M-101 Low Pass into High Pass with CP-251?

Alien8 · Post by **Alien8** » Sun Oct 29, 2006 8:09 pm

Hello All,

I'm new to the forums, and fairly new to MOOG noize.

As my first question I will be asking one that has been asked before but I feel has never been fully answered...(sorry if it has, I didn't understand it then...)

Can I make the low pass filter sweep in the oposite direction? ie from ow-ah-ow to ah-ow-ah (sweep from high frequency to low and back based on the envelope)

From what I have gathered so far, I see that you may be able to use the CP-251 to be able to create a "negative" control voltage to achieve this.

I am asking because I will be buying a CP-251 for this reason only, but do not have access to one for trial.

Can anyone confirm that this is possilbe? Provide me with the patch path? Even post a sound sample of it!?!

Any help is appreciated!!

Cheers!

CTRLSHFT · Post by **CTRLSHFT** » Sun Oct 29, 2006 8:39 pm

Alien8 wrote:Hello All,

I'm new to the forums, and fairly new to MOOG noize.

As my first question I will be asking one that has been asked before but I feel has never been fully answered...(sorry if it has, I didn't understand it then...)

Can I make the low pass filter sweep in the oposite direction? ie from ow-ah-ow to ah-ow-ah (sweep from high frequency to low and back based on the envelope)

From what I have gathered so far, I see that you may be able to use the CP-251 to be able to create a "negative" control voltage to achieve this.

I am asking because I will be buying a CP-251 for this reason only, but do not have access to one for trial.

Can anyone confirm that this is possilbe? Provide me with the patch path? Even post a sound sample of it!?!

Any help is appreciated!!

Cheers!

you could invert the way the low pass cutoff knob turns, but unfortunately the effect would still be a lowpass. hopefully moog will introduce a multiband resonant filter soon, i'd be mega fun. in the meanwhile, the closest moog items that could do this for you would be the filters on the voyager, or using the murf (although this would not give the full desired effect due to the start point on the low end of things for cutoff).

OysterRock · Post by **OysterRock** » Sun Oct 29, 2006 11:50 pm

Alien8 wrote: Can I make the low pass filter sweep in the oposite direction? ie from ow-ah-ow to ah-ow-ah (sweep from high frequency to low and back based on the envelope)

I think he's asking if you can invert the envelope. I don't have a CP, but doesn't the mixer have an inverter? If it does, you can invert the envelope signal and route it back to the filter cutoff.

Maybe someone who has a CP can tell us if it has an inverter.

asd · Post by **asd** » Mon Oct 30, 2006 12:20 am

I think you're asking two different questions, but both CTRLSHFT and OysterRock are right.

You can't turn the 101 into a high-pass filter. No matter what, it will filter out the frequencies above the cutoff and let the ones lower than it pass through.

You can have the envelope follower sweep in the opposite direction with the CP (starting at the cutoff frequency and then going lower rather than the default low to high). Patch the 101's Env Out to the CP's mixer and then send the "-" (inverted output) of the mixer to the cutoff in on the 101.

I think that even if you buy the CP just for that, you'll find a lot of other ways to use it, especially with the 101.

Alien8 · Post by **Alien8** » Mon Oct 30, 2006 12:21 pm

Thanks for your responses!

I too agree that a multi band filter would be a blast! I'm a filter freak!

Looking back, I did inadvertantly ask two questions. Both have ultimately been answered... let me explain:

I am trying to simply invert the envelope. In the past I had considered a High Pass filter - similar to the one on the Qtron - but had always found it to be missing something... ie the Moog touch! Inverting the envelope would accomplish what I had asked, but it would not be a high pass filter, which is what I may be searching for.

The more I think about the sound at the end of this quest, the more it becomes apparent that I'll have to find a CP-251 I can try out to see if it does produce the sound I am expecting.

Thanks for your help!

asd · Post by **asd** » Mon Oct 30, 2006 6:00 pm

Complete tangent, but.... are you Alien8 as in Alien8 Recordings?

eric coleridge · Post by **eric coleridge** » Tue Oct 31, 2006 5:30 am

I posted a similar question a few months ago asking if it was possible to electronically alter the MF101 Low Pass Filter so as to change it to a High Pass Filter response and someone responded with a description of a patch (with no electronic modifications), I believe, using the CP251's inverter, to actually change the Low Pass response to High Pass response.

I know this sounds unlikely, but I believe it is possible to do so.

I'm going to look for the post and re-post it here.[/i]

eric coleridge · Post by **eric coleridge** » Tue Oct 31, 2006 6:17 am

I found it. This comes from a very good, reliable source-- a very technically oriented Moog forum writer named "Till".

I asked:

Is there any way to convert a Low Pass, like the MF101, into a High Pass?

Till answered:

"You could invert the output of the lowpass and mix it with the ingoing signal of that filter in a simple mixer. This should cancel certain frequencies depending on their filter caused phase shift. Thus resulting in a highpass filter."

Although he doesn't mention the CP251 specifically, I believe it fits the requirements for this patch (inverter + mixer). I haven't tried it yet because I don't have a CP251 or equivalent. So, I'd love to hear if it works in practice.

asd · Post by **asd** » Tue Oct 31, 2006 9:08 am

Eric,
That's a really interesting idea... I wonder if it would actually work out in practice.

Unfortunately I don't think that the 251 would be the tool to use, you'd need to have something else to invert the audio and then mix the two signals. I tried using the 251's mixer with audio and there's a significant bleed-through click from the square wave. The mult, attenuator and lag processor work with audio fine, but not the mixer (or at least not the one I have).

Alien8 · Post by **Alien8** » Tue Oct 31, 2006 11:11 am

Complete tangent, but.... are you Alien8 as in Alien8 Recordings?

No, I'm not.

I have to agree that Eric's idea is quite a ground breaker... I guess you would just need a phase inverter...

I wonder if you used a humbucker and wired one side in phase and one out, then mixed as described, would this create the answer... probably not... the two sounds wouldn't be exact...

eric coleridge · Post by **eric coleridge** » Tue Oct 31, 2006 2:13 pm

asd wrote: Unfortunately I don't think that the 251 would be the tool to use, you'd need to have something else to invert the audio and then mix the two signals. I tried using the 251's mixer with audio and there's a significant bleed-through click from the square wave. The mult, attenuator and lag processor work with audio fine, but not the mixer (or at least not the one I have).

drag. I didn't know the CPs mixer isn't good for audio.

they should really make another control processor with an audio mixer, a VCO, and an ADSR... and a VCA...

But back to the High Pass Filter: I guess someone could (if you really wanted to) find these simple circuits (from a synth DIY site, for instance) and actually build them into a MF101. That would be a pretty useful mod (assuming it works)... to have a switchable High Pass/Low Pass MF101. Probably wouldn't even be a very complicated circuit...

Now I'd really like to know if this will work... any modular users here? isn't there a module that will do phase inversion? A VCA maybe? I'm just guessing...

OysterRock · Post by **OysterRock** » Tue Oct 31, 2006 4:20 pm

You would need an voltage controlled allpass filter with a center frequency that tracks to the LPF's cutoff frequency. Subtract the LPF's output from the allpass filter's output. Very possible, but not without modifications or external circuitry.

Impossible Sound · Post by **Impossible Sound** » Tue Oct 31, 2006 8:33 pm

I've tried the "inverted audio" trick to turn a LPF into HPF, and it definitely works. It's far from ideal, but possible.

CTRLSHFT · Post by **CTRLSHFT** » Tue Oct 31, 2006 9:13 pm

Impossible Sound wrote:I've tried the "inverted audio" trick to turn a LPF into HPF, and it definitely works. It's far from ideal, but possible.

what did you do to accomplish this?

Impossible Sound · Post by **Impossible Sound** » Tue Oct 31, 2006 11:54 pm

It's exactly what Eric said above - take the inverted output from the LPF and mix it with the original signal.

When I did it, I used the CP-251 and the filter on an MS-10, although really any LPF, inverter, and mixer would work. I've since added a real HPF to my rig, so there's no more need for this tomfoolery.

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M-101 Low Pass into High Pass with CP-251?

M-101 Low Pass into High Pass with CP-251?

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