Photonic guitar pickup has fiber Fabry-Perot cavity

[Joe Gwinn]

I swear, this is in fact the title of the article, on page 17 of the June 2010 issue of Laser Focus World.

Anyway, the pickup in question reads the body vibrations in the body of an otherwise unmodified acoustic guitar using a resonant fiber cavity (made of a pair of FBGs) tuned to a narrowband 1542.14 nanometer laser. No coils of wire or permanent magnets used. What is sensed is mechanical strain in the wood, not motion per se. In other words, it's effectively a strain gauge.

FIBER BRAGG GRATINGS: Photonic guitar pickup has fiber Fabry-Perot cavity - OptoIQ

They speak only Photonics there, worse than Electronics.

Think it will catch on the Music world? It's only $10,000 per installation.

[Mike Sulzer]

I swear, this is in fact the title of the article, on page 17 of the June 2010 issue of Laser Focus World.

Anyway, the pickup in question reads the body vibrations in the body of an otherwise.....

I saw that a couple of weeks ago. For $10,000 the best they can say is:

"Audio recordings of the acoustic guitar, as well as of some of the other instruments, showed that the photonic pickup's output approached the quality of a conventional high-quality pickup."

Since the device senses the flexing of the body, I assume the competition that they refer to is piezo.

[Joe Gwinn]

I saw that a couple of weeks ago. For $10,000 the best they can say is:

"Audio recordings of the acoustic guitar, as well as of some of the other instruments, showed that the photonic pickup's output approached the quality of a conventional high-quality pickup."

Since the device senses the flexing of the body, I assume the competition that they refer to is piezo.

Piezo is the named competition, but piezo units are almost universally accelerometers, not strain gauges, so they don't measure the same things.

I think that the main use of the photonic sensor will be for research, and that only if photonic sensors turn out to give information not obtainable using piezo units, which are far cheaper. And, not obtainable using an ordinary silicon strain gauge, which is also far simpler than photonic approaches.

[Mark Hammer]

Wasn't there a guitar synth system that read the strings optically, near the bridge?

[dai h.]

I was thinking there was some old Yamaha gtr. synth like what (which might be this) :

Synthony Music's Synth & Midi Museum

,but only the string bending is optically sensed apparently. (Anyway, sry if that is totally incorrect!)

[David Schwab]

Wasn't there a guitar synth system that read the strings optically, near the bridge?

There are a couple of optical pickup systems out there.

HOAG OPTICALGUITARS SITE

LightWave Systems - Bass and Guitars

[Mike Sulzer]

Piezo is the named competition, but piezo units are almost universally accelerometers, not strain gauges, so they don't measure the same things.

But if you want to measure the same thing, that is, bending with a piezo as with this optical device, one can use a two layer piezo device (Piezo Systems: Introduction to Piezoelectric Transducers, a bit more than a third of the way down down).

I do not know how the relative sensitivities compare.

[David Schwab]

Don't piezos also measure pressure? Electric guitar or bass bridges don't flex like acoustic guitar tops, but still work with piezos.

It's an interesting idea to measure the guitar with lasers. You could probably even do that from across the room from shining one on a small mirror on an acoustic guitar top and then reflecting that into some sort of sensor. Just have to make sure the person sits relatively still!

[Mike Sulzer]

Don't piezos also measure pressure?

They can; it depends on how they are cut, stacked or not, etc. Check the reference I gave above.

[Joe Gwinn]

But if you want to measure the same thing, that is, bending with a piezo as with this optical device, one can use a two layer piezo device (Piezo Systems: Introduction to Piezoelectric Transducers, a bit more than a third of the way down down).

Most of the guitar pickups I have seen used a polyvinylidene fluoride layer and a mass, or put the piezo layer under the bridge.

Polyvinylidene fluoride - Wikipedia, the free encyclopedia

I do not know how the relative sensitivities compare.

They all seem to be adequate.

[Xaar]

I wonder how much compressed the sound is on thes optical pickups -how sensitive they are to string attack- since they seem to be nothing else than light powered interrupters.

Which results do they yield through distortion? Seems more to me like a modern replacement for piezos than for distortion pups.

[Rick Turner]

It's not about technology, it's about location, location, location.

Optical pickups have no significant advantages over piezo pickups other than the "gee whiz, far out" science project aspect.

Piezos can be designed to very directly read string motion at the witness point of the bridge saddle...the ultimate node of string movement, as imperfect as that perfect spot may be. Optical pickups have the same screwed up "vision" of the string that magnetic pickups do; they read some weird spot on the string that is harmonically loaded weirdly, no matter what you do. The very placement makes harmonic accuracy impossible. And the bullshit about music at the speed of light...well, what can I say?

[Mike Sulzer]

It's not about technology, it's about location, location, location.

Optical pickups have no significant advantages over piezo pickups other than the "gee whiz, far out" science project aspect.

Piezos can be designed to very directly read string motion at the witness point of the bridge saddle...the ultimate node of string movement, as imperfect as that perfect spot may be. Optical pickups have the same screwed up "vision" of the string that magnetic pickups do; they read some weird spot on the string that is harmonically loaded weirdly, no matter what you do. The very placement makes harmonic accuracy impossible. And the bullshit about music at the speed of light...well, what can I say?

I do not see how such a piezo can measure string motion directly. The bridge material is always involved as well and the body to some extent, and in any case, it is the changing string tension that is transferred to the bridge.

You can actively add several magnetic pickups to get a wide variety of harmonic balances. The fundamental will be underrepresented for open and low frets, of course, but is that not what you want in any case?

[Rick Turner]

Mike, let's not talk about string motion, then, let's talk string pressure vectors acting on the bridge saddle...the very place from which acoustic instruments get their string signal "information" devoid of any major odd issues like a pickup dividing the string length and thus favoring some harmonics over others in a constantly changing fashion.

Of course there are other things going on...body resonance and damping, etc., but they are going on no matter what pickup technology you use. Everything the body and neck do winds up back in the strings.

I make several types of piezo pickups: classic "undersaddle" designs for mostly acoustic guitars (see Welcome to D-TAR! Acoustic Guitar Pickups and Preamps) and I also make very close coupled individual string pickups that respond both in pressure mode (for string motion perpendicular to the top) and in shear mode (for string motion parallel to the top), and these pickups do a nice job integrating all string motion vectors. With the right preamp input impedance, these pickups go sub-sonic in the low end, and way out past 20 K in the high end. I use these mostly for electric bass, though I've done a fair amount of custom work on some wonderfully strange instruments. To make them, I silver solder a tiny nickel silver or brass saddle witness point to the top of a piezo ceramic block, so the string is barely 3/32" from the piezo itself. That makes for pretty direct measurement of the string pressure modes. I suppose I could have piezo ceramic blocks custom made with string slots in them; that would be even more directly coupling the strings, but I'm not ready to go there...yet...though that's what the old Gibson C1E guitars from 1963 had.

[Mike Sulzer]

Mike, let's not talk about string motion, then, let's talk string pressure vectors acting on the bridge saddle...the very place from which acoustic instruments get their string signal "information" devoid of any major odd issues like a pickup dividing the string length and thus favoring some harmonics over others in a constantly changing fashion.

Constantly changing depending upon the fret. But this means that the frequencies emphasized, or reduced, are constant. This is what the sounding board and resonator of an acoustic instrument do: provide its character as well amplify the sound. The difference is that with an electric instrument sampling the string in one or several locations, the sets of special frequencies are different for each string. Thus one has a frequency filter for each string, while an acoustic instrument has one in common.

So if you use a type of pickup that does not have this filtering function, then you have the opportunity to provide it. This might be some transfer function constructed digitally, or even the speaker enclosure.