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 Forum index » DIY Hardware and Software » Thomas Henry designs
Ramp wave ugliness
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Thomas_Henry



Joined: Jul 24, 2009
Posts: 170
Location: N. Mankato, MN
Audio files: 3

PostPosted: Mon Feb 27, 2017 6:16 pm    Post subject: Ramp wave ugliness
Subject description: Will you do me a favor?
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Hi Gang,

If any of you have VCOs using the CEM3340, would you do me the great favor of looking at the ramp wave carefully and tell me if you see any undershoot, or other asymmetric behavior? I don't really care what the design is, homebrew or commercial. I'm just curious if your ramp is "perfect."

Many thanks, and in return I promise a new design in the offing.

Thomas Henry
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varice



Joined: Dec 29, 2004
Posts: 923
Location: Northeastern shore of Toledo Bend
Audio files: 29
G2 patch files: 53

PostPosted: Tue Feb 28, 2017 3:04 am    Post subject: Reply with quote  Mark this post and the followings unread

I have a Prophet-5 rev3.3 that uses the CEM3340 chip. I recall that the saw waveform was very free of distortion when viewed on an oscilloscope.

If you are interested, I recorded and uploaded a wavefile sample of the Prophet-5 saw wave. It is a recording of the output of the synth (with the VCO going through the mixer, the low pass filter set wide open with no resonance, and the VCA circuits). The digital recording was done with a sample rate of 44.1kHz. The sample is here (along with other waveform samples):

http://electro-music.com/forum/topic-12389-25.html

Using a wavefile editor, the saw waveform appears to me to be very clean, without any significant under or overshoot, or any other ugliness.

I hope this helps, and good luck with the new VCO design!

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varice



Joined: Dec 29, 2004
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Location: Northeastern shore of Toledo Bend
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G2 patch files: 53

PostPosted: Tue Feb 28, 2017 1:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

varice wrote:
...Using a wavefile editor, the saw waveform appears to me to be very clean, without any significant under or overshoot, or any other ugliness...

I guess I should have posted an image of the saw waveform sample in case you do not have a wavefile editor handy… Here you go:


Prophet-5 Saw Waveform Sample.png
 Description:
A wavefile editor image of a Prophet-5 rev3.3 saw waveform digital sample.
 Filesize:  10.7 KB
 Viewed:  180 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

Prophet-5 Saw Waveform Sample.png



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Blue Hell
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Joined: Apr 03, 2004
Posts: 22567
Location: The Netherlands, Enschede
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PostPosted: Tue Feb 28, 2017 2:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

When the wave editor is using sinc filtering on displaying wave forms (like cooledit did) you will always see 'ringing' - between quotes as it is not actually ringing but an artifact of the limited bandwidth of the recorded signal, i.e. an approximation of the signal as a sum of sines.
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varice



Joined: Dec 29, 2004
Posts: 923
Location: Northeastern shore of Toledo Bend
Audio files: 29
G2 patch files: 53

PostPosted: Tue Feb 28, 2017 2:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
When the wave editor is using sinc filtering on displaying wave forms (like cooledit did) you will always see 'ringing' - between quotes as it is not actually ringing but an artifact of the limited bandwidth of the recorded signal, i.e. an approximation of the signal as a sum of sines.

Exactly! The dead giveaway is the 'ringing' starting just before the peak of the waveform in the image. That just does not happen with the analog VCO waveform. So the slight 'ringing' in this image is for the most part a digital artifact.

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Thomas_Henry



Joined: Jul 24, 2009
Posts: 170
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PostPosted: Tue Feb 28, 2017 8:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thank you, lads! I'm not sure the older CEMs do this, but on 7 different new ones (Revision G) we (three different experimenters) were seeing an undershoot to -0.7V on the ramp. Not necessarily ringing, but a short glitch/fall below zero.

However, I'm happy to report that one of our team nailed a solution tonight, after several days of real sweat. This is going to be a cool design!

Thanks again,

Thomas Henry
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