Noob Questions About Filters

[Perfect Human Interface]

Hi all, I have some questions about filters that I hope someone might be able to share some insight on, specifically regarding steepness. I don't even know how these things are put together in code, so my understanding of them is only practical.

Is filter resonance simply feedback? What's the difference between resonance/feedback and a filter's inherent dB-per-octave measurement? How is a filter slope made steeper or gentler?

A current project of mine would benefit from steeper filters, but all I have to work with is several pre-made filter modules (put together by Dozius). Right now I have in there a butterworth that I think is 12dB/oct.; I've heard of filters used in plugins that go up to 96dB/oct. I know steep filters can sound less "natural" but I need something pretty surgical for this.

Much thanks in advance.

[KG_is_back]

Is filter resonance simply feedback?

Well, not really. Filter is a machine/circuit/program that conducts and reflects signals. They may have multiple feedback lines (for example biquad filter has two). Resonance causes the filter to (believe it or not) resonate more. That is usually caused by increasing feedback on at least some of the feedback lines of the filter, but also changing the feedforward.
To make the long story short: Resonance is far more complicated phenomenon and feedback plays a mayor part in it.

hat's the difference between resonance/feedback and a filter's inherent dB-per-octave measurement? How is a filter slope made steeper or gentler?

Slope can be made steeper/gentler by changing the filter resonance. that causes the filter to have resonant peak (which gets narrower and sharper until the filter becomes a sine wave oscillator). Another way is to Increase the order of the filter. That allows you to have sharper edges in filter response. However, this is not a simple task to do. Simplest way to do it is to chain multiple filters (for example two 12db/oct lowpass filters in series are effectively one 24db/oct lowpass filter).

[tester]

Basically, when you change the slope of a filter, keep in mind second thing. How it actually sounds. One order of filter with sharper slope will sound differently than 2x higher order that filter with 2x gentler slopes per order. The thing is. First - you affect phase across the spectra. Second - you affect the slope's shape (curve so to speak) and dynamic reaction of the filter to the sound that comes in.

As a non-programmer, but sound designer, my advice: play with filters you know from experience, and check which are good. And play with filter designs you can make in FS (use some examples to compare them), and check which sounds good for what purposes. It may be either on white noise, or on instruments, or with modulation. A good filter is a filter, that "sounds great", which means - even a crap design can become a great filter (...and theoretically great design may bring poor results in practice).

If you don't have good ears or studio hardware to test stuff like that - find someone who does the mixing and/or mastering tasks. Give them something to play with (even do realtime sessions), and ask them what they like, or what is missing.

As an example. I remember old Waves and I heard newer Waves. And to my surprise, the old ones were better, than the new ones (for things I do). The same EQ plugin. Sure, the new ones could bring more briliance to certain types of sounds, but they were no longer universal.

[Flemming]

Yes, Butterworth seems to go pretty steep. look at the picture here
http://zone.ni.com/reference/en-XX/help ... h_filters/

The steepest curve in that picture is a 20th order filter, which i believe can be achieved by chaining 10 of your 12dB Butterworth filters. Now, i have a theory, but it's only a guess: 10 filters in a row will eat a lot of CPU. If instead one programmed a 'cascading filter' with the possibility to cascade 10*12dB, it might eat up a little less CPU, considering that all the cascaded filters might be able to share some of the calculations.

[tester]

How sharp slope do you need? I have much higher orders x20 on board and it works fine on my old 2GHz C2D.

[Perfect Human Interface]

Simplest way to do it is to chain multiple filters (for example two 12db/oct lowpass filters in series are effectively one 24db/oct lowpass filter).

Ah, I've done that in the past and wasn't certain if it was correct to do so. I was thinking that was effectively like feedback, so wasn't sure if it would start to cause unwanted resonance or something. But if need be that should be a really simple solution. Thanks!

Basically, when you change the slope of a filter, keep in mind second thing. How it actually sounds.

Oh don't worry; I did the whole music production thing before getting into trying my hand at plugin development. For what I'm working on I need to be able to make precise cuts, for example to remove a fundamental frequency, so having something that sounds "smooth" and "musical" is less important.

How sharp slope do you need? I have much higher orders x20 on board and it works fine on my old 2GHz C2D.

I don't know. I'd have to experiment.

Thanks guys