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Extending the soundscape of a hexaphonic guitar pickup
sorveltaja:
Not so much of an aural testing today. Instead I've been drawing a breadboard version of a compressor, that uses led-ldr pair for gain control. If it makes noticeable improvement for speech processing, more about it later.
I also measured outputs of the band pass filters, one by one. 0.1 volt sine wave was used, at each one's peak frequency. Power supply at -15/+15 volts. Results:
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Quite a lot of deviation, to say at least. I started modding with the 560p one, to add more gain for it. Schematic of the mic-instrument pair:
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First I removed resistors Ra and Rb(both 4.7k), and replaced them with 10k multiturn trimpots. Still not enough, so I detached one end of the Rc and Rd(2.2M each), and added 2.2M resistors in series with them. Then I was able to get the max. output to 12.0 volts, which might be enough for now.
I just don't understand, why there is so much deviation between bpf's outputs. 560pf one had 072 dual op-amp, and I replaced it with 082, to make sure, that the ic is working, but still same results.
Those bpf's have identical components, besides frequency setting capacitors.
Modifications are quite simple to make. On the right side is modded one:
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sorveltaja:
Each of the band pass filters were adjusted to have ~12 volts(p-p) outputs. It alone made audible improvement to the overall output.
Just today I read from some forum, that this(Paia) vocoder, has an inherent property in form of deviation of the bpf outputs. When I was measuring the compander's output levels, it wasn't so dramatic, so that could the reason, why the designer of this device(Craig Anderton) did let them be, as they are.
But now it's been taken care of, and the previously mentioned compressor for speech signals may not be necessary at this point.
Optocoupler pair, as a module, and built one(1 of 4):
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There were two odd things, that I stumbled upon. First one was, that I had to swap mic/instrument connections for the led-ldr pairs 2-4-6-8. Otherwise they wouldn't give any output. Probably some logic error in the bus boards. I didn't want to spend too much time to find them(errors), instead it was easy to fix the problem, when they were on the breadboard.
Second one was utterly strange. When I built the above led-ldr pair as a module, there was usual testings, to see if it works, as expected. Procedure was this: feed a low frequency square wave(1Hz) to the led inputs, and higher one(audio range) freq. signal to ldr's. That way I expected to see, how the slowly flashing led turns on and off the signal, that goes through the ldr.
No such thing. The signal went through the ldr, and the flashing led did affect it only barely visible amount, when looking at the scope. I used signal generator for the signals, and dialed the levels up and down, but no visible reaction.
There I was, thinking: "bugger! this doesn't work!". Then I did the same tests with 'previously aurally tested and working' -setup, that was on the breadboard. Exactly similar results.
My conclusion was, based on level measurements, that it shouldn't work, but when connected to the rest of the circuitry, and listening to the output, it works, for some reason.
There must be some 'plan 9 from outer phase' -audiochemical processes going on.
But seriously, the way they work, isn't obvious(at least for me). I would most probably have skipped the whole concept, if I was counting on my skills to find out, what's happening, just by looking at the numbers or curves.
In the end, no off-topic ramble this time.
Makes one wonder, why the optoisolators aren't used on diy-vocoders(as far, as I'm aware of).
sorveltaja:
All the optocoupler boards are finished, and the whole device works, as expected. Now it's time to look, what needs to be added/modded, to get the most out of it.
First one is simple; 100k potentiometers(left from the arbitrary waveform generator -project) between bpf- and optocoupler boards, to adjust each channels levels individually:
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It works like an equaliser, but has rather subtle effect. That is probably because the leds on the optocouplers could use more 'juice', to make the ldr's more reactive/responsive. The outputs of the band pass filters, although their gains are cranked close to the max, are still lacking the grunt. But then again, they weren't originally designed to drive leds...
I discovered a possible trick to increase their current levels, while testing earlier with a single potentiometer. So far, I can't tell, what the cost will be(like roasted op-amps, and such).
As always, there is only one way to find out.
sorveltaja:
The previously mentioned trick to increase the current levels for the leds didn't work well at all. Leds flashed a lot brighter, but the audio output was not usable, being heavily distorted and muddy.
There I was, reminded of the thing, that everything doesn't have to go to eleven(like Spinal Tap). I encountered the very same thing, when building the electronics for the hex pickup.
In fact, the band pass filter's gain levels are so high, that the output op-amp clips/distorts way too easily. Maybe just replacing it with a pot(and perhaps with a buffer(which has a unity gain)) could be enough, as the output seems to be already 'hot' enough, to need any amplification.
But the idea of using the 100k pots between bpf's and optocouplers seems to work quite well, when inspected and listened closer.
It is true, what I've been reading about analog vocoders: it takes a lot of effort to get a grasp, how to operate the bugger, to suit one's taste(or get whatever usable effects).
Not an object for me, as I tend to go to extremes, when testing/abusing such diy-built devices.
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Partly off-topic philosophising:
When I got my first stereo system, some time went by, and I read that an equaliser would improve the listening experience. So I bought one(that I still have).
At first, when turning the knobs up an down, I wondered: "where is the beef?". Again, after some time, I started to notice slight differences in sound quality, when playing with the knobs.
Eventually that eq became an essential part of the stereo system. The key is to develop the hearing,
Same seems to apply to this device also: Hearing/ear training. Instead of turning the knobs to eleven, there are subtle things, of which the secrets are made of.
In the end, I have ideas about extending this vocoders range of usability further, by adding something like 4046-phase locked loops for each channel. We'll see about that.
sorveltaja:
I can now see, why there is a fuzz option for the instrument input, in the original schematic. It adds, when used very sparingly, a bit more harmonic content. So I breadboarded the 'tube sound fuzz':
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It uses a 4049 cmos hex Inverting Buffer -ic:
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I left the R6(10M) out of the circuit, as only very mild portion of fuzz is needed. Actually finding a sweet spot could be easier to dial in, if a multiturn pot was used insted for R5(1M).
They seem to cost at least five times more, than ordinary pots, so I rather look for other options. One could be to use gears to increase the ratio of rotation.
I have some plastic gears from printers etc. and also guitar tuning machines, that use brass/steel worm gears. Printing the gears with small teeth, like mod 0.5, and expecting them to mesh smoothly, isn't too good idea, as I found out on one of the past projects. In future it could be done, once the entry level printers develop enough.
Certain level of precision is preferable, so perhaps the guitar tuning machines(having something like 8-12 ratios) would be more than plenty. They also take less room, than similar ratios done with spur gears. I have a feeling, that it will be the next 'sub-project'.
In the end, an audio sample, where the fuzz was used for instrument signal:
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