seems to have become. I have a chinese-made harp, LEDs and sensors to hand; so I'll give this a shot..
This noise is a strange phenomena at first encounter in DC conditions. Go to a movie or try to recollect the experience (35mm film analogue sound), enjoy it. Notice the soundtrack. Where's the shot noise from the photodiode?
Components; Phototransistors are often connected internally as a photo-darlington, with a higher dark noise. SMD versions of phototransistors and photodiodes are only pennies compared to the metal can and leaded legacy items which may be inferior specs, the price reflects different volume production. SMD is the way to go. To get better specs you will need better specs.

 

[humblepie]

I'll eat some humility on my suggestion to access the end of the reeds. I took a harp apart and it was obvious there isn't more than a sliver of metal reed and access is impossible compared to an accordion.

However, tidying the kitchen worktop of all the bits and bobs that tend to collect; led me to a serendipitious discovery which would allow the system and installation you have already worked on to succeed.

 

[meBigGuy]

Just a few observations: Maybe most of this is obvious.

1. If you are going to do this with "DC" light, filtering each reed's contribution before summing is important. Increasing the signal from each reed is also important.

2. Summing multiple sensors to reduce noise only helps if there is a common coherent signal. For example, multiple sensors looking at the same reed and phased properly. The coherent power sums, the noise as sqrt of sum of sq.

3. I don't understand the hetrodyning or delta-sigma suggestions. If the noise is already present in the signal it isn't going to help. Maybe I'm missing the obvious.

4. Sophie is correct about dolby. It really isn't "noise reduction" as much as "pre-equalization" before a noisy media.

6. A basic flaw with your design is that the noise of 10 sensors are summed together and always present. (this gets back to 1)

Maybe someone said this already, but I would modulate the LED current with a high frequency and then filter and detect that in the receiver.

Modulate the light at a frequency of 10-20x above the highest reed frequency component you want to detect.

 

[humblepie]

Is there any change in the reflected signal?

I can only admire the OP persevering with this idea and undertaking some practical experiments.

I have found that the reflection is great from the surface of a reed, but that it hardly alters the current in the diode when the reed angle changes as it would when being played?

Like I say, there are several ways to get around this using the original 'face on' approach. If the OP is still interested I'll explain further.

I suggest the OP does a 'static' DC test depressing and raising with a slip of paper under a reed and a DMM to confirm how much the current changes with the angle of the reed.
On this change hangs the whole issue of getting a resonable signal to noise ratio.
If you mount a single detector/sensor on a small piece of PCB, you may be able to temporarily mount it suspended at your original design height and position and still get access to a reed.

What is needed is more like a 'light-gate' - the sensors you are using are in a family of gross light change sensors. Like sensing the reflective spot on a turning gear wheel for example. either fully on, or fully off. Very bright ambient light is likely to be the only issue in such a case. Which is where 'high frequency modulation' techniques are relevant ie IR TV remote controls.

See my link to noise calculations above.
On this change in current value hangs the whole issue of getting a resonable signal to noise ratio.

In scientific instrumentation the modulation with a higher frequency is used to help remove 1/f or flicker noise. Shot noise is white noise. This should not be a marginal signal detection situation, which is where all the AD630 sychronous detectors etc are very helpful. Modulation adds it's own noise as well.

De-emphasis (low pass filter) may reduce the white noise to 'pink noise' - removing some of the hiss. As the frequency increases with the same amplitude, the energy will double for each octave. This is effectively a large amount of 'pre-emphasis'. However, the movement of each reed may be less when ascending the musical scale. It's difficult to tell whether this is the case because the size and thickness of the reeds in my sample get smaller as frequency increases. And I can't see what the reeds are doing while I play the instrument. A fixed set-up with foot-pump, a strobscope and video would be ideal - but it's really just another point to consider in terms of filtering the output.

The original stated frequency range is not going to give any kind of fidelity as far as the harmonics and timbre are concerned. From a quick check on an oscilloscope, a harmonica has significant harmonics going up to at least 5 kHz.

I'm interested to see how this project develops.
It's a fun way to look from a different viewpoint at some existing projects I'm trying to resolve.

 

[sophiecentaur]

SNR is what it's all bout.
What about an entirely different approach; did you ever consider a piezo sensor? The mechanical impedance could be good for coupling to a sensor screwed to the frame. There would be very little breath noise.
Just an idea.

 

[humblepie]

sophiecentaur - see his history of the electronic harmonica, he already tried the piezo contact mike approach.
Several decades ago the avant garde electronic musicians were fixing these sensors to all sorts of instruments and vibrating objects. They need the mass of a larger body vibrating with surface waves, as in a guitar body, or have force transfer through them like an 'under bridge' pickup. There are also electric reed organs which used the 'metal detector' principle and therefore the brass/bronze reed worked fine.
Thank you for posting the links OP;-
I have finally been able to download your Patent, History of your ELX Electronic Harmonica and your research paper on reed behaviour.

It would seem you now have all the information you need, but lack the inspiration after 20 years of frustrated development for a new lateral thinking approach.

Try a comb, a diode and a light bulb, get a feel for deep modulation versus spotlight DC illumination.
It's irrelevant that you maximised the output when aligning the sensors - if that maximum is a fraction of what should be happening.
It is a shame your research paper only has 'normalised' figures for reed movement. I'm guessing you never made any attempt to actually measure that factor because you were using the audio output from the sensors as the data to analyse?

 

[meBigGuy]

This may not be clearly described, but addresses the optics.

Maybe what is happening with the reflection off the reed is that:
1. The border of the LED "beam" is not sharp (distinct) enough, It is "blurry" where it hits the reed and then the sensor.
2. The reed angle change is not enough to cause the reflection to pass through the blurry border. In other words, there is not much change in the total amount of light falling on the photo-diode.

The sharpest the beam can be is if the led image is projected on the reed via a lens. Then if the motion of the reed is enough to cause the reflection to move on/off the photo-sensor, you get maximum possible amplitude. I don't know enough optics to know whether a simple ball lens will do the trick.

Another possiblity is to mess with the surface of the reed. If only a small section is reflective, then possibly you can position the sensor to get a larger change.

Or maybe you need to position the sensor to be off the very end of the reed where there might be a more distinct border.

But, I stand by my original observation:

Each channel needs to be filtered before summing. This is true for "DC" or modulated light. Understand that modulating the light is as simple as varying the series drive current. (although every channel then needs a filter and detector)

 

[sophiecentaur]

It's a longtime since this thread started and I forgot the piezo idea had already been mooted.

It's almost certain that the best optical system would use oblique illumination because this would achieve a long 'throw' * and hence more movement of the 'light spot' as the reed vibrates. Perhaps a cylindrical lens could focus the light from a linear light source or a set of optic fibres onto the sensors. The lens would give an improved illumination of the whole row of reeds and increase the SNR significantly, the light level arriving on the sensor could be modulated right down to zero for full amplitude vibrations if the spot could be made narrow enough to be deflected right away from the sensor.
An ideal optical system could involve a single moulded light 'manifold' with individual, spaced exits, with lenses of the right focal length to form an image on each sensor.
* There would be five times the deflection across the sensor if the light grazed the reed at an angle of 10°, for instance. The reeds would need to be well polished, though, for good specular reflections. Regular cleaning would be needed; there's no knowing what spit would do to the performance.