analog filter question

technochris81 · Post by **technochris81** » Thu Jan 31, 2008 6:34 am

Can someone please explain the following to me in more detail? I understand what a basic filter does, but I can't exactly picture what this is doing in my head.

"The Slave switch below Filter 2’s mode chooser causes this lter’s cutoff
frequency to follow the cutoff of Filter 1. If this is enabled, Filter 2’s cutoff knob controls the
amount of offset between the two cutoff amounts. If any of Analog’s modulation sources
are controlling Filter 1’s cutoff, Filter 2 will also be affected by them when Slave is enabled."
-p322 L7 manual

thanks,
-chris

technochris81 · Post by **technochris81** » Thu Jan 31, 2008 3:40 pm

Zerobae · Post by **Zerobae** » Thu Jan 31, 2008 4:37 pm

not that I've tried this yet, but I think it says the following:

"The Slave switch below Filter 2’s mode chooser causes this lter’s cutoff
frequency to follow the cutoff of Filter 1. If this is enabled, Filter 2’s cutoff knob controls the
amount of offset between the two cutoff amounts.

when in "slave" mode the cutoff knob of filter 2 does not -- as it normally does -- control filter 2's cutoff frequency, because this is controlled by filter 1's cutoff knob (which controls cutoff 1+2 when "slave mode" is on). it does however control something: the offset between the cutoff frequencies of filter 1 and 2.

If any of Analog’s modulation sources
are controlling Filter 1’s cutoff, Filter 2 will also be affected by them when Slave is enabled.

with modulation sources like lfo's you can control f.i. cutoff frequencies. when you use say an lfo to control filter 1's cutoff and the filters are in "slave mode", the lfo will control filter 2's cutoff, too. (because the filters are linked).

technochris81 · Post by **technochris81** » Thu Jan 31, 2008 8:38 pm

Zerobae wrote:when in "slave" mode the cutoff knob of filter 2 does not -- as it normally does -- control filter 2's cutoff frequency, because this is controlled by filter 1's cutoff knob (which controls cutoff 1+2 when "slave mode" is on). it does however control something: the offset between the cutoff frequencies of filter 1 and 2.

so what exactly does "offset" mean then?

and how does high-pass vs. low-pass on either of the filters play into the equation? i mean, i know what HP and LP mean normally, but what does it mean if the slave filter is set to HP vs. LP?

thanks,
-chris

Machinate · Post by **Machinate** » Thu Jan 31, 2008 8:43 pm

the link is in the cutoff frequency - so if filter 1 is at 850hz, and the freq knob in filter 2 is set to 0 then the freq of filter 2 will ALSO be 850hz, since they are Linked.

If you then move the knob of filter freq 2 to 200hz, then the current cutoff frequency of filter2 will be at 1050hz.

laird · Post by **laird** » Thu Jan 31, 2008 8:48 pm

So are the filters set to be off by a certain amount,
or are they set by an off amount.

I find this conundrum offsetting.

technochris81 · Post by **technochris81** » Thu Jan 31, 2008 9:23 pm

Machinate wrote:the link is in the cutoff frequency - so if filter 1 is at 850hz, and the freq knob in filter 2 is set to 0 then the freq of filter 2 will ALSO be 850hz, since they are Linked.

If you then move the knob of filter freq 2 to 200hz, then the current cutoff frequency of filter2 will be at 1050hz.

ok, but i still have a few more questions:

1) the signal is essentially just getting "filtered" twice right?

2) what is the significance of HP vs LP on filter 2 if it is in slave mode?

-chris

laird · Post by **laird** » Thu Jan 31, 2008 9:37 pm

A HP+LP filter combo in series is also known as a bandpass filter.

Only requires 1 knob to adjust frequency... and the Q/width remains constant (the "offset")

elektrovert · Post by **elektrovert** » Thu Jan 31, 2008 9:38 pm

it makes it into a band pass filter!

laird · Post by **laird** » Thu Jan 31, 2008 9:44 pm

I agree!

Unless the HP is set higher than the LP, and it becomes a None Shall Pass Filter.

technochris81 · Post by **technochris81** » Thu Jan 31, 2008 10:07 pm

laird wrote:I agree!

Unless the HP is set higher than the LP, and it becomes a None Shall Pass Filter.

ok, but the F2 value would always be ADDED to the F1 value right, never subtracted?

-chris

laird · Post by **laird** » Thu Jan 31, 2008 10:12 pm

In your instance, yes.

But if you have two individual filters in series, (not in slave mode), then if you only let stuff above 1000hz pass, then remove everything above 1000 hz, you get nuzzing, Lebowski, nuzzing.

technochris81 · Post by **technochris81** » Thu Jan 31, 2008 11:27 pm

ok, good. thanks. i think i'm straight on that.

now...can anyone explain how a formant filter works?
or maybe just provide a link?

thanks,
-chris

tylenol · Post by **tylenol** » Fri Feb 01, 2008 12:41 am

technochris81 wrote:now...can anyone explain how a formant filter works?

A common model in speech production for the vocal tract is that it consists of a sound generator (for vowels, the glottis) plus some resonating tubes and adjustable parts (the tongue, lips, etc). The tubes and parts effectively form a physical filter for the sound produced by the glottis. When producing a vowel, glottal vibration along with this filtering effect is all the vocal tract does (this isn't true of non-vowel sounds). The characteristics of this filter are basically a series of several "formants" -- dominant frequencies, i.e. partials, that are let through by the filter. Wikipedia has a summary of the first few formants for a range of vowels.

To get a better intuitive idea of how this works, blow through your mouth as if you were whistling (without the whistle part). This produces something like pink noise. While doing this, move your tongue around (the body of the tongue and the tip being the two important parts), and listen to the filtering effect -- for instance, you can actually produce filter sweeps by holding your tongue body high and moving the tip up and down gradually.

A formant filter works by mimicking the filtering effect of the vocal tract in various configurations (tongue position being the most important), I think usually with a series of specially tuned band pass filters, where each band corresponds to a formant. To actually make this sound good with a wide range of inputs, it might not be tuned exactly like the vocal tract, which really only ever has the glottal vibration as its input.

Komplex · Post by **Komplex** » Fri Feb 01, 2008 6:50 am

change ur topic name, this aint no analogue filter.