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

asd · Post by **asd** » Wed Nov 01, 2006 11:19 am

Impossible Sound,
So you used the mixer on the 251 to get an inverted signal. Did you use another mixer to combine the inverted and original signals? Does your 251 mixer add an audible clicking (at the rate the square wave) when you use it for audio?

Impossible Sound · Post by **Impossible Sound** » Wed Nov 01, 2006 6:42 pm

First, let me clarify - you mix the output of the LPF with the the inverted output of the LPF. The way I worded it was unclear.

I used an external mixer to combine the signals, but the CP's mixer works fine for audio - I don't get any clicking.

eric coleridge · Post by **eric coleridge** » Wed Nov 01, 2006 10:39 pm

i have a few different sound processors with phase inverters, and i've been trying to aproximate this patch on my mackie mixer.

although i have been getting some unusual filter responces, i haven't been able to get anything close to a high pass filter response.

So, I think this idea might be a little difficult to execute in the real world. but, i'm sure the processors i'm using are not ideal...

Impossible Sound · Post by **Impossible Sound** » Wed Nov 01, 2006 10:51 pm

From what I remember, it took a lot of twiddling to get it to work right. Although it's possible, it's an extremely unpractical method.

A MoogerFooger 24db/Oct resonant HPF might be cool...

OysterRock · Post by **OysterRock** » Thu Nov 02, 2006 12:47 am

Its going to take more than just phase inversion to get it to work as a highpass filter. The phase shift must be frequency dependent, thats why an allpass filter is needed to track to the lowpass filter. It may work somewhat when the cutoff is at a certain frequency, but it won't work if you want to change the cutoff frequency.

When you subtract these two filter's outputs the phase shift causes the lower frequency to cancel leaving only the high frequencies. This is why it must be subtracted from an allpass filter, the phase shift caused by a LPF varies according to the input frequency. That means a signal at frequency A will have a different phase delay than a signal at frequency B. An inverter has a constant phase shift of 90 degrees over the entire frequency spectrum, not the varying phase shift needed. An allpass filter passes all frequencies equally, but with an varying phase shift.

Also, keep in mind that the user is going to want to change the cutoff frequency of the LPF. Therefore the allpass filter must track to this cutoff change to cancel properly.

I couldn't find much on the internets about the phase cancellation I was describing, but check out this neat little diagram I found. Its shows a discrete signal, but the basic idea is the same.

GregAE · Post by **GregAE** » Thu Nov 02, 2006 6:43 am

Impossible Sound wrote:I used an external mixer to combine the signals, but the CP's mixer works fine for audio - I don't get any clicking.

The mixer in the CP-251 has sufficient bandwidth for processing audio signals, and can be used as a simple audio mixer. But watch out for DC offset (the Mixer's Offset control), as this will add a DC component to the Mixer Output signal (AC). A DC offset can have a negative effect on audio circuits that the Mix Out signal is routed to.

-G

asd · Post by **asd** » Thu Nov 02, 2006 12:04 pm

Greg,
I've used my CP with audio so I know it's not a bandwith issue, but I get a bleedthrough from the lfo resulting in a continuous loud clicking. I assumed it was just a limitation of the CP, but it seems like it just might be my unit...

willi · Post by **willi** » Sun Nov 05, 2006 7:08 pm

Impossible Sound wrote:I've since added a real HPF to my rig, so there's no more need for this tomfoolery.

What HPF did you choose?

Impossible Sound · Post by **Impossible Sound** » Tue Nov 07, 2006 2:10 pm

Wiard Borg 2 and Frostwave Resonator.