Gallery, Projects and General > Project Logs
Diy optical pickup for guitar -- is it possible?
sorveltaja:
New setup almost ready for testing:
Once the short-ish legs of the components are bent, they fit in just fine. That way, the wires that are to be soldered, stay outside of the base.
By using the smallest size heat shrinking tube, that I have, the legs shouldn't short between each other. But we'll see.
While sketching and printing the parts, I remembered seeing a picture of those flat components used in optical pickup somewhere. So that's where I got the idea.
It was Ron Hoag's invention. I have no idea, of how old that picture is, but if judging it by the thickness of the optical components, they look a lot thicker, so probably not very recent:
Again, making an optical pickup to look that clean, without wires running around, there really isn't too much options, other than to drill holes for the wires to go to the back side of the guitar.
After all, there is very little information available of that pickup/setup. I have found only few sources, one which is on the same site, as the above picture:
https://news.softpedia.com/news/Light-Music-Technology-On-Sale-The-Optical-Pickup-Patent-52851.shtml
If only he had 3d-printer (or affordable cnc) technology available back in the days, whole electric(and why not bass and acoustic also) guitar scene could be a whole lot different, than what it is today.
Generally, as mentioned before, this project doesn't follow the same path, that commercially available products, which concentrate on removing magnetic pull, created by traditional electromagnetic pickups, to get the cleanest possible output/sustain from the strings.
Bit of (ranty, sorry about that)background; for a long time, I've been a fan of the analog syntesiser sounds. But, not a surprise, that to get those, one needs to be able to play keyboards(which could make whole thing much more simpler).
Although I have had some keyboards in the past, they ended up gathering dust and sold, as they don't feel as natural to produce sounds(pluck), as guitar does(personal preference).
Other options available today, in form of commercial products:
- Get the costly 'authentic' old analog guitar synth with a hexaphonic pickup, that had already tracking problems, when it left the factory.
- Get the digital guitar synth with a hexaphonic pickup, that uses the newest technology, to emulate analog synth(could do the trick, if you got the money).
It seems, that working, true analog guitar synths haven't developed at all in the past decades, to make them easier to use, as there hasn't been enough demand for the big manufacturers, that have plenty of engineers, and other resources, to develop them further.
I'm well aware, that those kind of devices require special playing techniques to suit the technology.
But no matter how good/bad it is, that analog guitar synth -stuff seems to be very marginal these days. Maybe it was always that way.
End of the rant, and back to the project.
sorveltaja:
New setup was tested with three of the thinnest strings (g-b-e):
Outputs from the strings are now bit closer to be usable, than with previous setups. When they are mixed together after Schmitt triggers, chord harmonies are more audible, than on the lower frequencies (thicker strings e-a-d).
There is still that ugly sputtering, octave-skipping effect. One reason for that could be 'double clipping', where the amp board clips the signals first, and after that, the schmitt triggers clip it again.
On the other hand, actual analog cmos-based 'octaver' -device(divides the input frequency by 2, like Roctave divider)for the guitar, has a very similar behaviour, but it appears only when the signal fades out, or the input signal is too weak.
To get around that, perhaps an amplifier stage, that doesn't clip so easily, while producing huge amount of output, should be used. Have to find one first, that suits for the purpose, though.
There is also one stage, that is a part of the currently used amp board, which is placed before the 386-amp. It boosts the phototransistor output. Not sure, if it's an amplifier, or something else.
Generally phototransistors seem to have max. collector-emitter voltage, that is at least 30 volts. So far, I haven't explored it that much, and have used it only with 9 volts(like the rest of the amp board):
At this point, one might think, 'where's the beef? let's hear the results!'.
I was merrily recording an audio clip, using trusty Windows 7- based laptop, that has an ancient version of Goldwave installed. I know, but it just works for simple tasks(or so I thought).
As usual, I listened the output with headphones, while recording. All seemed/sounded fine.
But when listening the resulting, saved audio file, it had a strong, phase shifter-like effect going through the zero-point, that 'cancelled' most of the audio, making it useless.
Previous audio clip was recorded using exact same setup, with no problem.
I have also a desktop pc, and another laptop to test, so maybe i'll manage to record a new audio clip, that is worth posting.
sorveltaja:
In the past few days I've looked a way to replace the 386-amp board with an op-amp based one. Test circuit(TL072):
It was just thrown together for breadboard, and probably lacks all of the some important components. But as it is, it gives enough output for testing.
Seems to work better by using a split power supply.
But in general, the lack of delivering enough output to keep the schmitt triggers 'squaring' long enough, to make the output to feel more like guitar-ish, still exists.
Then I had to rethink about the whole thing: I have already an overly powerful(and noisy) 386-based amplifier board, which should be more than enough for the purpose.
But in practice, is that much grunt for amplifying the phototransistor's output really even needed, as the end result is still far from usable?
So it was time to try a bit different approach to the problem. I remembered seeing something like positive- and negative input threshold voltages on the 4093's(quad schmitt trigger ic's, that are used so far in this project ) datasheet.
This picture is actually from 40106's(cmos hex schmitt triggers) datasheet, as it's much more clearer, than the old 4093's one:
One thing, that came to my mind, was to bring the input signal between the Vp and Vn, by using an offset, and then adjusting it, to see how low it would go to 'tickle'(trigger) enough both limits.
There might be something in it, but I'll have to admit, that after trying that with a function generator and a scope, there is much more in it.
No matter, which way I tried to test that out, I just couldn't get my head around, of how it could actually be done.
Threshold voltages, although only approximate ones, are on the datasheet, and as the supply voltage is lowered, they lower also.
So the next thing to test was obvious; decrease the schmitt trigger's supply voltage. At 1,8 volts, the input threshold was under 0,1 volts, when testing with a square wave.
Needless to say perhaps, but at that level, the quality of the output never meets any of the standards.
Besides of that, I kept on going, and tested the bugger with a g-string. Yes, the output was a lot lower, but for some reason, 'squaring' was there, and it just kept on doing it longer, than ever before.
And that was by using the above 072-op-amp based circuit.
So far, cons of using that low supply voltage:
- output waveform has some mutations(although not necessarily lethal), when compared to clean square wave
- needs to be amplified/gated afterwards, to get back to the cmos-level
Then the pros:
- overall bandwidth of the schmitt trigger chip decreases drastically(down to few kilohertz), which should/could be good enough to reject radio-like interferences
- less amplification is needed
There is a drawback, when using optical components, that are extracted from optoswitch(or at least the ones I have), though. The phototransistor doesn't have an ir-filtering in it, so it is easily affected by the visible lighting.
sorveltaja:
I decided to take a final test for that noisy 386-based amp board, to see if it still has any use, with 1,8V supply fed schmitt triggers.
It appears to have, once its output levels are turned down enough.
Testing was done with three thinnest strings(g-b-e). There was a strong (4-5V) ~25kHz noise signal, that made the schmitt triggers go nuts.
In previous post I claimed, that the bandwidth of the schmitt triggers had decreased with lower supply voltage. Maybe it has, but not down to a few kHz.
Still I wanted to see, if that noise level could be lowered. By using an online low-pass filter calculator http://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php#answer1, I ended up making a quick test-plugin, that has 10k resistor and 1,5nF capacitor in it:
Its cut-off frequency is at ~10kHz, and seemed to do the trick. After that filter, the noise level went well under 1 volt.
After some tinkering, the strings' outputs had quite decent rectangle shapes. Although the scope showed also more acceptable sustain of the plucked strings, it's only visual result.
I hate to go ahead of things, but if the results are still there, when listening to the outputs, the project has taken a tiny step to the intended direction.
But after all, if that goes well, the current optical/mechanical setup still needs a removable (as slim as possible)cover above the strings to be drawn and printed.
And the schmitt trigger board needs its own dedicated regulator, likely LM317-based, variable one. I have those already at hand.
What comes to audio recording setup, especially, after failing with the previous audio clip, I feel like 'meh, what else is not working, as it should'. Eventually that needs to be sorted out, though.
sorveltaja:
Latest setup ready for height adjustments of the pickups:
To get reasonably stable outputs from the strings, for adjusting the height of the pickups for max. outputs, perhaps most time consuming part (at least on my guitar) is to track the peak frequencies, where the individual strings resonate, when using the small speaker as a resonator(mentioned earlier).
Using low enough outputs, that don't clip at the amp stage, makes it easier to find sweet spots.
I used to write those frequencies down, but they change from day to day. Not so much between the strings, but all of them have gone in frequencies tiny bit lower.
I guess, that's the way the wooden construction(mahogany body, maple neck) reacts to changes in air humidity.
But enough of that. Only after the height adjustments, it's worth to move on to the electrical side.
One thing, that popped to my mind, when testing, if it's possible to 'palm-mute' the strings, while the cover for the optical pickups is in place.
It is almost possible, but as the printed cover is made of plastic, it has to have some extra thickness in it. If that cover was made of, say, out of 0,5 or 1mm brass sheet, it should allow a lot lower, 'slimmer' cover to be used.
What I mean by that, is that if one uses a traditional electromagnetic pickup, its usability could be expanded, when using it besides the optical pickup.
Making that kind of brass/other metal cover would require some serious skills, that I don't have, though.
On the other hand, one way could be making a mold, and casting some tin in it. Could be too smoky thing to do in my apartment. But, after all, who knows
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version