Oscillator mod output around zero vs going positive

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cretaceousear
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Oscillator mod output around zero vs going positive

Post by cretaceousear » Wed Apr 16, 2014 12:47 pm

I notice that on most current VCOs there is no dual option for sin and tri output around zero in addition to the standard going positive option.
My old VCOs boards provide both options.

Sorry - not sure of the technical terms, but those of you who know what I'm on about will know - so please clarify my terminology!

I need to add a VCO and VCLFO to my build but am wondering why new ones do not offer + and - around zero, or is not really that big a deal?
I'm not worried about reproducing exact tremolo or vibrato but I do want things to sound musical (sometimes).

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Post by sduck » Wed Apr 16, 2014 1:18 pm

What? Maybe your terms need refining. Most, if not all VCO outputs are centered on zero volts, with both positive and negative swings. I've never heard of any that are only offset so that they're all positive going. Or are you asking about the various mod inputs? It's hard to tell from your terminology.

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Post by daverj » Wed Apr 16, 2014 1:22 pm

Are you asking about the CV inputs, or the oscillator outputs? The outputs are generally bipolar (going plus and minus around zero). Some modulation inputs are bipolar and some are unipolar (going positive only from zero).

Neither of those effect how "musical" the sound is.

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Re: Oscillator mod output around zero vs going positive

Post by jupiter8 » Wed Apr 16, 2014 1:22 pm

cretaceousear wrote:My old VCOs boards provide both options.
Which ones ? I can't remember ever seeing that as an option on anything.

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Post by PrimateSynthesis » Wed Apr 16, 2014 1:23 pm

I have no idea what they are called in Lodnon, but the outputs of most VCO's are bipolar -- centered around zero. And if they weren't, most audio are paths would remove the DC-offset anyway.

Otoh, single-sided LFO's are useful, especially when modulating external equipment. Some LFO modules do give you the option of producing either, but I wouldn't say it's a common feature.
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Post by cretaceousear » Wed Apr 16, 2014 1:52 pm

oh.. :eek:
my elderly modular is a Digisound 80 and it has all those options

Surely a square (aka pulse) out is always 0-10 v (or 0 to positive anyway) - then it can act as a gate or trigger ?

You're not going to tell me envelope gate in and adsr outputs are not generally 0 to positive , are you ?

Maybe Charles Blakey's design philosphy has gone out of style - he clearly implies in the build notes that the + - output option for sin and tri is there as a useful extra for modding other inputs
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Post by PrimateSynthesis » Wed Apr 16, 2014 2:17 pm

cretaceousear wrote: Surely a square (aka pulse) out is always 0-10 v (or 0 to positive anyway) - then it can act as a gate or trigger ?
Actually, it's quite often bipolar (eg. -5V to +5V), but it can still act as a gate, trigger, clock, etc. since such inputs often ignore negative voltages.
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Post by daverj » Wed Apr 16, 2014 2:52 pm

There have been many "design philosophies" over the decades relating to what voltages are used. They generally are specific for each format of modular.

Square wave outputs of oscillators are often bipolar (plus and minus), assuming that the regular outputs are also bipolar.

ADSR outputs generally are positive only.

Gate and trigger inputs generally trigger off of positive voltage, but can generally handle negative voltages without a problem. So a +/-5v gate works in virtually any place that a 0-5v gate will work.

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Post by J3RK » Wed Apr 16, 2014 3:57 pm

All VCOs that I've used except my own are all bipolar. On mine I add a positive unipolar (0-10V) pulse wave. Basically the main bipolar pulse wave going into a comparator, so that it's more suited to logic operations. (not that there is much stopping a regular bipolar pulse wave from doing the same thing) I just like to have both.

I have however used a few LFOs that had some of their outputs shifted above 0VDC. The Blacet Micro LFO comes to mind.

This is a simple matter of using a differential amplifier or level shifter circuit. (usually an op amp, some resistors, and possibly a capacitor depending on which way you want to shift the signal) You can also do this if you have a CV processor that provides a reference DC voltage that you can use to shift the signal up and down with. Typically a 5VDC reference would allow you to shift a 10V P2P signal all the way above or below 0VDC. (say into a channel of an attenuverting mixer/CV processor)

I've used a lot of envelopes that go either positive, or negative, or both, but not centered bipolar.

I'm somewhat intrigued. I've never looked into the DigiSound synths much. It sounds like everything they do is the opposite of what's typical. I've seen some of the schematics floating around, but never took a look.
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Post by cretaceousear » Wed Apr 16, 2014 5:14 pm

right.. umm, glad I asked the question!

I have one Digisound oscillator working, the other oscillator's CEM chip malfunctions (over enthusiastic routing has deep fried it) and I'm baulking at importing an antique chip at £60 so figured I'd just get a new VCO.

Then I think the new VCOs better have unipolar as options, so I can mix it all freely at the VCMixer. I can do a bit of stripboard to add DC offset.
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Post by J3RK » Wed Apr 16, 2014 5:31 pm

Here's the schematic for the VCLFO:

http://electro-music.com/wiki/pmwiki.ph ... BasedVCLFO

From what I gather, it's putting out a unipolar signal from the CEM chip, then shifting it to produce the bipolar secondary outputs using those op amp circuits at the bottom.

That's interesting, and pretty cool. Buchla CVs are unipolar, so this would interface well with Buchla modules. :tu:

I'm guessing (though not completely sure since I have no real experience with CEM chips,) is that other designs just shift the signals to bipolar, and don't expose the unipolar signals from the chip.

Interesting.

This one's pretty crazy:

http://www.experimentalistsanonymous.co ... 20vcdo.jpg
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Post by Dave_P » Wed Apr 16, 2014 6:21 pm

For anyone who wants the full text of the Digisound VCO article (including, for the DIYers and circuit benders, an explanation of how the biploar waveforms are generated from the unipolar outputs from the CEM chip), this can be found at http://www.digisound80.co.uk/digisound/modules/80-2.htm (the VCO and VCLFO modules are the same basic design).

As noted above, the +/-5V sine and triangle waves are supplements to the primary 0-10V waveforms outputs (these being the norm for the Digisound 80-2 VCO and 80-3 VCLFO modules) and are derived from the 0-10V triangle output. The Digisound VCDO (80-21) and VCLFO (80-19) modules also, I believe, output 0-10V waveforms albeit I can't confirm these from experience as I've never actually used either of them in anger!

Most of the Digisound modules output 0-10V waveforms/signals and utilise 0-10V for the gates and other control voltages. However, and just to be contrary, the 80-C9 Voice Card is a bit of a mish-mash and uses a combination of +/-5V and 0-5V for control voltages - which means that modulating those Voice Card parameters which are externaly controllable with inputs from other Digisound modules either requires modifications to those inputs or the use of the +/- voltage outputs from the 80-2/3 VCO/VCLFOs.

So not even the Digisound modules were consistent across the range :cry:

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Post by davebr » Wed Apr 16, 2014 11:09 pm

sduck wrote:What? Maybe your terms need refining. Most, if not all VCO outputs are centered on zero volts, with both positive and negative swings. I've never heard of any that are only offset so that they're all positive going. Or are you asking about the various mod inputs? It's hard to tell from your terminology.
Aries square/pulse waveforms were unipolar and I believe Buchla square/pulse waves were also. Most modern VCOs are bipolar. You just need a level shifter. Run it through a mixer that has a bias control and bias it back down to +/- levels.

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Post by J3RK » Thu Apr 17, 2014 11:00 am

Dave_P wrote:For anyone who wants the full text of the Digisound VCO article (including, for the DIYers and circuit benders, an explanation of how the biploar waveforms are generated from the unipolar outputs from the CEM chip), this can be found at http://www.digisound80.co.uk/digisound/modules/80-2.htm (the VCO and VCLFO modules are the same basic design).

As noted above, the +/-5V sine and triangle waves are supplements to the primary 0-10V waveforms outputs (these being the norm for the Digisound 80-2 VCO and 80-3 VCLFO modules) and are derived from the 0-10V triangle output. The Digisound VCDO (80-21) and VCLFO (80-19) modules also, I believe, output 0-10V waveforms albeit I can't confirm these from experience as I've never actually used either of them in anger!

Most of the Digisound modules output 0-10V waveforms/signals and utilise 0-10V for the gates and other control voltages. However, and just to be contrary, the 80-C9 Voice Card is a bit of a mish-mash and uses a combination of +/-5V and 0-5V for control voltages - which means that modulating those Voice Card parameters which are externaly controllable with inputs from other Digisound modules either requires modifications to those inputs or the use of the +/- voltage outputs from the 80-2/3 VCO/VCLFOs.

So not even the Digisound modules were consistent across the range :cry:

Dave
Nice! Thanks for the link! I assume that's your site since there is a Dave contact at the bottom. I've only read the first page (history section) so far, but cool stuff.

:tu:
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Post by Dave_P » Thu Apr 17, 2014 12:12 pm

Nice! Thanks for the link! I assume that's your site since there is a Dave contact at the bottom. I've only read the first page (history section) so far, but cool stuff.
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Post by cretaceousear » Wed Apr 23, 2014 11:09 am

I 've been in email contact with Tony of Oakley Sound Systems and he gave me what seems an authoritative reply - he said I could paste it in here, so..
I've never really understood why certain designers used unipolar audio waveforms. As well as the Digisound the ARP2600 does it too.

Firstly, you end up having to remove the asymmetry with a capacitor before either the filter or the VCA, otherwise you end up with a
thumping sound on fast attacks and decays. The 2600 has this and many folk have had their 2600s modded with a cap, or use a patch lead with a cap built inside.

Secondly, you have a reduced dynamic range since you are only using half the available voltage.

Thirdly, whenever you control a parameter with a positive only audio signal the parameter will always increase in value. This means adding unipolar audio filter frequency modulation will always make the signal brighter rather than just modulating it. One would then have to compensate by turning the filter cut off down by hand as the modulation depth is increased.

I use bipolar waveforms for virtually everything with the exception of gate and trigger.

Tony
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