What's the difference between SVF and IIR filters?

[the_nihilist]

What's the difference between SVF and IIR filters? I've never actually seen an explanation between different filter types like that, and was wondering if anyone could shed some light on the issue?

[scuzzphut]

There are two fundamental types of filter algorithm
IIR - Infinite Impulse Response
FIR - Finite Impulse Response

Compared to IIR filters, FIR filters offer the following advantages:

* They can easily be designed to be "linear phase" (and usually are). Put simply, linear-phase filters delay the input signal, but don’t distort its phase.
* They are simple to implement. On most DSP microprocessors, the FIR calculation can be done by looping a single instruction.
* They are suited to multi-rate applications. By multi-rate, we mean either "decimation" (reducing the sampling rate), "interpolation" (increasing the sampling rate), or both. Whether decimating or interpolating, the use of FIR filters allows some of the calculations to be omitted, thus providing an important computational efficiency. In contrast, if IIR filters are used, each output must be individually calculated, even if it that output will discarded (so the feedback will be incorporated into the filter).
* They have desireable numeric properties. In practice, all DSP filters must be implemented using "finite-precision" arithmetic, that is, a limited number of bits. The use of finite-precision arithmetic in IIR filters can cause significant problems due to the use of feedback, but FIR filters have no feedback, so they can usually be implemented using fewer bits, and the designer has fewer practical problems to solve related to non-ideal arithmetic.
* They can be implemented using fractional arithmetic. Unlike IIR filters, it is always possible to implement a FIR filter using coefficients with magnitude of less than 1.0. (The overall gain of the FIR filter can be adjusted at its output, if desired.) This is an important considertaion when using fixed-point DSP's, because it makes the implementation much simpler.

i.e. it's to do with maths and hard sums and therefore best forgotten.



A State Variable Filter (SVF) is just a filter which has LPF,HPF,Notch and Bandpass all available simultaneously, like in this picture (see the bottom right for the 4 output jacks)

[atomota]

This probably won't help, but a SVF is a type of IIR filter.

An IIR filter is built out of digital delays with feedback, which causes it to have an Infinite Impulse Response: when you put in a single impulse sample, it keeps feeding back resulting in an infinite response that never decays completely to zero.

Contrast this to an FIR (Finite Impulse Response) filter, which can be conceptualized as a tapped delay line with the taps mixed together at various levels. Put an impulse into this and once it has traversed the entire delay and fallen out the end, the filter's response is zero.

FWIW, convolution reverbs are FIRs with a very long tapped delay. Comb-filter reverbs (freeverb, etc.) are IIRs.

The real DSP heads may say I've simplified things, but this is how I conceptualize this stuff and it works for me.

[Lunch Money]

Slightly on-topic, but more an excuse to 'sell' you some free software-- BetabugsAudio offers a few different filter plug-ins, including a handy SVF filter. Just click my sig and then "plug-ins" on the BBA page that appears.

[Christian Budde]

SVF is a special form of an IIR-Filter. It is much easier to use this kind of filter, if you really need these different outputs for processing. Otherwise a regular IIR-Filter will do a much better job.
Usually you may only calc some 'adds' too much, but that's for every sample, so if not needed you will anyway waste performance and it maybe not as numerical correct as an standard (one output only) IIR-Filter.
In the regular case you can also do some nice inplace Block-Processing, which gains more performance.
Use your brain to decide, what filter you need,

Christian

[edit]Ups, sorry i thought this was in DSP & Development...[/edit]

[griffin]

Could someone explain the difference between the svf filters on nitro and sytrus. On sytrus it allows you to balance the outputs between the low,band & high pass but why does it then have low,band highpass filters to choose from in the filters section:? But the svf filters in nitro dont have control over low,band & highpass they are just like low,band & highpass filters but they can go into self oscillation. I'm confused to say the the least, can anyone enlighten me?