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Discontinuity - Thresh CV bleed through?
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author Discontinuity - Thresh CV bleed through?
eljay
Just built my Discontinuity module, have done initial testing and calibration. I'm not sure if it is behaving correctly so would appreciate some feedback. With the Threshold knob set to the left of anything past approx. 2 o'clock I can hear the input Thresh CV modulation source beating at whatever frequency or shape (Sine, Triangle etc) I am inputting, All other knobs are centred. I don't need a signal in the Input socket to hear this when monitoring the Output socket, i.e if I use a VCO Sine wave at 220Hz to modulate the Thresh CV, I hear a 220Hz Sine wave coming out. Is this correct behaviour or have I got a problem I need to investigate?
Synthbuilder
I think that's right. If you turn down the Discontinuity pot, ie. away from the centre it should go quiet.

Tony
eljay
Setting the Discontinuity fully left (-ve) minimises the output signal but it doesn't go totally, and turning it fully right (+ve) increases the signal amplitude, so overall the Discontinuity pot behaves like an amplitude control in this instance. It can be heard and seen on a scope. The output waveform looks like the modulating input signal (sine, triangle saw) but clipped, top clipped if Polarity set Pos, bottom clipped if Polarity set to middle or negative.
Synthbuilder
Alas, I'm not sure. You'd think that as the designer I'd know what was up, but I haven't played with a Discontinuity module for a long time.

If you put a triangle wave into the input, set the mode to pos, all other pots to their central position, and sweep the threshold pot with no threshold CV, what do you see on the scope? You should see a triangle wave with the top taken off. The higher the threshold pot the more the triangle wave can be seen. There should not be a DC offset to the signal, ie. the triangle wave should remain in the centre of the scope trace.

Tony
eljay
I have conducted the test you describe. Between the central (up position) and fully right (+ve) sweeping the Threshold knob produces the fully clipped triangle through to no clipping at maximum. There is no DC offset.

Between the central (up position) and fully left (-ve) I get something that looks a lot like your logo.

It is quite possible that the module is working correctly, it's just that I don't know what to expect. I can generate all of the waveshapes shown in the builders/users guide correctly, I just wasn't expecting the bleed through from the modulating CV dependent on the setting of the Threshold pot. I also note this bleed through is present from the Discon and Fold CV inputs as well with the Threshold pot set left of center.

I have performed all the calibration actions again, with the same results. I can't get a perfectly level flat top to the wave when adjusting the Unity trimmer, but I can get rid of the pointy edges.
Synthbuilder
That first bit sounds right but I don't like what's happening when the Threshold is taken negative. That should still be a clipped triangle wave but the clipping just goes into the negative. If it looks like my forum avatar there's definately something wrong. That's got discontinuity turned up and fold added.

Try this: With no input, mode is positive, and all pots centralised. Sweep the threshold pot from min to max. What does the output do now?

Tony
eljay
With no main Input, there is no waveshape evident however there are a couple of glitches when sweeping the Threshold pot, these occur between approx. 7'o clock and 10 o'clock.

With no main Input but a 220Hz triangle Thresh CV input and the Thresh CV knob at center, the full modulating triangle can be seen when the Threshold pot is at it's minimum (fully -ve), as the Threshold pot is swept up the amplitude of this triangle reduces slightly until about 10'0clock when it starts to clip the top of the triangle, the clipping of the top of the triangle continues until the pot is swept further to about 2'o clock, at this point the whole of the top of the triangle is clipped (i.e everything above 0 volts), a slight increase of the pot beyond 2'o clock reduces the size of the remaining triangle bottom until it disappears at about 2.30.
Synthbuilder
That's odd. The Threshold CV should behave like the threshold pot. That is, a varying signal on the Thresh CV input would work just like wiggling the Threshold pot.

It could be a grounding problem. If MOTM do you have both pins 2 and 3 on your power header connected back to the power supply? If Dotcom have you fitted the link LK?

Does turning the threshold CV pot have any impact on the signal you are seeing at the output?

Tony
eljay
Some progress has been made - the Unity trimmer is harder to set correctly than I thought (even with a scope) and has a profound effect when set incorrectly. Now it is set correctly or at least a lot better than it was I get the expected results sweeping the Threshold pot with a Triangle Wave input and get the expected clipping over the full range of the pot - hurrah!. I have to say it was easier to set the Unity trimmer using a triangle wave input and then manipulating the Volume and Discontinuity pots so that I could see the 'spikes' more clearly and then adjusting the trimmer.

The 'bleed' problem on CV input remains but does almost totally go if the Discontinuity pot is set to minimum and can be removed totally by setting the Threshold pot to about 3'o clock. This may now be 'expected behaviour'?

Power supply and the rest of the equipment is all Oakley so there shouldn't be any grounding or power supply issues.
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