LFO waveform symmetry and sweep symmetry

[markphaser]

How do i change the LFO's waveform symmetry(ratio and percentages)?

How do i change the LFO's sweep symmetry(ratio and percentages)?

[Mark Hammer]

They are the same thing (the two types of symmetry).  Look here: http://www.musicfromouterspace.com/analogsynth/coolnewlfo.html and you will see a pot with a couple of diode in the upper left hand corner of the schematic.  That's how it is most often done.

[Dave_B]

That's such a cool site, thanks Mark. 

[Mark Hammer]

You're welcome.  Some of the most productive and giving people on the net are the modular analog synthesis people.  Ray Wilson has a page of links on his site.  You should check some of those out too.

[markphaser]

Thanks Mark Hammer

What does the Waveshaper do ? its a diode shaper circuit but how does it work?

How does the Symmetry parameters work in this circuit? what does it do please?

[Mark Hammer]

The current from IC1-C comes through R8, the diodes after it and whatever portion of the pot is presented.  That current charges up the caps in the feedback loop of IC1-A.  If the pot is set to the midpoint, then as much curent on the positive half cycle charges the cap as on the negative half-cycle.  If the pot is set somewhere other than the middle, then you get more current for one part of the cycle than for the other.  So, the going up part might happen faster (if the resistance is low and lots of current can charge the cap up quickly) than the going down part.

[markphaser]

So the symmetry is adjusting the charging and discharging times or the on/off times?

whats the differences between symmetry VS duty cycle? they both are controlling the charging and discharging times or the on/off times

[gez]

So the symmetry is adjusting the charging and discharging times or the on/off times?

Symmetry is changed by varying the charge/discharge time of the cap.  This is done by 'switching in' different values of resistance (the diodes function as simple switches).

whats the differences between symmetry VS duty cycle? they both are controlling the charging and discharging times or the on/off times

The duty cycle varies with symmetry and visa versa.  The output of the square wave provides the voltage to ramp the cap up/down, and this ramp is fed back to the Schmidt Trigger's input causing it to snap between V+ and ground.  So yes, varying the duty-cycle affects the symmetry of the ramp, and varying the symmetry of the ramp affects duty-cycle, the two are interlinked.

[markphaser]

Thanks for the information

Symmetry is changed by varying the charge/discharge time of the cap.

Doesn't duty cycle change the charge/discharge time of the cap?


Where in a LFO circuit/stage do u put the duty cycle parameter at mostly?

Where in a LFO circuit/stage do u put the symmetery parameter at mostly?

[gez]

Symmetry is changed by varying the charge/discharge time of the cap.

Yes

Doesn't duty cycle change the charge/discharge time of the cap?

Yes, they're interrelated.  Please read through my last post again.

Where in a LFO circuit/stage do u put the duty cycle parameter at mostly?

Don't really understand your question.  Mostly, with this oscillator type, you want a symmetrical waveform so the duty cycle of the square wave output is (ideally) 50:50.  Any deviation from this will create asymmetry in the triangle waveform.

Where in a LFO circuit/stage do u put the symmetry parameter at mostly?

The symmetry control is used to adjust the symmetry of the triangle output, which is the output most commonly used with this type of LFO (there are easier ways to produce just a square wave).  The duty cycle of the Schmidt trigger's output is varied too as a result of any changes in symmetry to the triangle.

[markphaser]

If i have a triangle wave oscillator how do i add a duty cycle and symmetry parameter to it please?

[gez]

If i have a triangle wave oscillator how do i add a duty cycle and symmetry parameter to it please?

Exactly as it's done in the schematic Mark linked to.  A pot and two diodes - CR1, CR2 and R1 (50k) in the schematic.

[markphaser]

What does the  two diodes do ? what does the pot do to the circuit does it add DC offset or time offset?

[gez]

What does the  two diodes do ? what does the pot do to the circuit does it add DC offset or time offset?

http://www.musicfromouterspace.com/analogsynth/coolnewlfo.html

There's a description of how the whole circuit works in the above link, including the asymmetry part.

Remember that diodes only conduct current in one direction - when the anode is made at least 0.6V positive of the cathode for Si devices - so they can act as switches.  The output of the Schmidt part of the circuit is slamming from V+ to V- so only one diode will conduct at any one time relative to ground. 

Remember this, work through the above schematic, read the comments and hopefully things will click.

[markphaser]

so only one diode will conduct at any one time relative to ground.

So its Rectifing the waveform? the waveshaper?

The LFO waveshaper has a 50K pot with 2 diodes this must clip either the positive or negative cycles

the ratio of charge to discharge current ,ratio and thus the rise and fall rates ,Top to Bottom Symmetry' trim pot

The symmetry pot is a variable resistor for the +12 and -12 rails does this add DC offset to the output of the LFO?

[Transmogrifox]

This circuit doesn't really control symmetry about the "t" axis (commonly the x axis on generic plots) like we would think of a distortion pedal controlling symmetry.  This circuit controls the symmetry about the 'y' axis.  It's essentially a triangle wave on the output. As you start to crank the symmetry pot, then the rising slope is steeper than the falling slope, and this begins to look like a ramp generator as you get to the extreme.  When you crank the pot the other way, then the falling slope is steeper than the rising slope, and approaches an inverted ramp.

Duty cycle only applies to a square wave.  It's a measure of time high to time low.  With a triangle wave put into a schmitt trigger, the square output is 50% duty.  As one of the slopes gets steeper, then there is less time required to dive to the threshold that pops the schmitt trigger the other way, and this effects the duty cycle somewhat. The greater the hysteresis on the schmitt trigger, the more this type of waveform will effect duty cyle. If it was an outright comparator (no hysteresis) then changing the triangle toward a ramp would not change the duty cycle at all.  In this case, changing the comparator reference would modify the duty cycle.  For clarity, this last bit about the comparator does not apply to this circuit since this oscillator would not work with a comparator--it must be a schmitt trigger so changing the y-axis symmetry does change the duty cycle some.

The alternate symmetry would be to put the triangle wave through a tubescreamer type clipper with one of the feedback diodes removed.

[markphaser]

Thanks alot Transmogrifox for the information

How does the Hystersis of the comparator affects the duty cycle?

Are u talking about when there is "noise" on the input of the compartor?

Hystersis means- 2 different trigger levels create an offset or lag in the switching action
Hystersis gives an op-amp immunity
Hystersis means- a higher ref level when the input voltage goes from a lower to higher value than when it goes
                         from higher to a lower value

Are u saying a Comparitor with Hystersis? with positive feedback?