BiQuad frequency response for visualisation

robert bristow-johnson wrote:

> On Jun 30, 3:16 pm, Vladimir Vassilevsky <antispam_bo...@hotmail.com>
> wrote:
> 
>>Greg Berchin wrote:
>>
>>>In the last audio EQ that I designed, the customers demanded boost/cut
>>>symmetry in EQ filters, so I (re)defined Q accordingly.
>>
>>Exactly. The similar adjustment has to be applied for the shelving
>>filters for tone control.
> 
> 
> i guess i don't know what you mean, Vlad.  do you mean to adjust the
> value of Q (when the shelf gain changes) so as to keep the steepest
> possible monotonic slope of the shelf?
> 
> otherwise, what Q adjustment are you guys referring to?

As noted by Greg, they want the boost and cut responses to be 
symmetrical. I.e. the cut response should look like a mirror image or 
the boost response. This applies to the EQ sections as well as to the 
tone control sections. With EQ sections, we have to adjust Q by gain, 
and with the shelving sections, we have to adjust the corner frequencies.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

Btw. I'm pretty newbie in this, what is exactly the bilinear transform in
this case? I've read about the pole-zero method, but this does not seem too
close. Just please give me a hint :-).

And one more not very celver question : What is exactly the difference
between filters like Chebyshev, biquad etc. I don't know if it is
chebyshev, but it seems, that most of those filters are simply IIRs. So I
would think, that the difference is basically the way how to compute filter
coefficients to achieve certain frequency/phase responses. Am I correct?

Thx.

On Jun 30, 7:20&#4294967295;pm, robert bristow-johnson <r...@audioimagination.com>
wrote:

> but that's the correct EE Q anyway for a notch. &#4294967295;it's what's in the
> cookbook (for notch, not for boost/cut peaking EQ).

Agreed, but non-intuitive for the customers, who were not EEs.  Once
they saw for themselves just how narrow a 1/12 octave notch was
(compared with a 1/12 octave boost/cut EQ), they were sold.

Of course, then there was the problem of making 20 Hz 1/12 octave
notch filters work at 96 kHz sampling rate with fixed point
arithmetic.

Greg

On Jul 1, 6:57 am, "jungledmnc" <jungled...@gmail.com> wrote:
> Btw. I'm pretty newbie in this, what is exactly the bilinear transform in
> this case? I've read about the pole-zero method, but this does not seem too
> close. Just please give me a hint :-).
>
> And one more not very celver question : What is exactly the difference
> between filters like Chebyshev, biquad etc. I don't know if it is
> chebyshev, but it seems, that most of those filters are simply IIRs. So I
> would think, that the difference is basically the way how to compute filter
> coefficients to achieve certain frequency/phase responses. Am I correct?
>
> Thx.

http://en.wikipedia.org/wiki/Bilinear_transform

It's a method usually used for transforming an analog prototype filter
(Chebyshev, Butterworth, elliptic, Bessel, etc.) into a digital filter
that has a similar response. Google for these filter types and you can
learn all about them.

Jason

On Jun 30, 11:34&#4294967295;pm, Vladimir Vassilevsky <antispam_bo...@hotmail.com>
wrote:
> robert bristow-johnson wrote:
> > On Jun 30, 3:16 pm, Vladimir Vassilevsky <antispam_bo...@hotmail.com>
> > wrote:
>
> >>Greg Berchin wrote:
>
> >>>In the last audio EQ that I designed, the customers demanded boost/cut
> >>>symmetry in EQ filters, so I (re)defined Q accordingly.
>
> >>Exactly. The similar adjustment has to be applied for the shelving
> >>filters for tone control.
>
> > i guess i don't know what you mean, Vlad. &#4294967295;do you mean to adjust the
> > value of Q (when the shelf gain changes) so as to keep the steepest
> > possible monotonic slope of the shelf?
>
> > otherwise, what Q adjustment are you guys referring to?
>
> As noted by Greg, they want the boost and cut responses to be
> symmetrical. I.e. the cut response should look like a mirror image or
> the boost response.

i get that, and had since 1994.  the cookbook peaking EQ does that,
but the Q or BW of that maybe needs a mapping to relate it to yours.
there are an infinite number of ways to accomplish this boost/cut
symmetry.  my question is what basis did you use to chose one of those
ways.

> This applies to the EQ sections as well as to the
> tone control sections. With EQ sections, we have to adjust Q by gain,
> and with the shelving sections, we have to adjust the corner frequencies.

depends on how the corner frequencies are defined.  since it's kinda
hard to define it as the 3 dB frequency when the boost/cut is less
than 3 dB, i've been defining it at the shelf-midpoint frequency
(since that exists for all boost and cut amounts).  in the cookbook,
the boost and cut shelving filters are also symmetrical in the same
manner.

it's the same idea for the peaking EQ to define the bandedges at the
midpoint gain.  those bandedges are consistently defined and exist for
any boost/cut gain.

r b-j