Joined: Feb 28, 2005 Posts: 4044 Location: England
Audio files: 32
G2 patch files: 15
Posted: Tue Apr 18, 2006 12:08 pm Post subject:
4LP Doped (AR) filter Subject description: What exactly is one of these?
Hi Everyone (especially filter gurus),
I don't know if any of you have the Photone synth that runs in Reaktor but it has a really nice sounding filter called "4LP Doped (AR)".
I have searched on the internet for Doped Filters and have not got very far (something to do with erbium maybe?), I have also looked at the internals of the filter in Reaktor but its all greek to me!
Joined: Mar 29, 2004 Posts: 580 Location: The Hague/Netherlands/EC
G2 patch files: 109
Posted: Wed Apr 19, 2006 1:05 pm Post subject:
BobTheDog wrote:
Hi Guys,
Here is a wav @ 24 bit 48K
Cheers
Andy
Sounds and looks (the waveforms) like a pretty standard 24dB ladder filter emulation to me.
/Rob
Btw, it made me change my avatar...
(In fact, this little cactus that few of you might recognize as a Lophophora Williamsii decided to blossom again after twelve years, and as the flower lasts only one day it was especially special that this just happened on my birthday yesterday. A very special gift I would say and probably a sign that the great Mescalito will help patching quite a few doped filters the coming year.)
it was especially special that this just happened on my birthday yesterday. A very special gift I would say and probably a sign that the great Mescalito will help patching quite a few doped filters the coming year.)
from one old fart to another, Hey! happy birthday! _________________ Dave Peck
Lophophora Williamsii decided to blossom again after twelve years, and as the flower lasts only one day it was especially special that this just happened on my birthday yesterday. A very special gift I would say
What a coincidence that this just happens on your birthday!
Happy birthday by the way!!
Wow Rob, what a fantastic looking cactus. If flowering on the anniversary of your birth isn't a sure sign that it's ready to guide you, I don't know what is!
Joined: Mar 29, 2004 Posts: 580 Location: The Hague/Netherlands/EC
G2 patch files: 109
Posted: Sat Apr 22, 2006 12:36 pm Post subject:
BobTheDog wrote:
Hi Guys,
Here is a wav @ 24 bit 48K
Cheers
Andy
Bob, as it now looks like I hijacked your thread, which I suppose is not very gentlemen-like, I thought it proper to have another look at that wave file.
There is definitely two resonances going on, which suggests some sort of filter with e.g. two second-order poles. The idea of such a filter is to create two or more slightly resonant and detuned filters, where the resonance peak of the higher tuned filter sort of sharpens the corner of the cutoff slope of the lower tuned filter at the cutoff point. Math geeks name these Chebyshev filters, btw. But in this case the second resonance is too high for emulating a Chebyshev filter, so there is something fishy going on here.
Another observation is that when a pure square wave is fed into a filter the resonance swings around the horizontal lines of the square wave. But here the flanks of the square wave seem to have been emphasised. This normally suggests AC coupling in an analog synth, which can be emulated by a highpass filter set lower than the fundamental of the wave to be filtered. But this might already be in the waveform before it is fed into the filter, so there is not much to say about this.
Anyway, I fiddled around a bit on the G2 and came up with two types of filters that are not really like what you are after, but that might still be of interest.
The first type is two state variable filters in series, which is similar to many of the analog filters used in Roland synths. On the first filter outputs is a mixer that mixes the LP and HP output in a way that it creates a notch at the third harmonic of the fundamental when the filter is tuned to the fundamental of the waveform to be filtered. This appears to make the cutoff slope sharper. But it also leaks a bit of the very high from the HP output, which is conveniently filtered away in the second filter. The filters are tuned in a way that they sort of create a "3-dB passband ripple" Chebyshev filter.
The second type is two detuned classic filters in parallel, but with a phase-shifting allpass filter set before the input of one of the filters. When the ouputs are mixed the result is better considered to be the interference between the two filters than just a sum of what they pass on. The phase shifting allpass filter can alter the interference by pushing the waveform harmonics forwards or backwards in time. Each harmonic will have its own amount of shift. This will shift harmonics in or out of phase with the output of the other classic filter, which sort of makes it between a lowpass filter or a bandpass filter. Changing the frequency parameter of the allpass filter (made with the simple and non-resonant 24 dB LP filter module) changes the character of the filter sound.
Have fun,
/Rob
DopeyFilters.pch2
Description:
Just two more exotic types of resonant filters to sweep.
Joined: Feb 28, 2005 Posts: 4044 Location: England
Audio files: 32
G2 patch files: 15
Posted: Sun Apr 23, 2006 9:48 am Post subject:
Hi Rob,
First of all there is no problem hijacking the thread and a happy birthday to you from me as well.
Thanks for the two filters, I particularly like the type I one.
I am just beginning to realise that the filters play a big part in the sound of different synths, do you have a recommendation of a book or good web site that would enable me to remove some of my ignorance surrounding the matter?
I have also included a pic of the Reaktor structure for you, I'm sure it will make a lot more sense to you than it does to me!
Once again thanks for your help
Andy
SMAxdoped.JPG
Description:
Filesize:
78.86 KB
Viewed:
657 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.
Joined: Mar 29, 2004 Posts: 580 Location: The Hague/Netherlands/EC
G2 patch files: 109
Posted: Sun Apr 23, 2006 4:46 pm Post subject:
BobTheDog wrote:
I have also included a pic of the Reaktor structure for you, I'm sure it will make a lot more sense to you than it does to me!
Andy
As I don't work with Reaktor I cannot say much about what is going on there. I don't know the peculiarities of those Reaktor modules, but next to a resonant LP filter an allpass filter and a 6dB LP filter seem to influence the sound.
Aah well, there is soo much variations one can try oneself by combining more than one filter that it seems more fun to just experiment and hear what comes out.
A book that tells a lot about filters is Don Lancaster's Active Filter Cookbook, already around for some thirty years and recently reprinted. It contains all the basics in accessible language. Regrettably it is all about actual analog circuit, so you would have to learn how to read analog opamp-based circuit diagrams. (Which isn't really that hard.)
Imho it is also very well possible to use a 'common sense approach' to filters, without having to use complex math. The important thing to realize is that filters also cause frequency-dependent phase shifts and both the cutoff and the phase shifts of a combination of filters act together to combine into a final 'sound' specific for that configuration. E.g. in that Type I filter I simply used a sinewave osc tuned three times higher as the cutoff frequency of the filter to set the balance of the mixer to get a notch at the 'third harmonic'. Simple to do and much quicker as calculating what that balance should be, which is actually quite complex math. A cheap trick that works like a charm on the G2.
Joined: Jun 20, 2004 Posts: 701 Location: Brisbane, Australia
Audio files: 12
G2 patch files: 79
Posted: Tue Apr 25, 2006 2:52 pm Post subject:
Really liking those dopeyfilters Rob. The first one sounds alot like the "phat" virtual analogue sound common in the nova/virus range. The second one without any resonance sounds really cool too - it's like the width of a 12db bandpass with the steepness on either side of a 24db one. They've given me some ideas for further experimentation
Maybe this patch could be copied to the building blocks section? They both seem to have that "new" sound people search for when they first get their G2.
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4LP Doped (AR) filter
