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Old 1st November 2010, 06:24 PM   #11
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Been busy reseaching and planning what to do. The Phase-locked loop idea is an interestingone, something also mentioned to me by my lecturer. Although right now something that is still beyond my knowledge

I am currently waiting for components to arrive, I plan on initially using a square-wave with a set frequency with a LM331 chip.

Click the image to open in full size.

I will then test with other frequencies and make sure Im getting a linear output. Im expecting this to go okay and give results as expected.

Something my attention is turning to now (hoping the above goes without glitch of course) is how to extract the fundamental freq from the input.

I intend on using a comparator circuit to change the audio input into a square (please point out if you see any complications I might get here) I was thinking of using a 741 ic unless anyone can tell me why not to / a better suggestion?

What I want is the fundamental freq reaching the f/cv part of the circuit, rather than a mess of harmonics etc ( i think anyway!)

Any further suggestions / ideas alwas welcome!
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Old 7th December 2010, 10:53 PM   #12
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Quote:
Originally Posted by tjs2010 View Post
Been busy reseaching and planning what to do. The Phase-locked loop idea is an interestingone, something also mentioned to me by my lecturer. Although right now something that is still beyond my knowledge
Hi,

I hope my answer does not come too late. I have created some guitar synthesizers myself, using phase locked loop AND ramp-sample/hold, and have some schematics... somewhere. I have researched this a lot years ago, the schematics of the Roland GR-300, the Electro Harmonix Rackmount Guitar Synthesizer, the Moog patents, the frequency-to-voltage converters, etc.

Believe it or not, there is little information about this on the internet, since today everybody does the digital way.

Brace yourself, everything you needed to know about analog guitar synths and were afraid to ask!!

I'm assuming you are using a normal pickup, not a divided (hexaphonic) pickup. The building blocks are as follows:

GUITAR IN ---> COMPRESSION --> FUNDAMENTAL DETECTION ---> PERIOD/ZERO CROSSING DETECTION ---> PERIOD TO VOLTAGE ---> VOLTAGE processing

1. Compression: You need to "level out" the amplitude of the waveform so it's constant. This is easy.

2. Fundamental detection: You want to remove the harmonics and stay with the fundamental of the guitar. For this the mediocre way is a simple low pass filter. The good way is to use an adaptive filter that "locks" to the guitar note. I will find some schematics, they come from this guitar synth:

"Electro Harmonix Guitar Synth"
hammer.ampage.org

Download the schematics, lots to learn. BTW, the component values for the adaptive filter are wrong, and the pinout of the opamps in that section are wrong, that's why i need to find my notes. Also, this is a guitar synth that uses a PLL, that is its achiles heel.

3. Zero crossing detection: You want to find out when the sinewave crosses zero, or in other words, detect where a cycle begins and end. This is as simple as doing zero crossing detection, since you already have the fundamental filtered after step 2.

Observation: Step (2) introduces a time lag, some Moog patents exist for a period detector that does not require filtering out the fundamentals, with a corresponding speed increase. However, the time lag in (2) was of no consequence in my opinion.

The output of this step is a pulse whenever a cycle start.

4. Period to voltage... You can do it:

a. The bad way: Using a tachometer circuit (that is: use the pulses of step 3 to trigger a fixed-duration pulse, and add a low pass filter at the output), this will give you a voltage linearly proportional to frequency. THIS IS A BAD APPROACH, IS TOO SLOW. Sadly, almost all the "do it yourself guitar synth" circuits uses this approach.

b. The mediocre way: Use a PLL to "lock" to the frequency of the pulses, this will give you a voltage proportional to frequency. I'm not an expert in PLL, but the Electro Harmonix uses this scheme. The problem is the PLL filter used there: If you want fast detection, then the PLL should be quick acting, which in the case of this particular synthesizer, gave trembling pitch; if you smooth out the PLL filter you have steady, in-tune pitch but slow detection. The EH Guitar Synth uses a switch that puts the PLL filter in "fast mode" when the guitar string is plucked and then after about 20ms, it puts it in "slow mode" so the pitch stays steady...

... the problem is that this requires reliably detecting the start and stop of the notes and this is a problem onto itself.

Anyways, this gives you a voltage linearly dependent to the FREQUENCY of the fundamental.

c. The best way: Using a ramp-sample/hold circuit. You need a constant current source, and with this current source you charge a capacitor (the voltage ramps up linearly on the capacitor). With each pulse from step (3), you "reset" (discharge) the capacitor so it starts charging again, with the next pulse, you sample the voltage value (using a simple sample-hold), and this is a voltage linear to the PERIOD of the fundamental.

d. (The modern way --- count the period digitally and do the rest in the digital domain, midi, etc.) BORING...

5. Voltage processing:

If you have a voltage depending on the period, you have to convert it into a voltage depending of the frequency. For this you need an inverter (that is, a circuit that does the 1/x function). Such a circuit is not so easy to do since it needs temperature compensation or else the pitch will drift.

Anyway you finally get a voltage LINEARLY depending on the frequency. To drive normal analog synths you need to output a signal that is 1V per octave. Which means that it is not linearly dependent on the frequency...

So you need a log converter, who does the Log(x) function. This also needs temperature compensation.

The alternative is to create the syntesizer part yourself and use VCOs that take a linear voltage. This is what is done on the EH Guitar Synth.

The other alternative, if you use approach (4c) is to have sawtooth VCOs done using ramp-sample/hold circuits, and use the voltage obtained in (4c) as the "top limit" of the ramp, then change the slope of the ramp [and thus the frequency of the output waveform] by changing the charging current of the capacitor. With this, you can have many VCOs that can be tuned as you like [x octaves up, down, etc], and track perfectly and musically to your guitar. This is exactly what it's done in the Roland GR-300 (google it.)

The only problem is that the volume of your waveform VCOs will vary with frequency!! So you need to compress the output. This was done on the GR-300 using simple diodes as limiter.

A further problem if you go this route is that your opamps have to be perfectly linear over a wide scale of voltage to cover all the fretboard, otherwise the output will be out of tune at the highest part of the fretboard (high pitch = low period = low control voltage). For the GR300 this is not a problem since it treats each string independently (using a hexaphonic pickup), so the range that needs to be tracked is small.

VOILA!! Have fun.
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Old 7th December 2010, 11:00 PM   #13
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Now, my above response assumed you only had 1 normal guitar pickup and want the synth to be monophonic.

If you want to go polyphonic, you need an hexaphonic pickup. The advantages are:

1. Polyphony!!

2. Fundamental detection is easier since the range of frequencies that you are working with is narrower, even a simple low-pass-filter can be acceptable.

BTW when you pick up a guitar string you first get a strong fundamental and then the 2nd harmonic starts to appear. The GR-300 takes advantage of this using an "adjustable" low pass filter that, once the fundamental is locked, "adjusts" the LPF's cutoff so no subsequent harmonic distracts the converter. Basically the LPF in the GR-300 only has two cutoff frequency: low and high. "Low" filters out the upper half of the fretboard for the string.

3. Adjacent strings that are accidentaly plucked pose no problem.

4. Less voltage precision needed in your circuits since the frequency range for each string is narrow.

Drawbacks:

1. You need to have SIX of everything.

2. ... So you have to simplify everything, also the audio part of your synthesizer would have to be either (a) very simple, or (b) huge.
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Old 7th December 2010, 11:22 PM   #14
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Ok, so let's continue with "everything you wanted to know about guitar synths but were afraid to ask"...

Some classic analog guitar synths examined:

ARP Avatar: This one was a flop because it supposedly tracked terribly bad. It used a hexaphonic pickup, a "selector" that detects which string is the one you are plucking, and selects the audio of said string. Then a "digital" frequency counter that finds out the frequency.

You don't want to check out the schematics.
Sound Samples: Don't bother searching for them.

Roland GR-500: I think i had the schematics but couldn't understand how the pitch to voltage was performed. It also uses a string selector (the proper synth part is monophonic). This is IMHO the most cool of the guitar synths.

Sound Samples: There is a cool japanese guitar player, Ryu [insert last name], that has some youtube videos using the GR500. It sounds interesting. Jeff Beck also used it, but haven't heard recordings of him with the GR500. Sadly, because he's my favorite guitar player.

Roland GR-300 came later than the GR-500 and it's fully polyphonic. This is used by Pat Metheny with great success. It uses a hexaphonic pickup, and a ramp-sample/hold converter that drives sawtooth VCOs using directly the linear, period-proportional voltage from the converter. It tracks quickly and wonderfully. It is a benchmark in this regard.

Sound Samples: Guitar god Pat Metheny has used it extensively. Listen to "Song For Bilbao", "Language of Time", "Are you coming with me?", "Third wind", etc.

Roland GR-700 came later than the GR-300 and as far as i know uses a digital frequency counter. For some reason, probably because the processing needed to filter out the fundamental and count the period reliably, it suffers from a bit of lag. The good part is that the synth part of this machine is KILLER, with excellent sounds. To get a stronger fundamental frequency from the strings, it required an odd-looking special guitar with had a stabilization bar (google it).

Sound Samples: On youtube.

Electro Harmonix Rackmount Guitar Synthesizer is monophonic and it's peculiarlity is that it uses the normal output of the normal, common electric guitar. Uses a PLL, as described above. It allows you (and frankly requires to) mix a bit of guitar-distorsion-sound from your guitar with the synthesizer sound, because the synth part is a bit slow (the PLL is a bit slow). The good thing is that the synth (a) does not require clean plucking technique (= tracks well), and (b) does not require a special pickup. The drawbacks have already been discussed.

Sound Samples: The middle solo in the song "Wildest dreams" by the great guitarist Steve Howe on the awful group "ASIA".

BTW do not confuse this synth with the "electro harmonix micr0 gu!t@r synth" which is nothing else than a standard fuzz box with a killer-sounding 4-pole VCF, a crude octave-down (using flipflops), and a crude octave up (using diodes). I've built one too. It's fun but too limited.

/ 360 Systems Polyphonic Guitar Syntesizer: This was the earliest (1976?) polyphonic guitar synth. Uses an hexaphonic pickup, and AFAIK uses the ramp-sample/hold method. It was HUGE and really cool.

Sound Samples: "Captain Fingers" by Lee Ritenour. Also there is a group called "Symphonic Slam" with Timo Lain, who supposedly was the first guitarist to use this mammoth synth. The record is in the public domain, download it! It has a lot of guitar synth on it. VERY COOL!

360 Systems Slavedriver : Mono pitch-to-cv using the same principle, requiring an hexaphonic pickup.
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Old 7th December 2010, 11:32 PM   #15
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NED (New England Digital) Synclavier Digital Guitar also known as "Synclavier Guitar Interface": Used an analog fundamental detector for which there is a patent, and then the rest of the processing is digital. Suffered from a bit lag on the lowest (and thus, longer period) strings. It connects to the Synclavier. If you don't know what's a Synclavier, google it now, because that's a wonderful machine.

Sound Samples: Again, Pat Metheny uses it. He also has a modified guitar for the GR-300 that can drive the GR-300 and the Synclavier GUitar Interface simultaneously. Why? Because he can afford it.

finally... entering the modern era:

Roland GR-33 and others: Have no idea how they do it internally but they are still limited in speed at the lower (thickest) guitar strings, because the period of the waveform is longer.

If you need SPEED you need to use other approaches, such as:

Synthaxe: Google it and write back to me what do you think about it. I want one.

AXON guitar synth: Now this is the state-of-the art. When you pluck a string, before the musical note sounds, there is a pulse train (heard as plucking noise) that is actually related to the length of the string (= position of the finger in the fretboard), and to the place where you plucked the string.

This pulse train/"plucking noise" is processed digitally by a neural network that spits out (a) what is the pitch of the note, and (b) where did you plucked the string (i.e. near the bridge, near the neck). And it finds out this actually BEFORE the sound starts to come out of the guitar. SIMPLY AMAZING!!

Sadly, seems that they haven't sold too many units.
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Old 8th December 2010, 01:34 PM   #16
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This is the patent for the NED Synclavier Guitar Interface period detector, instead of detecting the zero-crossings, it detects the peak of the waveform and "memorizes" the last peak.

Pitch extractor apparatus and the like
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Old 8th December 2010, 01:50 PM   #17
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This is the patent of the Roland GR-300, check out the "variable bandpass filter":
Guitar synthesizer

The sample&hold circuit is very simple in this synth.
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Old 8th December 2010, 09:01 PM   #18
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thanks for the infomation flavio, been really busy today so only had time to skim read it. Going to print it all out and have a good read over it tomorrow. From what i can gather some really useful stuff there, although im sure that some of it is going to be beyond my current level of understanding so going to have to have a read elsewhere to get my theory up to scratch!

I have just ordered another LM331 (think my old one might of been damaged) Hopefuly should be here tomorrow or friday. I know you have said the tachometer is the worst way to do it, but if i get this working with a few other parts of the puzzle then at least I could have something working (albeit slow) Then I can impove the designs? Hopefuly getting onto a sample & hold method.

I have until around april to have this done, although a semifunctioning prototype would be good by the new year.

Will get on to answering your questions, just wanted to reply now since you have put alot of effort into providing me with someuseful info!
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Old 8th December 2010, 09:04 PM   #19
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By the way my intention is just to use the standard guitar pickups so monosynthesis is my current ambition
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Old 10th December 2010, 03:50 PM   #20
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Don't use the LM331, you'll waste your time. Go for the ramp sample/hold approach or the PLL approach (if you have good PLL theory).
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