Optical Pickup

[m1kem00re]

Yesterday I repaired an optical smoke detector. It has the assembly as you describe, with the proper mounting cradle for the IR emitter/receiver.


Now, to optically read a vibrating string; perhaps you have ideas, project has merit, go for it.
I do not know if a string will produce an useable response. As such assembly detects smoke microparticles suspended in air, give it a try with a string in the middle. Perhaps a string wrapped in shiny foil by the sensor.

----> https://i.ytimg.com/vi/5zkLkhVK_p4/hqdefault.jpg

----> https://people.ece.cornell.edu/land...2008/nbc5_emc47/reportfiles/smokedetector.jpg

Thanks, I might try that

 

[m1kem00re]

It won't work.
The IR beam is much wider than a guitar string. Then shadow of the vibrating string will not change the amount of light so the output of the IR detector will not have a signal of the vibration.

If the IR beam is the same width or wider than the guitar string then the output will be a constant level squarewave or nothing. The guitar will sound like a buzzer.

If the pickup is a magnetic generator then its output faithfully follows the amplitude and harmonics of the guitar string. A HUGE difference.

Thanks for you reply, I know that it is possible, just need to know how!

 

[m1kem00re]

Explore the use of a laser instead of infrared wide beams... Optical CD readers work well on reflection of tiny pits.

Laser devices can be dangerous, but I would like to find out more about the principle and componenets.

 

[m1kem00re]

I don't see how an optical sensor will give a signal proportional to the string amplitude, which is what you need. As AG says, the signal will be more like a square wave and will sound awful. Or perhaps that's what you're aiming for? Built-in fuzz?

I want to faithfully reproduce the sound of the string

 

[m1kem00re]

This appears to be an ill-conceived project. Magnetic pickups have been used for decades to convert steel string vibrations to proportional electrical audio signals. The only reason I can think of NOT to use a magnetic pickup is if the instrument is strung with something other than steel strings, plastic or animal gut for example. In which case it is an acoustic instrument and an appropriate acoustic pickup (microphone) attached to the sounding board or positioned inside the body of your bass guitar is indicated.

I am attempting to create a pickup system which is different to the norm. One disadvantage with Magnetic pickups is that they exert a "drag" on the metal strings, thereby reducing the amount of sustain. The trouble with contact mikes or peizo devices is that they pick up everything.

 

[m1kem00re]

I think all of you are misinterpreting what the poster wants. He says he wants to detect the frequency of vibration and decode it. I think this means he will use the frequency to set the frequency of some kind of synthesizer that produces the actual sound.

bob

Thanks. All I want to do is reproduce the pure sound of the string through an amplifier, using a pre-amp if necessary.

 

[m1kem00re]

nobody have mis-interpreted what the OP wanted

read his second line again ....

that's what a magnetic pickup already does

I like your metaphor

 

[m1kem00re]

Yes, but read this paragraph:

A normal pickup does not "detect frequencies" or require "decoding" to produce an audible tone "identical" to the vibrating string.

Only the OP can clarify it, but it does not look like he is coming back. Perhaps I am reading too much into his inaccurate description of what a pickup does, but perhaps not.

Bob

Thanks, I Perhaps I have described my project clumsily. All I want to do is faithfully reproduce the actual sound of the vibrating string. I know that it is possible, just don't know how!

 

[m1kem00re]

As @Audioguru stated in post #5 directly measuring the vibrations of a string by optical means will be an enterprise next to impossible without super precise optical components for the reasons given by audioguru.

A method that is known to work employs laser interferometry, also know as laser microphone. You'd bounce the light of a laser off the string. The reflected light's time of flight is modulated by the vibrations of the string. By interference with the original beam the modulated signal can be retrieved. This setup will require good alignment of laser, string and receiver and will probably very costly.
You'll also have to take into consideration safety measures to prevent damage e.g. to the eyes of somewone who might happen to look into the laser beam.

I'l look further into this idea. Thanks.

 

[m1kem00re]

Some singers (Drake and many more) read the lyrics of a song and the electronics of "Autotune" can make anybody sing accurate musical notes. Is that what this optical guitar needs to do?

It's up to the player to play in tune.

 

[m1kem00re]

In case anyone's interested, this picture is of my design, sill work in progress. Can't finish and get on with the prototype until I sort out the pickups. As you will see, I am trying to to come up with something completely different. Re-inventing the wheel?

 

[xryz]

Nice project. I did something similar a few years back. I ended up reflecting the light off the string and modulating the light source at ~100KHz and using the modulation to gate the photo transistor output to reduce stray pick-up from florescent lights etc. the original design had the strings partly blocking the light beam, with the sensors over the string. I abandoned that approach because the player didn't like the strings covered by the fixings for the sensors although this arrangement was simpler to get working well and sounded better than the reflection method.

 

[wazuupster]

So, I'm no expert on guitars but it seems to me that:
There may be an issue with the directionality of the vibration of the string. That's to say, a guitar sting does not (as far as I'm aware) vibrate in a planar fashion, but will oscillate in any normal to the string. As a result, I would suggest that a single beam that the string 'cuts' would produce varied results for each (strum? Pluck? or even throughout the sustain of the note?)
To get round this, I would suggest lasers orthogonal to the string, with detectors that pick up the reflected light.

By detectors, it's possible that LDR's would do the trick (for a really cheap solution) or photo transistors are only slightly more pricey but may give a better result. Both devices give an analog output (with the correct circuitry) which is relative to the amount of light incident on them. Therefore, I would position them such that they are at full output when the string is stationary. The string being plucked would then give an AC waveform in either one or both of the sensors (dependant upon the angle of the plane on which the string oscillates). A capacitive coupling of this signal through a summing amplifier would give you amplitude and frequency in the analog domain, which could essentially be directly connected to an amplifier.

As an electronic engineer, the daunting aspect of the project seems to me to be the perfect positioning of the laser transmitters and the optical sensors which I'm sure as a mechanical guy you'll be able to work your way around. I think the other posters here are being a little critical, the positioning is definitely the hardest aspect of what you're trying to do.

I would suggest that finding a laser with a beam width that's specified such that it perfectly matches the strings amplitude would be a costly thing so as a start to this project, I would be buying 'mr cheap price laser pens' from eBay and starting there. Get a breadboard, a few quad opamps (lm347 or similar), a hobbyists capacitor and resistor set with a few values and a couple of variable potentiometers for experimenting with the gain of the amplifiers and see how you go. WRT the electronics side of this, the hardest bit will be knowing when you have a working system. If you want to test with an amplifier, your circuit MUST have an AC output. If there's a DC component to the electrical signal at the output you can seriously risk damaging the amplifier / speaker you're using. If you can beg steal or borrow even the cheapest of oscilloscopes and get a rough idea of how to use it, you'll find this project a million times easier.

Good luck, Google well and shout when you reach a point where your circuit isn't quite doing what you wanted and I'm sure the guys here will help you out.

If you're really at a basics level with electronics / opamps / gain setting you can do no better thing than checking out EEVblog's tutorials on Youtube.

 

[ayalara]

Thanks, I expect to require a pre-amp

Consider this ...
The music business is extremely large and it attracts some of the the best & most creative, innovative engineers around. If this were a good idea from a music perspective it would have been developed and marketed decades ago. Are you perhaps hoping to do something different with it?

 

[hevans1944]

Thanks, I have approached both RS components and Farnell for advice on which type products to try, but both decline to assist. I'll keep trying!

I am not surprised. Both RS Components and Farnell sell components to people who already know what they are going to do with those components, and can therefore specify what they want. It would help if you were to tell us what you are trying to DO, then move on from there to possible methods of HOW to do it.

Optical pickups, based on sensing the shadow cast by the string, using one or more photo-detectors, when the string is illuminated by an infrared LED, seems to be a simple enough concept, but the devil is always in the details. I would begin this quest by using visible light LEDs so I could see what was going on. I would also suggest modulating the light source at an ultra-sonic frequency to allow more sensitive synchronous demodulation of the photo-detector signal. Then fiddle with LED placement, focusing, and orientation with respect to the guitar string, while examining the photo-detector signal with an oscilloscope and/or audio amplifier and earphones or loudspeaker. Expect to spend a few weeks (at least) getting the optical pickup designed and working. Then a few more weeks refining the design with smaller components to fit on your guitar.

BTW, if you ultrasonically modulate the LED illumination, the photo-detector will become insensitive to ambient light interference. Not immune, because you can always overwhelm the photo-detector with a strong interfering light source, but changes in room lighting should have no effect.

 

[FuZZ1L0G1C]

Hi m1kem00re.
I gather you're not looking at producing a square-wave, as feeding this into an audio amp would create major distortion (side harmonics), like over-driving a loudspeaker.
Running my 6-string acoustic through an analyser, the average wave shape for guitar is somewhere between triangle and sine (each peak and trough onion-shaped).
Never heard of optical pickups.... interesting.

 

[FuZZ1L0G1C]

Sorry, I didn't read every post first time round.
I realize now what u want (faithfully reproduced audio waveform using optics).
Also building an electric in spare time - with inductive pickup for steel strings.

 

[BobK]

What do you hope to achieve by faithfully reproducing the motion of the string in a an electrical signal? Musical instruments, like an acoustic guitar for example, do not do this. The sound box resonates and filters, greatly modifying the wave produced by the string, making it sound far richer than the motion of the string would be.

If you take the direct output of an electric guitar and run it through a good accurate sound reproduction system, like a good home stereo it sounds completely awful. The sound of an electric guitar is a combination of the waveform from the string, the characteristics of the pickup, and the acoustics of the speaker box, much like the sound box on an acoustic. I found this out when I tried to record my guitar playing. I first used the direct output for the pickup through a preamp. This gave a flat uninteresting sound. I then did some research on how guitar is recorded in the studio. Counter-intuitively, they use a mic in front of the guitar amp speaker box.

Bob

 

[hevans1944]

There are a LOT of subtle issues involved in reproducing musical instruments electronically. I think, even today, the most "accurate" studio reproductions still rely on well-placed microphones, although there are acceptable transducers available for most instruments. In the end, the "proof" is in the hearing. Most of us know what a guitar "should" sound like, so the trick is to make that guitar sound come from an electronics amplifier and loudspeaker.

As for magnetic pickups increasing the dampening of a plucked string, I seem to recall that a few decades ago just the opposite effect was created: one set of pickups detected string vibrations and these were amplified and applied to a nearby set of other pickups. By adjusting the gain of the circuit, you could create "infinite" sustain. Some musicians dispensed with the pickups and held their guitar bodies near the loudspeaker to create positive audio feedback. This works for both electrical transducers as well as acoustic instruments, but it turns out to be quite tricky, since it depends on so many factors as to what effect is produced. Young rockers of my generation loved to experiment with their instruments and their music, and weren't afraid to try new things.

 

[BobK]

Some musicians dispensed with the pickups and held their guitar bodies near the loudspeaker

Jimi rules!

Bob