tap.filter~ questions

[bkshepard]

I'd like to try using [tap.filter~] to create a highpass filter, but
have a couple of questions.

1. What are the default values for the object?
2. Am I correct in assuming that a highpass.butterworth.1, . 2, .3 or .
4 has a slope of 6dB, 12dB, 18dB, or 24db per octave respectively?
3. Does "q" have any effect if you choose a highpass.butterworth
filter?

Thanks.

[Trond Lossius]

Hi,

> I'd like to try using [tap.filter~] to create a highpass filter, but
> have a couple of questions.
>
> 1. What are the default values for the object?

The default value seems to be 1000 Hz for all Butterworth filters. I
guess I set this default frequency for the simple reason that the
filtering would be noticable in all filter types.

> 2. Am I correct in assuming that a highpass.butterworth.1, . 2, .3
> or .
> 4 has a slope of 6dB, 12dB, 18dB, or 24db per octave respectively?

Yes.

> 3. Does "q" have any effect if you choose a highpass.butterworth
> filter?

For the butterforth high- and lowpass filters, no. Butterworth filters
are characterised by having maximally flat frequency response in the
passband region, and do not have resonating peeks. For the bandpass
and pandreject filters though, q will influence how wide the bandpass
region is.

If you want resonating peeks for highpass/lowpass filters, you should
look for other filters than Butterworth.

Best,
Trond

[bkshepard]

Thanks for the clarification! I'm building a variable-slope highpass
filter and the butterworth seemed like the way to go. I was curious
about the Q, though, since it defaults to "18." I didn't think Q had
an effect, but I wanted to confirm. Thanks again for the info.

[Timothy Place]

Internally tap.filter~ is loading a variety of different filters with different properties.  Some have resonance/q/bandwidth, while others don't.  The Max object has to make an attribute available for controlling it regardless of whether it applies to the current filter.  This is the same as with the standard filtergraph~ object in MSP.

Many of the filters, in case you are interested, are implemented in the Jamoma DSP FilterLib.  The source code for these can be found @ http://code.google.com/p/ttblue/source/browse/#svn/branches/active/extensions/TTFilterLib .

best,
 Tim

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