tuning a parametric equalizer

Jon Harris wrote:

> Also, I just remembered there is an app note that discusses graphic EQs in 
> detail:
> http://www.rane.com/note101.html
> I think that may address many of your questions.
> 

I think these RaneNotes ( http://www.rane.com/rnumber.html ) rate a 
mention in FAQ's some where. Not explicitly DSP but point out things us 
newbies should be at least aware of.

in article 1107562092.243811.38420@z14g2000cwz.googlegroups.com, AdamMil at
adamm@san.rr.com wrote on 02/04/2005 19:08:

> I changed the EQ from bandpass-in-parallel to peaking-in-series and it
> appears to be working. Now I'm back to the original problem of tuning
> it. Are there any guidelines for choosing the Q value for a given
> frequency spread? (Again, it's a 10-band equalizer doubling from
> 31.25hz to 16khz.)

this appears to be one band per octave.  is that so?

if it is so, i would recommend starting with the bandwidth, BW, set to 1
octave.  the bandwidth parameter is a direct function of Q (and a little
dependent on the center frequency, f0, due to frequency warping of the
bilinear xform).

it won't be exactly flat, but a decent goal would be to set BW or Q so that
if all levels of the graphic EQ were raised by the same dB amount, that the
resulting frequency response would be flat.  in this case it won't be flat,
but it *will* be approximately flat to two points for each band (if the BW
in octaves is equal to the octave spread of neighboring f0).  because of
overlap of bands, the gain at the band centers will be a little larger than
the gain setting of the peak, so maybe setting the BW to be slightly less
than 1 octave would be better.  that could use a little experimentation.

-- 

r b-j                  rbj@audioimagination.com

"Imagination is more important than knowledge."

"robert bristow-johnson" <rbj@audioimagination.com> wrote in message
news:BE299294.448F%rbj@audioimagination.com...
> in article 1107562092.243811.38420@z14g2000cwz.googlegroups.com, AdamMil at
> adamm@san.rr.com wrote on 02/04/2005 19:08:
>
> > I changed the EQ from bandpass-in-parallel to peaking-in-series and it
> > appears to be working. Now I'm back to the original problem of tuning
> > it. Are there any guidelines for choosing the Q value for a given
> > frequency spread? (Again, it's a 10-band equalizer doubling from
> > 31.25hz to 16khz.)
>
> this appears to be one band per octave.  is that so?
>
> if it is so, i would recommend starting with the bandwidth, BW, set to 1
> octave.  the bandwidth parameter is a direct function of Q (and a little
> dependent on the center frequency, f0, due to frequency warping of the
> bilinear xform).
>
> it won't be exactly flat, but a decent goal would be to set BW or Q so that
> if all levels of the graphic EQ were raised by the same dB amount, that the
> resulting frequency response would be flat.  in this case it won't be flat,
> but it *will* be approximately flat to two points for each band (if the BW
> in octaves is equal to the octave spread of neighboring f0).  because of
> overlap of bands, the gain at the band centers will be a little larger than
> the gain setting of the peak, so maybe setting the BW to be slightly less
> than 1 octave would be better.  that could use a little experimentation.

RB-J has hit on some of the areas of difficulty with graphic EQs composite
frequency response matching what the sliders show.  It's a trade-off.  Consider
the case of 2 neighboring filters being boosted the same amount:  if the filters
are narrow enough so the resulting boost is about the same as the each band,
there will be a significant dip between the bands.  If they are wide enough to
completely eliminate the dip, the composite boost will be significantly larger
than the individual band settings.

But now that DSP is replacing static filters for graphic EQs, there have been
some advancements in this area--basically tweaking the filter boost and width/Q
dynamically so that the composite shape better matches the slider settings.
Even if these "smart adjustments" aren't made, most digital EQs show a composite
frequency graph so the user can at least tweak their settings to achieve the
composite curve they are looking for.