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Friday, November 30, 2007
In the Galaxy
YouTube via katafeyka. Sent my way via fabio.
"Dmitriy and Katafeyka studio." Anyone make out the soft synth on the screen?
Update via Monokit: "uses Propellerheads Reason, which you can better see in his other video:"
Music sound.
Sequential Circuits Prophet VS Vector-Synthesizer (audio)
YouTube via retrosound72.
"audio demo of the SCI Prophet VS Vector Synthesizer by RetroSound. more info: www.retrosound.de"
David Rogoff on VCOs
David Rogoff sent the following into the Yamaha CS80 list. I asked him if I could put it up and he gave me the OK.
"This touches on a big, somewhat technical, issue of what kind of VCOs the CS80 uses. The VCO III chip is a linear VCO, sometimes called Hz/Volt, as opposed to the more common exponential (Volts/Octave) VCOs (e.g. MiniMoog, Curtis & SSM chips in SCI and Oberheim polys).
Here's a pretty good explanation: link
Here's a (I hope) quick one:
The most basic VCO is a sawtooth one, which can be a capacitor charged by a current. For non-EE types, here's my modified toilet analog (and you though the Metasonix vacuum-tube VCO was weird) : The capacitor is like the water tank of a toilet. The water filling it up is the current. The height of the water is like the voltage across the capacitor. Now, modify the float valve so that when the tank is full it automatically flushes. Then the cycle starts again. If you double the water filling rate ( = double the current), you double the frequency of the flush cycles.
The is a basic, linear VCO (actually Water-CO). It shows a couple of things. First, it's not actually voltage controlled, but current controlled. Ignore that for now. Also, the filling time is adjustable, but the discharge/flushing time is fixed. This is an issue with all sawtooth VCOs and is why many (e.g. Moog) VCOs have a high-frequency-tracking adjustment, which helps cancel this out. Here's the CS80 VCO: link
Ok, so why don't all synths use linear VCOs? As the above link explains, human ears don't hear frequency linearly. A above middle C is 440Hz. An octave about is 880Hz, or double the frequency. The next octave would be 1760Hz: double that. If you graph this, it's an exponential curve. So, the space (in Hertz) between two notes keeps getting bigger as we get to high pitches. If you had a modular synth with linear VCOs (like that old Paia), the top key might output 5 volts. One octave down would be 2.5volts. The next 1.25volts, followed by 0.625v and 0.3125v. This is a pain to generate. Also, as you get to lower notes, smaller voltage inaccuracies start becoming bigger pitch errors to our ears.
To avoid all this, someone (anyone know who? Dr. Bob? Tom Oberheim? Don Buchla?) came up with exponential VCOs. Basically, they're just a linear VCO with a circuit in front of them called (big surprise) an exponential converter. This is just a circuit that takes a linear input (1volt/octave) and outputs the doubling voltage (actually current...) that the VCO wants. Now, everything is simple.
So, why did Yamaha go for the linear? Two reasons, I'd guess. First, adding the exponential converter to each VCO adds more cost to the chips, since there's more circuitry. A bigger issue is temperature stability. As we've been talking about lately, all circuits are affected (i.e. knocked out of tuning) by temperature changes. The exponential converter, for reasons I won't go into, is really sensitive to this. People have been complaining about the tuning stability of the CS80, but it's rock solid compared to any poly-synth with exponential VCOs (P5, OBX, A6, etc). They all need computer-controlled auto-tuning routines to have any chance of staying in tune.
So, what issues/problems/advantages does the CS80 having linear VCOs create?
Good things:
1) modulation - linear vibrato sounds a bit different than v/oct vibrato, probably closer to acoustic vibrato (e.g. violin). Also, as the modulation speed increases, you start getting into F.M. land, which requires linear modulation (you don't want to know the math!). This is why some modular VCOs have linear FM inputs in addition to the normal v/oct controls.
2) sweep to D.C. - my favorite. If you start a pitch bend at the right end of the ribbon and slide all the way to the left, the pitch of the VCOs all go down to 0Hz / D.C. / flat-line. This is because the input to the VCOs goes to 0 volts and the frequency equals the voltage times a constant. With a exponential VCO this is impossible. Going 1 volt less on the control input goes down one octave. Mathematically, you can't get to zero Hz. You'd need to input -infinity volts! Also, many other limitations in the circuit block the VCO from even getting close. Big win for linear VCOs!
Bad things:
1) Keyboard voltages - as I wrote above, the keyboard has to generate exponential voltages. This is a big pain. In a digitally-controlled analog (like the CS80, P5, etc), the keyboard voltage comes from a DAC (digital-analog-converter). 99.99% of DACs are linear. The CS50/60/80 (and others in the family) have bizarre, custom exponential DACs. This makes interfacing the CS80 to other synths and/or MIDI-CV converters a pain.
2) CV mixing. Finally, we get to the original question of adding a pitch-bend input to the CS80. In the volts/octave world, everything is easy: you just add voltages together. Adding voltages is simple to do - just an op-amp and a few resistors. Let's say you had the following voltages come out of a v/oct keyboard: 1v, 2v, 4v. This could represent a low C (c1), C one octave up (c2), and C two octave above that (c4). To make it simple, let's say we have a pitch wheel or pedal add 1 volt to this (2v, 3v, 5v). This would be c2, c3, c5, so we've just transposed the sequence up an octave.
Ok, what happens if we try this with a linear voltage. For the same c1, c2, c4 notes, we might have 1volt, 2volt, 8volt. Adding one volt gives 2volt, 3volt, 9volt. The first note is correctly up an octave, but the next is only up about a 5th and the third note is only transposed up about a semitone. This, obviously, doesn't work. What we need to do, instead, is multiply the voltages. To transpose up an octave, double the voltages. To transpose down an octave, halve them. This is easy for a fixed transpose, but if you want a variable, like a pitch-bend pedal input, you need to multiply voltages. Just like it's much, much easier for people to add and subtract than multiply and divide, so it is for analog (and digital) circuitry.
If you follow the schematics or block diagram of the CS80, you can see that the voltage to the VCOs comes through a long chain of multiplications. The ribbon is actually the initial voltage source for the whole instrument. If the ribbon isn't pressed it outputs some fixed voltage (not sure the actual value - call it 2 volts). If the ribbon is slid up, all the way, from the left to the right, it would output double this voltage, which corresponds to one octave up. If the ribbon is slid the other way, it outputs zero volts, as mentioned above. Next, the voltage is sent through the concentric pitch knobs. Any normal potentiometer is a voltage multiplier, which can multiply the input by anything from zero to one.
This voltage then becomes the reference input to the exponential DAC on the KAS board, which multiplies it by it's exponential resistor network to create the CVs for each of the either voices. These voltages go to the VCO chips on the M-Boards. Are we done - nope - one more CS80 weirdness. In a v/oct synth, the octave/foot switches would just generate a voltage that would be added to the keyboard CV (e.g. MiniMoog). The CS80 VCO, instead, has a special footage input that needs an exponential current for each feet setting. Because this is difficult to do accurately over a wide range, we end up with the wonderful VR4, VR5, and VR6 trimmers to get the feet switching calibrated separately for each of the 16 VCOs. Yuch!
Getting back to the original question (remember Alice? There's a song about Alice...), a pitch bend input would need to control a voltage multiplier. This could be an added circuit, after the ribbon circuit, or could probably be merged with the ribbon voltage. I haven't figured out the details, but it's not rocket science. However, it is a lot more work than it would be on something like a Prophet 5.
Ok, I guess that wasn't quick, but at least I didn't have an graphs or get into transistor curves or Bessell functions.
David"
"This touches on a big, somewhat technical, issue of what kind of VCOs the CS80 uses. The VCO III chip is a linear VCO, sometimes called Hz/Volt, as opposed to the more common exponential (Volts/Octave) VCOs (e.g. MiniMoog, Curtis & SSM chips in SCI and Oberheim polys).
Here's a pretty good explanation: link
Here's a (I hope) quick one:
The most basic VCO is a sawtooth one, which can be a capacitor charged by a current. For non-EE types, here's my modified toilet analog (and you though the Metasonix vacuum-tube VCO was weird) : The capacitor is like the water tank of a toilet. The water filling it up is the current. The height of the water is like the voltage across the capacitor. Now, modify the float valve so that when the tank is full it automatically flushes. Then the cycle starts again. If you double the water filling rate ( = double the current), you double the frequency of the flush cycles.
The is a basic, linear VCO (actually Water-CO). It shows a couple of things. First, it's not actually voltage controlled, but current controlled. Ignore that for now. Also, the filling time is adjustable, but the discharge/flushing time is fixed. This is an issue with all sawtooth VCOs and is why many (e.g. Moog) VCOs have a high-frequency-tracking adjustment, which helps cancel this out. Here's the CS80 VCO: link
Ok, so why don't all synths use linear VCOs? As the above link explains, human ears don't hear frequency linearly. A above middle C is 440Hz. An octave about is 880Hz, or double the frequency. The next octave would be 1760Hz: double that. If you graph this, it's an exponential curve. So, the space (in Hertz) between two notes keeps getting bigger as we get to high pitches. If you had a modular synth with linear VCOs (like that old Paia), the top key might output 5 volts. One octave down would be 2.5volts. The next 1.25volts, followed by 0.625v and 0.3125v. This is a pain to generate. Also, as you get to lower notes, smaller voltage inaccuracies start becoming bigger pitch errors to our ears.
To avoid all this, someone (anyone know who? Dr. Bob? Tom Oberheim? Don Buchla?) came up with exponential VCOs. Basically, they're just a linear VCO with a circuit in front of them called (big surprise) an exponential converter. This is just a circuit that takes a linear input (1volt/octave) and outputs the doubling voltage (actually current...) that the VCO wants. Now, everything is simple.
So, why did Yamaha go for the linear? Two reasons, I'd guess. First, adding the exponential converter to each VCO adds more cost to the chips, since there's more circuitry. A bigger issue is temperature stability. As we've been talking about lately, all circuits are affected (i.e. knocked out of tuning) by temperature changes. The exponential converter, for reasons I won't go into, is really sensitive to this. People have been complaining about the tuning stability of the CS80, but it's rock solid compared to any poly-synth with exponential VCOs (P5, OBX, A6, etc). They all need computer-controlled auto-tuning routines to have any chance of staying in tune.
So, what issues/problems/advantages does the CS80 having linear VCOs create?
Good things:
1) modulation - linear vibrato sounds a bit different than v/oct vibrato, probably closer to acoustic vibrato (e.g. violin). Also, as the modulation speed increases, you start getting into F.M. land, which requires linear modulation (you don't want to know the math!). This is why some modular VCOs have linear FM inputs in addition to the normal v/oct controls.
2) sweep to D.C. - my favorite. If you start a pitch bend at the right end of the ribbon and slide all the way to the left, the pitch of the VCOs all go down to 0Hz / D.C. / flat-line. This is because the input to the VCOs goes to 0 volts and the frequency equals the voltage times a constant. With a exponential VCO this is impossible. Going 1 volt less on the control input goes down one octave. Mathematically, you can't get to zero Hz. You'd need to input -infinity volts! Also, many other limitations in the circuit block the VCO from even getting close. Big win for linear VCOs!
Bad things:
1) Keyboard voltages - as I wrote above, the keyboard has to generate exponential voltages. This is a big pain. In a digitally-controlled analog (like the CS80, P5, etc), the keyboard voltage comes from a DAC (digital-analog-converter). 99.99% of DACs are linear. The CS50/60/80 (and others in the family) have bizarre, custom exponential DACs. This makes interfacing the CS80 to other synths and/or MIDI-CV converters a pain.
2) CV mixing. Finally, we get to the original question of adding a pitch-bend input to the CS80. In the volts/octave world, everything is easy: you just add voltages together. Adding voltages is simple to do - just an op-amp and a few resistors. Let's say you had the following voltages come out of a v/oct keyboard: 1v, 2v, 4v. This could represent a low C (c1), C one octave up (c2), and C two octave above that (c4). To make it simple, let's say we have a pitch wheel or pedal add 1 volt to this (2v, 3v, 5v). This would be c2, c3, c5, so we've just transposed the sequence up an octave.
Ok, what happens if we try this with a linear voltage. For the same c1, c2, c4 notes, we might have 1volt, 2volt, 8volt. Adding one volt gives 2volt, 3volt, 9volt. The first note is correctly up an octave, but the next is only up about a 5th and the third note is only transposed up about a semitone. This, obviously, doesn't work. What we need to do, instead, is multiply the voltages. To transpose up an octave, double the voltages. To transpose down an octave, halve them. This is easy for a fixed transpose, but if you want a variable, like a pitch-bend pedal input, you need to multiply voltages. Just like it's much, much easier for people to add and subtract than multiply and divide, so it is for analog (and digital) circuitry.
If you follow the schematics or block diagram of the CS80, you can see that the voltage to the VCOs comes through a long chain of multiplications. The ribbon is actually the initial voltage source for the whole instrument. If the ribbon isn't pressed it outputs some fixed voltage (not sure the actual value - call it 2 volts). If the ribbon is slid up, all the way, from the left to the right, it would output double this voltage, which corresponds to one octave up. If the ribbon is slid the other way, it outputs zero volts, as mentioned above. Next, the voltage is sent through the concentric pitch knobs. Any normal potentiometer is a voltage multiplier, which can multiply the input by anything from zero to one.
This voltage then becomes the reference input to the exponential DAC on the KAS board, which multiplies it by it's exponential resistor network to create the CVs for each of the either voices. These voltages go to the VCO chips on the M-Boards. Are we done - nope - one more CS80 weirdness. In a v/oct synth, the octave/foot switches would just generate a voltage that would be added to the keyboard CV (e.g. MiniMoog). The CS80 VCO, instead, has a special footage input that needs an exponential current for each feet setting. Because this is difficult to do accurately over a wide range, we end up with the wonderful VR4, VR5, and VR6 trimmers to get the feet switching calibrated separately for each of the 16 VCOs. Yuch!
Getting back to the original question (remember Alice? There's a song about Alice...), a pitch bend input would need to control a voltage multiplier. This could be an added circuit, after the ribbon circuit, or could probably be merged with the ribbon voltage. I haven't figured out the details, but it's not rocket science. However, it is a lot more work than it would be on something like a Prophet 5.
Ok, I guess that wasn't quick, but at least I didn't have an graphs or get into transistor curves or Bessell functions.
David"
Oberheim Double SEM Modular
images via this auction via Tec."For auction worldwide a pair of Oberheim SEMs in a solid oak box draw with modularised jack connections to the board points.
Powered by an original Four voice supply,set for 230V use.
Many of the 1/4" jacks are normalised such that the MASTER CV will input on all four VCOs and also the MASTER GATE will input both modules.Inserting to individual jacks will break the master connection to that part.
Jacks in for VCO1:CV1 CV2 SYNC ,VCO2: CV1 CV2 SYNC, VCF:CV1 CV2 ,VCO1 : MOD, VCO2: MOD ,VCF:EXT INPUT 1 & 2 ,VCF MOD ,ENV1 GATE,ENV2 GATE and VCA CV.
Jacks out for VCO1 SYNC OUT,VCO2 SYNC OUT,VCO1 SAW PULSE,VCO2 SAW PULSE,VCF HP BP LP,ENV 1 OUT,ENV 2 OUT,VCA OUT.
28 jacks.....per SEM.56 total.The four VCF CVs have attenuation pots with the same production grey knobs.
The jacks are seated on 3mm Perspex acrylic with card paper backing for decals.It would not be hard to replace if you have an aversion to yellow.
The SEMs are in good cosmetic condition...unfortunately one dual pot cap is missing on the right hand module.
I built these into the box before i started on my modular build and as that took over this piece was put in storage for some years.I brought it out to sell and we must consider that the modules are 30 years old.I've tested both modules by the master cv/gate and both VCOs are outputting saws/pulses/syncs.
Some pots could certainly do with a clean.Filter outputs and envelopes are working.I have not checked the alternate inputs but i don't see why they shouldn't be working.
The main niggle is the right hand LFO isn't working(probably a dead 741 or the jfet)but since it was a simple device anyrate and i had plenty of better LFOs offboard i never repaired it."
New & Back In Stock at Analogue Haven
I just got my subscription email from Analogue Haven. In it was a slew of new and re-stocked goodies, some of which I have not seen before, including this Diamond Memory Lane Delay Pedal."As you can probably guess already, this isn't your typical analog delay.
the memory lane has a number of innovations that bridge the feature gap between analog and digital delays while retaining the warmth and unique sound of bucket brigade delay devices in an all analog signal path.
offering up to 550 ms delay time, tempo can be set either by dial or by foot tap switch. when you don't need foot control of tempo, you can use the second foot switch as an on/off switch for the modulation feature simply by switching the small 'feature' toggle switch from 'tap' to 'mod'."
Scroll to the news section on Analogue Haven's main page for the list of new and back in stock items.
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MATRIXSYNTH - EVERYTHING SYNTH
© Matrixsynth - All posts are presented here for informative, historical and educative purposes as applicable within fair use.
MATRIXSYNTH is supported by affiliate links that use cookies to track clickthroughs and sales. See the privacy policy for details.
MATRIXSYNTH - EVERYTHING SYNTH
