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Diy optical pickup for guitar -- is it possible?
Will_D:
I would think that the optical method will work with any string as opposed to the electro-mechanical pickup that only works with a metallic string
As I am in no way musical the above may be pure BS
sorveltaja:
Thanks for the replies. RussellT, optical pickup isn't necessary better than the ordinary electromagnetic one.
Reason, that I chose optical pickups, is that they are much more compact, and ready to be used as sensors.
Electromagnetic pickups require very precise winding of really thin copper wire, to produce good output. Naturally, space between the strings limit the size of those type pickups.
Not only that, but when one wants to make a polyphonic pickup, six separate elements are needed for guitar.
What do I mean by polyphonic pickup, and what is its purpose?
The consept comes from commercial guitar synthesizers, that have a specific, polyphonic pickups.
Although that kind of pickups can be bought separately, unfortunately they require a model specific synthesizer device to work, which are rather expensive.
Difference between mono- and polyphonic pickups:
So the polyphonic(or hexaphonic) pickup has six outputs, which each could be processed independently. For example, signal from one string could be panned to the left, and signal from other string to the right in the stereo field. Or each string could drive different oscillators. Almost endless amount of options.
What comes to the current setup, I placed the optical pickups near the end of the strings(bridge), so that they are not in a way, when plucking strings.
Obviously, string vibration is very faint at the end of the string, but even that minimal movement is enough for the IR receivers to pick up even the tiniest deviations in the IR light, that the leds produce.
Russ57, that is exactly, what I thought at first. That idea came from optical encoders, that use slotted wheels to interrupt the IR beam.
Then came the idea of using narrowed laser beams as a light source. But I think, that the light source should be infrared, according to commercial optical pickups.
Infrared laser is definitely no-no, as it is plain dangerous, making serious damage for the eyes, without proper safety precautions.
Philf, thanks for adding the quote. It pretty much tells the essential things about optical pickups.
Does the ordinary listener detect the difference? Not necessarily.
But If the guitar or bass has only optical pickup(s) in it, there isn't any magnetic pull from the electromagnetic pickups, that might dampen the sustain of the steel strings.
So I assume, that to tell the difference, you have to be either musician, or any other person, that had to develop their auditory senses, while working with the music.
Will_D, That's not BS. When using optical pickup(s), the string material can be something like nylon or rubber, or practically anything in a form of a string, that vibrates.
BillTodd:
If you want linearity from optical sensors, you might try to get a difference signal from two sensors (such that the motion of the string covers one while exposing the other ) the differential approach will also remove overall illumination variance .
might also be worth modulating the light with a carrier and then synchronous rectification to recover the signal to remove any external interference.
fixed and tapered apertures around the light source and sensors may also help
bill
sorveltaja:
Bill, thanks for the suggestions. Could the differential approach be used for more than one pair, at the same time. Like for all the six outputs?
I'll have to admit, that it is grey area for me, even though there might be just simple math or logic involved.
But on the other hand, I have made a very simple square wave generator, to feed the IR leds, instead of DC. It is based on a 555-timer IC, that runs at ~120KHz.
That frequency is just a sum of the components, that I had on the shelf:
I have noticed, that by using plain DC for those leds, they tend to hog a lot of current, without actually "feeding" the receivers. I see those receivers, as bird cubs, that scream for food all the time.
When they are fed hundreds(or thousands) of times per second, they don't have the time to observe the environment, or make noise.
I'm not sure, if it is any form of modulation, but rather pulsing, to keep the receivers busy, so that they notice only the aberrations in the IR beams.
Also, current consumption is minimal, when compared to DC.
Some specs: my current setup has 3mm IR-leds (IR204), and 3mm IR-receivers (PT202B).
BillTodd:
Your generator is effectively sampling the position of the string (like strobe light) , it is sufficiently fast (at 120kHz) to catch all audible frequencies (has to be at least twice the maximum desired frequency or it will produce differential , alias, signals ). You may be able to reduce power consumption or reduce noise by reducing the on period while increasing the drive current (such that the average and peak current is within the diodes range). (look up shannon sampling theory)
An effective differential pickup would require at least one opamp per string (but they come four to a box so it's not such a problem). The hardest bit is the optical layout , are you relying on reflection from the strings or trying to have the beam cut by it?.
I wonder if you could use a pair of standard opto switches at slightly different heights to catch the top and bottom (or left and right) of the string??
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