Forums

All-pass filter specified by phase shift amount?

Started by Jon Harris November 4, 2003
In my experience 1st and 2nd order all-pass filters are usually specified by
their frequency, i.e. the frequency at which the phase shift is half of the
maximum. For example, see r b-j's audio cookbook
(http://www.harmony-central.com/Computer/Programming/Audio-EQ-Cookbook.txt)
for a 2nd order digital implementation.

However, often times in audio, I see all-pass filters specified by the
amount of phase shift. This is usually on a sub-woofer or other active
crossover.  For example, http://www.hothousepro.com/products/sbx.html
features a phase control described as follows:
"This control uses a potentiometer and an all pass filter to scale the
subwoofer's phase from 0 to 180 degrees. This can be one of the most useful
tools in blending a sub into any system."

What I don't understand is at what frequency or frequencies the phase shift
is at. Let's say you set the control to 90 degrees.  I assume that it's not
implementing a Hilbert transform!  So what frequency is shifted 90 degrees?
And how would I convert this phase adjustment to something I could realize
with say r b-j's AP filter?

--
Jon Harris
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In my experience 1st and 2nd order all-pass filters are usually specified by
their frequency, i.e. the frequency at which the phase shift is half of the
maximum. For example, see r b-j's audio cookbook
(http://www.harmony-central.com/Computer/Programming/Audio-EQ-Cookbook.txt)
for a 2nd order digital implementation.

However, often times in audio, I see all-pass filters specified by the
amount of phase shift. This is usually on a sub-woofer or other active
crossover.  For example, http://www.hothousepro.com/products/sbx.html
features a phase control described as follows:
"This control uses a potentiometer and an all pass filter to scale the
subwoofer's phase from 0 to 180 degrees. This can be one of the most useful
tools in blending a sub into any system."

What I don't understand is at what frequency or frequencies the phase shift
is at. Let's say you set the control to 90 degrees.  I assume that it's not
implementing a Hilbert transform!  So what frequency is shifted 90 degrees?
And how would I convert this phase adjustment to something I could realize
with say r b-j's AP filter?

--
Jon Harris


Jon Harris wrote:

> In my experience 1st and 2nd order all-pass filters are usually specified by > their frequency, i.e. the frequency at which the phase shift is half of the > maximum. For example, see r b-j's audio cookbook > (http://www.harmony-central.com/Computer/Programming/Audio-EQ-Cookbook.txt) > for a 2nd order digital implementation. > > However, often times in audio, I see all-pass filters specified by the > amount of phase shift. This is usually on a sub-woofer or other active > crossover. For example, http://www.hothousepro.com/products/sbx.html > features a phase control described as follows: > "This control uses a potentiometer and an all pass filter to scale the > subwoofer's phase from 0 to 180 degrees. This can be one of the most useful > tools in blending a sub into any system." > > What I don't understand is at what frequency or frequencies the phase shift > is at. Let's say you set the control to 90 degrees. I assume that it's not > implementing a Hilbert transform! So what frequency is shifted 90 degrees? > And how would I convert this phase adjustment to something I could realize > with say r b-j's AP filter? > > -- > Jon Harris
As a practical matter, it seems that the cross-over frequency is what matters. If you adjust the control to match the phases of woofer and sub- woofer where each shows significant energy, that's where you'll end up. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
"Jerry Avins" <jya@ieee.org> wrote in message
news:bodkc4$lbu$1@bob.news.rcn.net...
> Jon Harris wrote: > > > In my experience 1st and 2nd order all-pass filters are usually
specified by
> > their frequency, i.e. the frequency at which the phase shift is half of
the
> > maximum. For example, see r b-j's audio cookbook > >
(http://www.harmony-central.com/Computer/Programming/Audio-EQ-Cookbook.txt)
> > for a 2nd order digital implementation. > > > > However, often times in audio, I see all-pass filters specified by the > > amount of phase shift. This is usually on a sub-woofer or other active > > crossover. For example, http://www.hothousepro.com/products/sbx.html > > features a phase control described as follows: > > "This control uses a potentiometer and an all pass filter to scale the > > subwoofer's phase from 0 to 180 degrees. This can be one of the most
useful
> > tools in blending a sub into any system." > > > > What I don't understand is at what frequency or frequencies the phase
shift
> > is at. Let's say you set the control to 90 degrees. I assume that it's
not
> > implementing a Hilbert transform! So what frequency is shifted 90
degrees?
> > And how would I convert this phase adjustment to something I could
realize
> > with say r b-j's AP filter? > > > > -- > > Jon Harris > > As a practical matter, it seems that the cross-over frequency is what > matters. If you adjust the control to match the phases of woofer and sub- > woofer where each shows significant energy, that's where you'll end up. > > Jerry
That would make sense logically. However, I would guess that in many analog active crossovers with adjustable frequency, for simplicity the all-pass circuit is not coupled to the crossover frequency. In that case, the phase shift setting should be treated more qualitatively rather than an exact amount. Since this parameter is often tweaked by ear or by watching measurement gear, this is probably just fine.