A quick answer before I hit the sack, I'm sure more details will emerge
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Originally Posted by sctt_stone  1 How does CV work? |
It's a voltage (electric potential difference between two points) measured in Voltage (V), in most synthesizers from -5 V to +5 V, or 0V to +10V depending on the circuit.
That means the electrons are going this or the other way and what shows up on a measuring device is the difference between two points (not the amount of electrons travelling).
For instance, the green line represents the DIRECTION of electrons measured in Voltage (Y axis) over time (X axis). This picture is a triangle wave. It's called that because of the shape. Others are Square, Saw, and Sinus.
This voltage is not the current. Current (symbol I) is measured in Ampere (mAh in synths not that much), this is the rate of the flow of electric charge (the amount) - that's why the Amperage will kill you if you cause a short, not the voltage
Say a module requires 200 mAh in current. That means the powersupply must be capable of keeping a steady push of this amount of power.
There's Direct Current (DC) and Alternating Current (AC) which was invented by Nicola Tesla b.t.w.. DC is a one direction stream of electrons. From 0 to 10V. Alternating current can then be negative, then be positive, since it travels back an forth.
Using a so called "rectifier" one can "flip" the negative half of the AC to the positive and thus it becomes DC since it doesn't go through 0 Volt and only travels one way, not back and forth. Some circuits require DC, and so a rectifier is placed in the path of the electrons coming in.
Electronic circuits have been made that can take variable voltage (be it DC or AC) and with it processes inside this circuit can be changed. Take for instance a VCA circuit. It's a voltage controlled amplifier. It amplifies (mostly through the use of transistors) as much as there's a control voltage coming in: Feed it more CV, then the audio on the input will be amplified more at the output.
Now here's the kicker:
That stream of electrons going back and forth can be manipulated. The difference can be changed slightly or radically. This is called "modulation".
Modulation can be AM - Amplitude modulation - how big the difference, the Voltage is can be changed. And it can be FM - Frequency modulation - which is how often the electrons reverse polarity; change direction.
Look at a basic Oscillator. It outputs a sinus. By changing the amount of FM there will be more Oscillations. This output can be used as CV (control voltage) to drive another circuit (like an LFO - low frequency oscillator), which makes the sound "wobble" when it's fed to a VCA. The DIFFERENCE between polarity changes faster or slower in time. (think Skrillex
)
By AM (amplitude modulating) the sinus driving the VCA it can be made bigger or smaller (on the oscilloscope screen) wich means the VCA amplifies less (small sinus CV) or more (large Sinus).
Most VCA's can take AC but the parts (operation amplifier or OpAmp) require DC.
Now, the osciallator is set that it outputs a sine with a very high rate FM. And this output is connected to a speaker.. which makes it into a tone.
How much FM is put on the oscillator determines the tone. this must be quite precise, or the oscillator will sound flat.
There are two basic standards and they're how much voltage in FM is set to the Oscillator results in what frequency in air pressure (through the speaker) and for our ear/brain in what kind of pitch. Basically they divided the hearable range of notes by the amount of voltage. Low voltage FM results in a low note. Every scale needs a middle point and that is in modern western tuning the frequency of 440 Hz.
Then there's the Hz/Volt method wich is practically only used by Korg on the MS-10 MS-20 and by freaks like Pigtronix on some of their effect pedals.
Here's how it's arranged in the 1V/Octave PITCH CV system:
Note---V--------
A1 1.000
A2 2.000
A3 3.000
B3 3.167
C4 3.250
D4 3.417
E4 3.583
A4 4.000
A5 5.000
As you probably understand by now... audio is voltage too, inside an analogue synthesizer. So... that means that audio can be modulated as well. Or filtered, or flipped with a rectifier (since it's normally AC) or folded (so the FM is changed) etc. etc.
Bitcrushers chop off the top of the waveform. Since it's then more resembling a square wave, there's a more sudden difference between amplitude (than a gentle sloping sinus) and thus more overtones (it has to change suddenly). In a speaker more distortion means a more complicated waveform is generated. These overtones in the square wave inside the synth are in harmonic relation with the base frequency, and have different order. Our ears hear these orders differently and can differentiate. Clipping opamps or converters are generally percieved as nasty. Tubes and saturating transformers create other overtones yet again (2nd and 3rd order).
A low pass filter smoothes out these sudden changes, and turns the square back into a sinus. Thus the overtones are filtered out. And we only hear bass, since the overtones are higher frequency, and thus higher pitch. Violà.
We hear DC too with our ear/brain. Here's an example how a variable rectifier circuit (circuit is a square of rectifiers) can do fun things with audio:
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Originally Posted by sctt_stone 2 Why are the voltages different from one brand to another? |
Buchla was/is one of the great pioneers of modern synthesizers, which came along with the invention of transistors. He invented and made stuff for NASA, but then lost interest in boring megalomaniacs with fascination for huge metallic fallusses (rockets) and started inventing and making synths. He came up with a 1,2 V per octave system not unlike above 1V/Oct.. So... when others joined the party and the synthesizer system invented by the aimable Bob Moog, and a little later Serge Tcherepnin, as well as the mad geniusses at EMS (and others) was recieving traction (through use by rock musicians) Buchla stuck with his 1,2V per Octave one, to ensure backwards compatibillity. Why would he change? It worked fine.
Korg came up with the Hz/Volt system because it was more linear and early oscillator designs tracked pitch much better with this system.
Now we have temperature stable oscillators and the Hz/Volt system is mostly obsolete. It can be generated with midi-cv interfaces like the Kenton so there's not really any problem.
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Originally Posted by sctt_stone 3 What is Gate, Trigger, pitch, etc? |
Gate is a square wave that tells a module to do something as long as the voltage is high. Instead of that it looks at the rising edge (the left side of the square on the oscilloscope), it simply needs acontinuous positive voltage. It is often used to open an envelope generator, which is a circuit to introduce a voltage curve that roughly mimics the amplitude of when a note on an acoustic instrument is struck, like a piano (more CV at the envelope output routed to the VCA means that there's more amplification of the audio signal passing through the VCA)
A trigger is a short pulse, the circuit connected reacts on a positive voltage ramp up and then starts doing something. Often a square can fool a trigger circuit, but sometimes not, it needs something with more amplitude. Muy Complicada.
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Originally Posted by sctt_stone 4 What else can be controlled through CV? |
everything that has a CV input. Often there's an attennuator (which is a circuit with variable resistor - a potentiometer - a pot) that allows the amplitude of the CV signal to be adjusted (you'd want a little bit of that LFO going into your filter, since you are not Skrillex
) Also one can have a so called "attenuverter" which also can introduce a negative voltage, so the electrodes (to go back to the basic idea of electrons travelling back and forth) are going one way (back) more. That sounds complicated but in practical terms it means you're just giving the module a differently formed signal... On the scope the waveform now has more amplitude downward.
That's obviously different than adding (or substracting) a fixed amount of voltage. On the scope the whole waveform, intact then just moves up or down on the Y-axis.
It's useful to have these (mundane) processes in modules in the synth (apart from all the flashy filters, waveshapers, folders and shifters) since it's these basic building blocks that give more control over the whole process. The simple modules of Doepfer are therefore often overlooked.
Oh yeah.. stuff can have a delay of course too. Then the waveform is shifted as a whole over the X-axis, just like moving audio files on a timeline in a DAW. This goes as much for audio as for CV, it's basically the same soup.
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Originally Posted by sctt_stone 5 Are CV jacks simply attached to a straight wire soldered to a trace on the circuit board or is there some sort of chip or conversion process in the line? |
CV Jacks are just like unbalanced jacks, they have one voltage rail and a ground. The ground is connected to the chassis (jacks) or theres a seperate ground when banana plugs are used. The electrons travel throug the cable and into the module's circuit and back again if it's AC, and the two connected modules become ONE circuit, through the patch cable. So patching means you connect different parts of the circuit together to form one big one. this can be an audio path (which then has to be connected to a mixer and then speaker) or a CV path which has a party in the module connected and then basta, off to ground.
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Originally Posted by sctt_stone 6 Can any old analog synth be tricked out with a CV jack for every function if space allows? If no, why not? If yes I hope the answer to number 5 is a yes  |
Hmmm it depends. Basically.. yes, but some circuits are designed in such a way that they expect anotether part of the circuit next to it. They need a load, in case of power amplifiers for speakers suddenly switched to nothingness that can be troublesome. So this is best left to technicians who have done some research on that particular synth. Old Arp Axxe's are good candidates for this type mods, since they were fairly cheap and not so picky...
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Originally Posted by sctt_stone 7 Why CV? What are the pros and cons vs midi and are there other options that never really gained in popularity? |
midi is a machine language. CV is just voltage. In the case of CV it depends totally what kind of parts of the circuit (modules or external gear) can be connected for what the CV actually does. In the case of midi there's a little receiver and sender on each end that en/decode the serial messages (little voltage blips). This can be lots of things depening what message code is sent.
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Originally Posted by sctt_stone 8 When you have a synth that accepts both CV and midi is there any audible difference between the method of control that affects the character of the synth? An example would be the Oberheim Xpander. Will there be less stepping when controlled through CV? |
yes. since voltage is at the speed of light, and on molecular level (electrons moving back and forth) the effect on a connected circuit is inmediate. Changes will be handled by the circuit on the molecular level as well. There is a very fine resolution. Midi is not. It's 127 little steps but can be decoded in multiples of this (and smoothed out) into a CV. Just like digital audio in a DAC. Here's a page about midi:
http://home.roadrunner.com/~jgglatt/ Quote:
Originally Posted by sctt_stone Thank you, |
yerrr welcome. no one wrote this down here before, and I thought it needed to be done.
please note that it's night time, I'm off to bed, and that I can have made mistakes that need correction or further explanation?
I hope others will expand on this, and further explain 1-2--4 pol filters. Overtones and fundamentals, and what happens when something is slightly out of phase (unison).. etc. etc.
perhaps use videos of existing modules to make it palatable? the intarweb is way more suited for this than a book.
Oh yeah, read up on electrical circuits? I'll post some book titles tomorrow.
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