Optical Pickup

[m1kem00re]

I am a mechanical engineer with minimal electronics knowledge. Therefore I am seeking advice on which products to select.

My project is to design an optical pickup (instead of the usual magnetic devices) for use with a bass guitar that I am building.

I wish to select 2 components: An infra-red emitter and an infra-red receiver.

The two components can be positioned facing each other about 20/30mm apart, with a vibrating string running between them.

The receiver needs to be able to detect the frequency and be capable of generating a small voltage which can be decoded and fed through an amplifier to produce an audible tone at a pitch identical to that of the vibrating string.

Any advice on any additional circuitry would also be appreciated

 

[duke37]

You can get a device with a slot between an emitter and detector for counting purposes. If this is a digital switch it will be no use for you, you will need an analogue version with the string attenuating the signal.

 

[davenn]

My project is to design an optical pickup (instead of the usual magnetic devices) for use with a bass guitar that I am building.

I wish to select 2 components: An infra-red emitter and an infra-red receiver.

The two components can be positioned facing each other about 20/30mm apart, with a vibrating string running between them.

interesting thought but I see 2 significant issues that you may not have considered

1) you realise you will need 4 sets ? one for each string .... I suspect you will find this extremely difficult to do due to the lack of space

2) I also suspect that you will find that you will get nowhere near the output that you would from a magnetic pickup

 

[Externet]

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

I wish to select 2 components: An infra-red emitter and an infra-red receiver.
The two components can be positioned facing each other about 20/30mm apart,

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

 

[Audioguru]

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.

 

[Externet]

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

 

[Alec_t]

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?

 

[hevans1944]

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.

 

[BobK]

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

 

[bushtech]

Would a piezo vibration sensor give you something to work with. You would have to shield it from the adjoining strings. If I'm talking nonsense please ignore me. Way out of my depth here

 

[hevans1944]

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

That's pretty generous of you, Bob, given this is a mechanical engineer with minimal electronics knowledge who probably knows didly about synthesizers. Also, the proposed infrared receiver is only vaguely described in terms of its functionality, without a clue as to how said functionality will be achieved. From the OP's post:

The receiver needs to be able to detect the frequency and be capable of generating a small voltage which can be decoded and fed through an amplifier to produce an audible tone at a pitch identical to that of the vibrating string.

This statement appears to indicate a straightforward analog path, although I have no idea what "can be decoded" means in terms of the infrared signal seen by the infrared receiver. How is the signal encoded in the first place? Does the vibrating string somehow "encode" the receiver signal, albeit at a small voltage level, which must be fed through an amplifier to produce an audible tone at a pitch identical to the vibrating string? What does it mean that "the receiver needs to be able to detect the frequency?" Is there some sort of signal processing going on, inside the receiver, that encodes the infrared detector signal?

 

[davenn]

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

nobody have mis-interpreted what the OP wanted

read his second line again ....

My project is to design an optical pickup (instead of the usual magnetic devices) for use with a bass guitar that I am building.

that's what a magnetic pickup already does

 

[BobK]

Yes, but read this paragraph:

The receiver needs to be able to detect the frequency and be capable of generating a small voltage which can be decoded and fed through an amplifier to produce an audible tone at a pitch identical to that of the vibrating string.

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

 

[hevans1944]

that's what a magnetic pickup already does

As many of us responding have been quick to point out. An optical pickup sounds sexy and "state-of-the-art," but if such a thing existed it would be a "solution in search of a problem to solve" because magnetic pickups allow a simple and elegant solution to the problem of how to convert a vibrating string into an audible electronic signal.

Sometime back in the last century, when vinyl was king, I tried to "invent" a cartridge pickup based on optical principles: two orthogonal beams of light would bounce off opposite walls of a record groove and be "received" by photovoltaic detectors whose outputs would faithfully reproduced the undulations of the groove walls. The whole contraption would be mounted on a radial arm and driven radially by a servomotor to make sure the optical head tracked over the inwardly spiraling groove. The only thing coming in contact with my precious vinyl records would be photons.

Well, my idea never got beyond the concept stage with me, but apparently the idea was not unique. A laser head was eventually developed and sold as part of a rather pricey Laser Turntable system by a Japanese company, ELP Corporation. This occurred in 1989, a few years after my initial considerations. By then, my vinyl collection was getting a little long in the tooth, as was my hi-fi stereo system. My next "project" was to create an electronic juke box that would provide me random access to any track on any album in my collection. I dropped that idea, too, because about that time CDs were becoming popular and I didn't see how my hundreds of vinyl records could ever be compressed down to fit on a single hard drive. How short-sighted of me. The two terrabyte drive I have in my wife's desktop computer would do the job nicely.

Point of all this? Sometimes "good" ideas need a little time for current technology to "catch up" before those "good ideas" can be successfully deployed.

 

[m1kem00re]

Thanks to all of you who have tried to help me. I will bear all of your comments in mind. From my internet research I know that both laser and IR devices have been produced successfully, I am attempting to discover how it can be achieved using "economical" components. I look forward to seeing any more of your ideas.

 

[hevans1944]

Well, using the Google search string "optical guitar string transducer" reveals there exists at least one patented pickup, manufactured and sold by Lightwave Systems through a guitar manufacturer, Wilcox Guitars. There are a number of You Tube videos that demonstrate these guitars. The guitars are quite pricey, and not everyone likes the way they sound. One characteristic they do have is a long sustain after a string is plucked. Guitars with conventional magnetic pick-ups do not have the sensitivity, required for a long sustain, that optical pick-ups enjoy.

The cited text string above yielded a Google patent search result, which may not be definitive, but cites a long list of patents describing prior art. This particular patent application appears to be oriented more toward capturing the essence of guitar string vibrations by digitizing the transducer output for processing that eventually results in, say, a MIDI representation of the instrument, or input control waveforms/commands to a digital synthesizer, or processing that yields an automatic hard-copy musical score or a fret fingering music score.

It has been a sort of holy grail of amateur musicians, especially those without formal music training, to have their performances automatically scored for reproduction by other trained musicians who can read music scores. This effort seems to be trivial, given modern computers and their sound cards, but it is quite difficult. It is possible that an electro-optical pick-up is better suited fo this than a magnetic pick-up.

The OP has a long row to hoe in getting a practical optical pick-up to work with his bass guitar. A lot of hands-on experimentation with various infrared LED emitters and infrared-responsive photodiode detectors will be required. Since its infrared, it will be difficult to visually verify optical collimation, focusing, and light paths for purposes of adjustment and alignment. A cell-phone or other digital camera usually is sensitive to near-infrared LED emitters and can be of some help.

There are literally thousands of choices of infrared emitters and infrared photosensors available. If I were the OP, I would begin by searching on-line catalogs of distributors such as Digi-Key to find something of appropriate physical size to mount in near proximity to the string(s). There are a lot of SMD (Surface Mount Devices) available. Then it will be up to the OP to figure out the appropriate optical design and necessary signal conditioning. Good luck with that!

 

[Harald Kapp]

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.

 

[Audioguru]

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?

 

[m1kem00re]

You can get a device with a slot between an emitter and detector for counting purposes. If this is a digital switch it will be no use for you, you will need an analogue version with the string attenuating the signal.

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!

 

[m1kem00re]

interesting thought but I see 2 significant issues that you may not have considered

1) you realise you will need 4 sets ? one for each string .... I suspect you will find this extremely difficult to do due to the lack of space

2) I also suspect that you will find that you will get nowhere near the output that you would from a magnetic pickup

Thanks, I expect to require a pre-amp