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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
VCO (MFOS SLMS Mark II)
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El Mop



Joined: Aug 20, 2012
Posts: 54
Location: Germany

PostPosted: Sun Nov 24, 2013 1:56 pm    Post subject: VCO (MFOS SLMS Mark II) Reply with quote  Mark this post and the followings unread

Hi,
I built a synth containing three VCO from the
MFOS Sound Lab MiniSynth Mark 2.
The synth is hardwired but build modular inside, so that each module has its own PCB.
I´m still at the beginning of the debugging phase.
Today I was running the Synth through an osilloscope patch of Native Instruments Reaktor for checking the waveforms just for fun, because to me they sounded right.
I turned everything off to get a pure as possible waveform sound and didn´t get to see what I expected.
Pulse Waveform looks like a pulse with a saw on the top.
PWM works as it should under this circumstances.
The Saw Waveform is not symmetrical and do not fall off linear, insterad more in a logarithmic way.
Does somebody have any idea what cause this? (Maybe it is just the Reaktor patch?)
I do have another question.
How do I achieve that when I press a key corresponding to the note C, the VCO is also playing the note C?

Thank you in advance
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varice



Joined: Dec 29, 2004
Posts: 719
Location: Northeastern shore of Toledo Bend
Audio files: 22
G2 patch files: 49

PostPosted: Sun Nov 24, 2013 5:07 pm    Post subject: Re: VCO (MFOS SLMS Mark II) Reply with quote  Mark this post and the followings unread

El Mop wrote:
...Today I was running the Synth through an osilloscope patch of Native Instruments Reaktor for checking the waveforms just for fun, because to me they sounded right.
I turned everything off to get a pure as possible waveform sound and didn´t get to see what I expected.
Pulse Waveform looks like a pulse with a saw on the top.
PWM works as it should under this circumstances.
The Saw Waveform is not symmetrical and do not fall off linear, insterad more in a logarithmic way.
Does somebody have any idea what cause this?...

Anytime that you look at a waveform through a high pass filter you will see the effect that you describe. Normally, synth outputs and audio input circuits will have AC coupling capacitors to block undesirable DC voltage offsets. These capacitors will form a high pass filter which will distort the waveform as viewed on a scope, especially at very low frequencies. As long as the high pass filters formed by these capacitors have a corner frequency below 20Hz, the widely accepted low frequency limit of human hearing, the low frequency rolloff of the AC coupling caps should not be audible.

Here is a thread where this distorted o’scope synth waveform view is discussed:

http://electro-music.com/forum/viewtopic.php?highlight=atc&t=52030

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El Mop



Joined: Aug 20, 2012
Posts: 54
Location: Germany

PostPosted: Mon Nov 25, 2013 5:41 am    Post subject: Reply with quote  Mark this post and the followings unread

Thank you for your fast reply.
So everything is fine and the distorted waveform comes from the AC coupling?
Or, if the waveform is distorted that (heavy) way, it has to be something different?
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varice



Joined: Dec 29, 2004
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Location: Northeastern shore of Toledo Bend
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PostPosted: Mon Nov 25, 2013 9:53 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, AC coupling is the most likely cause of the waveform distortion that you describe. You mentioned that you used a Reaktor software o’scope patch. I assumed that you use an audio interface to get the signal into your computer. Most audio input interfaces use AC coupling caps, so it is fairly safe to assume that unless it is mentioned in the specifications that the interface uses DC coupling.
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El Mop



Joined: Aug 20, 2012
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PostPosted: Mon Nov 25, 2013 10:00 am    Post subject: Reply with quote  Mark this post and the followings unread

Thank you again!
If your suggestions are right, then you helped me a lot and saved much of my time.
Without you, I would have gone on a big bug hunt, but maybe there would have no bug been in the woods. Smile
I hope it is as you say und will do no changes.
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elmegil



Joined: Mar 20, 2012
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PostPosted: Mon Nov 25, 2013 11:46 am    Post subject: Re: VCO (MFOS SLMS Mark II) Reply with quote  Mark this post and the followings unread

El Mop wrote:

I do have another question.
How do I achieve that when I press a key corresponding to the note C, the VCO is also playing the note C?


It is not clear how much of Ray's VCO you have pulled out. If it includes the exponential converter, then the main answer would be to provide a calibrated keyboard input. The expo converter is the portion of the circuit shown below.

This VCO responds to a standard 1 Volt per octave, which means 1V of change in the control input (KBV in Ray's diagram) will lead to one octave change, or a doubling, of the pitch.

If you're expecting a rigid "C on the keyboard == exact C pitch" you will have to tune the synth with the coarse and fine adjustments to get there, but once you have calibrated the VCO's V/Oct response, this should be doable with a tuner.


Screen shot 2013-11-25 at Nov 25 12.42.15PM.png
 Description:
SLMS MkII Expo Converter
 Filesize:  31.42 KB
 Viewed:  1234 Time(s)

Screen shot 2013-11-25 at Nov 25 12.42.15PM.png


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El Mop



Joined: Aug 20, 2012
Posts: 54
Location: Germany

PostPosted: Mon Nov 25, 2013 4:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thank you too.
Yes, the expo converter is included.
To make it more clear:
I have an Octave Switch and Pitch Pot with a
range of 2 octaves (-1V <-> +1V).
What I meant is:
If I feed the (calibrated) VCOs CV Input with 0V/1V/2V (and so on),
do I get automatically the note C?
I calibrated one VCO by ear today and get to this result.
So, maybe my question is already answered with this.
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elmegil



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PostPosted: Mon Nov 25, 2013 4:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

Whether you get a C or a D or whatever on even volt boundaries will depend on the setting of the coarse and fine frequency controls.
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El Mop



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PostPosted: Tue Nov 26, 2013 1:12 am    Post subject: Reply with quote  Mark this post and the followings unread

I know.
But if I set te pots that way, that, for example, 1V (or 2V/3V/4V) goes in,
does it make a 'C'?
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elmegil



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PostPosted: Tue Nov 26, 2013 6:21 am    Post subject: Reply with quote  Mark this post and the followings unread

Of course Very Happy
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El Mop



Joined: Aug 20, 2012
Posts: 54
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PostPosted: Tue Nov 26, 2013 2:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

That´s all I want to know. Very Happy
Thank you!
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Skrog Productions



Joined: Jan 07, 2009
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PostPosted: Wed Nov 27, 2013 10:23 am    Post subject: Reply with quote  Mark this post and the followings unread

Reading the thread reminded me of a tv sketch from a long time ago...........

https://www.youtube.com/watch?v=eVbRX6njhrE


Very Happy
Dave
Very Happy

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