4093 NAND Gate as a filter

[gasboss775]

Last night when I was reading the thread about PHOBoS' Nandulator I noticed a comment that the last Nand gate oscillator prior to the output behaved more like a filter than an oscillator so I decided to do some experiments.

In the figure below The Left side oscillator is just a normal Schmitt trigger square wave oscillator, to provide a suitable test signal. The "oscillator" to the left is the filter.

With the values Shown the output is akin to a bandpass filter with high resonance and when the pot is varied it tunes in what are presumably the harmonics of the input square wave.

Other Findings.

If the input frequency is higher than that of the second oscillator ( acting as the filter ) the 2nd just tends to lock onto the input with limited variation in the sound when the 10K pot was varied.

It also works well when fed with this with white noise where again the Nand oscillator behaves a bit like a resonant filter. It needs to be heard, particularly with the noise , as it is quite a distinctive sound.

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[gasboss775]

By adding a vactrol I have been able to make this into a VCF.

See below:

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[blue hell]

It is a gated oscillator .. which sort of reminds me on VOSIM ... I'd not call it filtering but wave shaping rather (although that's both a bit .. dunno .. for digital circuits). But like filtering it does change harmonic content ... and it can give very nice results.

I had been experimenting with a variation on this for my soft synth with a pulse stretching thingie (basically a Monostable MultiVibrator (MMV)), and after reading this thread decided to make a a new mode for it in which it is a gated oscillator. With some good results :-)

BTW the MMV is less wild, but still interesting too.

[CMOS: CD4047 - Mono/A -Stable Multivibrator, CD4098 - Dual Monostable Multivibrator or CD4538 - also a dual MMV]

[gasboss775]

Today I put a 4024 counter after the 4093 "filter" the results were quite interesting the divider changing the characteristics of the filter as well as the expected reduction (1/2, 1/4, etc ) of the fundamental frequency.

It is interesting that putting the 4024 before the "filter" has quite a markedly different effect on the timbre of the sound in comparison to placing it after the "filter"

Do you know if anyone has coined a term for this type of "filter," gated oscillator sounds clumsy.

[gasboss775]

I wonder if doing it with a Schmitt inverter would sound any different?

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[richardc64]

[gasboss775]

[blue hell]

Goschillator, nice one

[gasboss775]

Some more thoughts on this circuit.

In the attached image the usual gated oscillator is shown as (1) when it is gated off it actually interrupts the oscillation cycle as the output is forced high for the duration that the gate input is low.

In the circuit marked (2) the gate input has no effect over the oscillation cycle but simply determines whether or not that it is passed through the 2nd gate.

In the context of the Goschillator (<<<<<are we really calling it this!!!) I would think this would actually influence the nature of how the signal input to the gate comes out at the other side. I will need to test this to see. It is too late now to do so tonight but I'll be interested to hear it for myself.

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[blue hell]

I was wondering about that difference too .. as the original configuration would need some extra charge time (as the capacitor would end up more discharged after having been off for a while) it should produce less (very) short glitches .. probably.

[gasboss775]

The difference in sound between the two circuits above are like night & day. The 2 gate version is more extreme and similar to ring modulation using an XOR gate. On this basis I don't think that the 2 Nand gate circuit can qualify as a goschillator!!

[blue hell]

Righto - so now I will need to simulate things better

Could you post some short mp3s?

[gasboss775]

[blue hell]

No worries ... I know about mess ...

I should just dig out a breadboard and a soldering iron and stuff ... from my mess .. didn't do hardware in many years .. but it should all be .. somewhere still

Anyways, I still like reading about it and tinkering a bit .. I only did software for .. hmm decades almost

[gasboss775]

I got some great results feeding two goschilators with a 55Hz square wave, then XORing the outputs of the two goschilators. Still to do a schematic for this. I want the two goschilators to be voltage controlled ( probably by using vactrols ) as the magic happens when the goschilators are being tuned. The resultant sounds are quite vocal in nature.

[gasboss775]

Today I made a 2 Stage Goschillator Filter with Voltage control using 2 VTL5C1 vactrols. I was very impressed with the sound.

The switch is to change the range of the filter, with the 3K6 resistor switched in it works better for higher frequencies and for lower frequencies it is better to switch to the 12K resistor.

Notes: Power supply is +12 V

Input & Output are digital however it could be adapted for analogue systems with some additional circuitry.

NAND gates are 4093 ( TI brand )

The reference voltage is +3 Volts attached to the bottom end of VR2. I haven't included a circuit for generating this.

Do you think that this is getting too complicated to fit into the Lunetta category?

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