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DCO has hard reset on normal analog expo VCO?
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hedefalk



Joined: Aug 29, 2017
Posts: 51
Location: Stockholm, Sweden

PostPosted: Wed Nov 22, 2017 5:17 am    Post subject: DCO has hard reset on normal analog expo VCO? Reply with quote  Mark this post and the followings unread

I just had a thought about VCO/DCO's that I haven't been able to google satisfactory. I guess there is nothing novel and that this idea has either been implemented or rejected, but I'm noob and suck at googling Smile No really, I just want to ask here since I have got so great answers before Smile

Anyway, I've been getting the feeling that it's a bit of a mess with temperature stability of analog VCO's in the expo converter. So I searched the webs about doing just that digitally and found the term DCO. So the idea there is to have a high freq (many MHz) clock that you subdivide to the asked freq and use this as the pulse for resetting the timing cap. However, the wikipedia article then go on:

"For a given capacitor charging current, the amplitude of the output waveform will decrease linearly with frequency. In musical terms, this means a waveform an octave higher in pitch is of half the amplitude."

Sure, of course. Then:

"In order to produce a constant amplitude over the full range of the oscillator, some compensation scheme must be employed. This is often done by controlling the charging current from the same microprocessor that controls the counter reset value."

If I understand, that is whan the Juno does: https://electricdruid.net/roland-juno-dcos/

But that sounds like we're actually going a little bit too digital here and that it might take the analog "umphf" away in some fashion.

So what I wonder is this, why not just use a normal expo too but tune it a bit low so that the digital reset always trumps it, but that the analog expo follows the digital hard sync pretty closely? This will push the tuning/stability problems of an analog vco into amplitude space instead of freq and would be pretty negligible?

Also, fun fact that its actually cheaper for me to buy say an Arduino Nano and a cheap 12-bit ADC than a tempco resistor. Weirds times Smile

Should I try on a breadboard or is it a bad idea?
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hedefalk



Joined: Aug 29, 2017
Posts: 51
Location: Stockholm, Sweden

PostPosted: Wed Nov 22, 2017 5:27 am    Post subject: Reply with quote  Mark this post and the followings unread

Hm, come to think of it, most vco's have hard sync anyways? Maybe just make one really decent 1V/oct to sync pulse then…
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Grumble



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PostPosted: Wed Nov 22, 2017 7:52 am    Post subject: Reply with quote  Mark this post and the followings unread

There is also a thing called DDS (Direct Digital Synthesis) and I use that a lot in my synth (see my signature below).
Indeed I use an Arduino (clone) to program an Analog Devices AD9833, and this device has an output where you can choose sine wave, triangle and square wave forms (nót simultaneous though).
If you use the 16 mHz clock of the Arduino, the output frequency can be programmed to vary between about 0.060Hz up to 8MHz.
read more about the Analog Devices DDS at ADIsim DDS

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gdavis



Joined: Feb 27, 2013
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PostPosted: Wed Dec 06, 2017 8:57 pm    Post subject: Re: DCO has hard reset on normal analog expo VCO? Reply with quote  Mark this post and the followings unread

hedefalk wrote:

So what I wonder is this, why not just use a normal expo too but tune it a bit low so that the digital reset always trumps it, but that the analog expo follows the digital hard sync pretty closely? This will push the tuning/stability problems of an analog vco into amplitude space instead of freq and would be pretty negligible?


There's no need to "tune it lower", you replace the comparator with the digital clock sync pulse, so the expo is out of the equation as far as resetting the integrator. Yes, this is exactly what the Juno does (uses the counter reset value to generate a corresponding V/oct CV output to the DCO to control the ramp rate to maintain a consistent amplitude).

What takes away from the analog "umphf" is the fact that the frequency is very accurate. The ear is much more sensitive to frequency than amplitude. The analog expo conversion is accurate enough to make the volume sound consistent, but not frequency, so that's why the digital clock is used to reset (frequency) and the analog expo for the ramp rate (amplitude).

The inaccuracy of the expo converter in a "traditional" VCO is exactly what gives the VCO much of the analog quality. The more accurate you make it, the more you lose that quality. The trick is to find the right balance so that it sounds in tune without being too perfect.

So what do you do? Make the analog expo better? Make the digital sync "worse"? Both approaches have their advantages and disadvantages.

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hedefalk



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Location: Stockholm, Sweden

PostPosted: Thu Dec 07, 2017 2:29 am    Post subject: Re: DCO has hard reset on normal analog expo VCO? Reply with quote  Mark this post and the followings unread

gdavis wrote:


There's no need to "tune it lower", you replace the comparator with the digital clock sync pulse, so the expo is out of the equation as far as resetting the integrator. Yes, this is exactly what the Juno does (uses the counter reset value to generate a corresponding V/oct CV output to the DCO to control the ramp rate to maintain a consistent amplitude).


Ah yes, you're right of course, since I wouldn't actually even use the analog expo for resetting the timing cap, I wouldn't need to tune low. I think I was in the mode of thinking about ADDING this "perfect pitch" module into an existing ramp core VCO.

Sorry if I'm a bit thick and verbose here but just want to be thorough on what I understand and not:

I'm actually thinking about doing a ramp core while as I understand it the Juno is basically a divide-by-x square core. But the problem for that saw-tooth/ramp would be the same.

Let me continue though. As for a ramp core, there are two different things needing to be done, resetting the timing cap and controlling the charging current. Normally the resetting would be done by a static/fixed comparator and only charging current would be effected by the CV, therefore the amplitude being constant.

If one would only use the digital clock to reset the timing cap but leave the charging current constant, one would achieve perfect pitch but 440Hz would reach twice the amplitude of 880Hz for instance.

So one really need to control the charging current too from the CV. So if no expo, then one need to to use that counter reset value to also control the charge rate as the Juno does. But how does it do that? With a DAC?

I can't seem to find exactly how it does it, on that page I linked I only read:

"The CPU provides a control voltage (DCO CV) which is designed to compensate for this effect by increasing the voltage that the capacitor charges from, thereby increasing its rate of charging. Thus the DCO CV keeps the sawtooth amplitude nearly constant (approx 12Vp-p) over the whole frequency range. In order to do this, the DCO CV has to include LFO, BENDER, PORTAMENTO and TUNE data as well as key value."

Sounds like a DAC?

What I'm suggesting is pretty simple - instead of doing whatever that is that Juno does, why not just use a normal expo for the charging current but leave the resetting to the digital clock?

I do understand that the digital coldness vs. analog warmth thing is very much about inaccuracies in pitch. I'm just thinking, if I could control this digitally but keeping everything else about charging the timing cap and waveshapers analog I'd get the best of two worlds.

I mean by doing some simple analysis of what I think are "nice" inaccuracies in pitch, I could just imitate that in with microcontroller at the expo stage. The only crossing of D and A here would be the ADC to read the CV and the crystal decided moment at which the timing cap is reset. Feels like I really would keep a lot of analog "oumphf". Am I naive here?
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gdavis



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PostPosted: Fri Dec 08, 2017 8:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

I think I'm starting to understand where the confusion might be coming from. You seem to be thinking that in the Juno the digital part controlling the ramp current is in the DCO. It's not.

The microcontroller in the Juno (and many other synths like the Jupiter 8, OB-X etc.) controls everything by outputting digital control values to a DAC which converts them to CV's for all the audio generation circuits - Pitch CV, VCF CV, VCA CV etc.

So in the Juno, there is a CV going to the DCO, and the DCO has a normal expo converter to generate the charging current from the CV. The only difference between the Juno DCO and a normal VCO is that the comparator reset circuit is replace with a reset pulse from the microcontroller.

So this is the same as what you want to do (as I understand it) except that you don't have a master microcontroller controlling the whole synth, you want to add one to a VCO to make it into a DCO.

So ya, like you're saying, remove the comparator and reset and replace it with a microcontroller to control the reset. Feed the CV to the expo and to an ADC connected to the microcontroller to detect the desired frequency.

And yes, you could model frequency variation in the microcontroller.

Hope that's clear. I think you understand what you need to do in order to add this to a VCO, you just don't quite seem to realize how similar this actually is to what the Juno does.

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