Re: Audio Shelving fliters

gusflit@xxxxxxx wrote:
Hi all!

I'd like to implement 1st and 2nd order shelving filters - which are
generally called 6 db/oct and 12 db/oct filters.

I had a look at the 'Audio EQ Cookbook', but it doesn't say a word
about 1st order filters,

That's because first order shelving filters don't exist.

and for 2nd order filters it's not clear to me
how the slope parameter S (S <= 1) relates to the filter slope
expressed in db/oct.

S is similar to the Q in lowpass filters. Around the 3dB-point, the Q
of lowpass filters significantly affects the slope of the filter. It
reaches the 12dB/oct roll-off only further away from the 3dB frequency.
Similarly, the slope of shelving filters is affected by the S
parameter, even though the order remains constant.

Matlab simulations showed that RBJ's second order
filter could be a 12dB/oct when S is set to 1 - but in order to achieve
such a rolloff, the amount of boost or cut must be significant (about
30dB) so i keep doubting (Sorry RBJ your document is just great - by
the way thanks for making it available online - it's just that i don't
know how to use it).

Perhaps you don't need a shelving filter at all? Shelving means that at
DC and Nyquist, the filter is flat, with a certain gain difference
between the two flat sections. The slope of the transition band is
controlled with the S parameter, and can be much steeper than 12dB /
oct.


I would greatly appreciate if someone could point a useful document -
common analog transfer functions ready for a bilinear transformation or
- even better! - digital transfer functions (yeah i'm a lazy bum ;) )

The cookbook has digital transfer functions.

Regards,
Andor

.

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