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

[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.