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Invert audio signal before or after VCF?
MUFF WIGGLER Forum Index -> 5U Format Modules  
Author Invert audio signal before or after VCF?
Rex Coil 7
Hey all! cool

I have two filters in my system, one is a Q107, the other a Q150. I'll primarily use them in series, but they'll also be used in parallel.

Member *Dave Peck
thoughtfully advised me to be sure that I invert the Q107 signal since it's output is inverted. If I run the two filters in parallel, and the output signals have opposite polarities it sets up a situation that may cause phase cancellation. Ok, I get that. Great suggestion, btw.

That having been said, which works "best" .... inverting the signal pre-VCF, or post-VCF?

For those unaware, my synth is presently disassembled due to the hot rodding project in progress. Otherwise, I'd just "try it".

I am attempting to make some decisions about the next phase of changes, and I'm at the point where decisions are being made regarding the placement of a few modules, as well as some signal routing issues.

At my disposal are two Q147 Distributors (which have 3 channel outputs ... each with an attenuverter to change each output signals' polarity) and a Q114++ Mixer, which also provides the ability to invert each channel. I also have a Suit And Tie Guy "Mixer", which is derived from the R.A. Moog CP3 mixer, and offers one Pos output, one Neg output, and a Main output with an attenuverter to invert that output's signal.

I ask you folks because I value the opinions of the good people in the 5U subforum. If I were able to round up all of the positive suggestions handed my way in this subforum, it would probably amount to a twenty chapter *How To* book!

Thanks! cool
ranix
my q147 has a weird quirk you might want to test on yours

it does not function properly unless there is a cable in the Input

instead it produces 500khz noise when any knob is turned over about 3

plugging a cable into the Input jack, even a short one, totally eliminates the problem seriously, i just don't get it

maybe a good candidate for something normalled to it or a switch, if you have the same issue
JohnLRice
I don't know if before or after would be better or make any difference but, I just has a thought, have you see Low-Gain's Bipolar Attenuverter Chiclets? A complete tiny little bugger for $15 that you could mount in your system where ever you might need attenuation and/or inversion? I haven't tried them but they look super handy!
http://lowgain-audio.com/



milkshake
Inverting before or after the filter doesn't matter.
Rex Coil 7
ranix wrote:
my q147 has a weird quirk you might want to test on yours

it does not function properly unless there is a cable in the Input

instead it produces 500khz noise when any knob is turned over about 3

plugging a cable into the Input jack, even a short one, totally eliminates the problem seriously, i just don't get it

maybe a good candidate for something normalled to it or a switch, if you have the same issue
500khz? Whew! That's far higher than my DOG can hear. I don't believe either of my Q147s do that. Even if they did all of the gear I own has no morethan roughly 20khz frequency response. I know that I certainly can't hear anything that high.

If you don't insert a plug, the Q147 is supposed to produce 10v- to 10v+ of offset voltage from each output jack. Beyond that, neither of mine exhibit any odd ball behavior.

Just to get it said, there's a jumper position round back of the module that will make each output produce 200% of the input voltage. Factory configuration is set to 100% (unity gain). Maybe yours is set that way and what you're hearing is a massive increase in voltage going in to something that's only expecting 10v instead of 20v.

After that, I got nothing ... sorry. seriously, i just don't get it
Rex Coil 7
JohnLRice wrote:
I don't know if before or after would be better or make any difference but, I just has a thought, have you see Low-Gain's Bipolar Attenuverter Chiclets? A complete tiny little bugger for $15 that you could mount in your system where ever you might need attenuation and/or inversion? I haven't tried them but they look super handy!
http://lowgain-audio.com/



This seems very usable, John. I could probably find some tiny little corner on a module panel to place a couple of those.

As usual, you, our resident synth sherpa, have brought the goods. thumbs up
ranix
Rex Coil 7 wrote:
[500khz? Whew! That's far higher than my DOG can hear.


the peak-to-peak on this is really high and it never reaches 5V, I recommend testing with a multimeter before using it as a voltage source blindly or else you may have trouble figuring out which part of your huge synth is causing the artifacts wink
Rex Coil 7
ranix wrote:
Rex Coil 7 wrote:
[500khz? Whew! That's far higher than my DOG can hear.


the peak-to-peak on this is really high and it never reaches 5V, I recommend testing with a multimeter before using it as a voltage source blindly or else you may have trouble figuring out which part of your huge synth is causing the artifacts wink
What on Earth are you talking about? "My huge synth" is not causing artifacts ... you're the one that posted the claim about having a 500khz "noise" emanating from your Q147. Presenting your own words ....

ranix wrote:
my q147 has a weird quirk you might want to test on yours

it does not function properly unless there is a cable in the Input

instead it produces 500khz noise when any knob is turned over about 3

plugging a cable into the Input jack, even a short one, totally eliminates the problem seriously, i just don't get it

maybe a good candidate for something normalled to it or a switch, if you have the same issue


All I'm inquiring about (in this thread) is if there is a sonic difference, possibly a preferred one, between inverting a signal before a filter, or after a filter. You then popped in to this thread and claimed your Q147 is making some kind of 500khz noise, I replied by saying 500khz is an extremely high frequency, far FAR higher than musical sound reinforcement gear is even capable of reproducing (begging the question of how you even know that it's doing that).

Your comments above are very baffling. hmmm..... I'm sorry but I cannot understand what you're getting at here. Clearly I am missing something.

seriously, i just don't get it

Yup, I just re-read everything .... I'm totally lost.

Sorry man. cool
Rex Coil 7
Ok wait .... are you suggesting to anyone that owns a Q147 that they should (somehow) test the output of their own Q147 to be sure that their's isn't producing some sort of 500khz ~squelch~ before they end up chasing their tail in an attempt to locate the source of some kind of mysterious racket in their "huge synth"?

Ok ... I guess. If that's what your vague post is getting at. But I don't understand your assertion .... "the Peak to Peak voltage is really high but never reaches 5 volts" ... it's either really high ... or ... it never reaches 5 volts. The two things are mutually exclusive (the way you've described it).

I think a more productive action would be to write to Roger Arrick at Synthesizers.com and present the question to him yourself about your Q147.

(I'm still confused here)
ranix
this is not a really complex idea I am communicating here

it does this: https://imgur.com/NTtioqA

maybe check to make sure yours doesn't do that before you use it for weird shit?

next time I'll keep it to myself
josaka
rex if you are totally lost why are you writing long rambling semi rude replies..?
Rex Coil 7
josaka wrote:
rex if you are totally lost why are you writing long rambling semi rude replies..?
So I'm to remain ignorant for the sake of brevity and silence? Asking questions about something I don't understand is somehow bad? Now I'm really lost.

Member *ranix, had you included the following two statements first off, this misunderstanding would have been alleviated .....

"it does this: https://imgur.com/NTtioqA

maybe check to make sure yours doesn't do that before you use it for weird shit?"

I think it's clear that I was the one that was having a problem understanding things, and I had no problem making it visible that I was the doofus with the problem ... y'know ... allowing myself to be made fun of. As I often times do.

The only way to gain understanding of one another is through communication. Running off and hiding is not a good way to handle things. EG;

"next time I'll keep it to myself"

No, that's not a good solution. Do you honestly feel that will solve anything? We are adults here, best I can tell. All I ask is perhaps next time just be a little more clear for those of us that are less able to read between the lines. I simply did not understand what you were getting at. Where's the sin? Other than me exposing myself to sarcasm and scrutiny ... willingly.

It's obvious I was having difficulty ... anyone ever consider that maybe ... just maybe ... I'm not as "sharp" as others? I don't always sleep very well, sometimes for days on end due to having severe nightmares that become so out of control that I'm actually afraid to go to sleep. In itself the lack of rest causes me to become dumb as a box of hair.

As for my "long rambling" posts, my apologies for anyone that feels forced to read them. I often catch crap for my long rambling posts, as well as for posting too many pictures. It's clearly time I make efforts to stop that. I suppose I misunderstand the purpose of a public forum, I honestly thought it was for sharing ideas and communicating with others that are like minded in our love for a given activity.

I'll offer my honest and sincere apologies. That's all I can do at this juncture. The wheels have obviously fallen off of this thread.

Ok, so to those that answered my initial questions about placing inverted signals before or after a filter, thanks a lot for helping me out. Solutions that are within the functions of the gear I already own have been found, so thanks again for the inspiration. It really helped!

Be well everyone.

Brian.


(... unsubbing because I need to thin out the threads I monitor ... there's just so many!! ... if anyone needs to contact me regarding these issues, please feel free to PM me ...)
ranix
Sorry, Rex, I thought it would be sufficient to tell you that something unusual happens when nothing is plugged into Input. All I intended to communicate is that I had reason to believe you should test it to make sure it does what you want before designing any new panels around it or deciding to use it without a panel for signal distribution. The module is not causing me any trouble because I have already characterized its behavior and know how it acts.

As an aside, I oft partake of the Devil's Lettuce before posting to this forum
megaohm
Phase inversion before or after the VCF won't make much, or any, audio difference. It's when you mix the signals where it comes into play.
I often patch a VCO to parallel VCF modules and mix the results. If you add a VCA to one of those parallel paths you can get interesting movement in the final result. It's different than pitch/freq mod and even amplitude mod.
But other times I am caught by surprise by phase inversions. If I don't know they are inverted and mix them I'm wondering where that sound went.
hmmm.....
Oh yeah! They're cancelling.

If I'm remembering right, The DotCom xfade module uses inversion for the xfade function.
Input signal feeds a VCA and an invertor in parallel and their outputs are summed.
When the VCA is open the two signals mix. Since they are out of phase with each other, they cancel and you get silence.
When the VCA is closed the signal from the invertor path gets through to the output unattenuated.
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