EQ Group Delay and Ringing

Archived from groups: rec.audio.pro (More info?)

"Mark" <makolber@yahoo.com> wrote in message
news:1107489607.048203.126790@z14g2000cwz.googlegroups.com

> This ring will occur even if the filter is
> implemented as an FIR and has a flat group delay. An FIR filter with
> a flat GD will pre-ring and post ring. A min phase IIR will only
> post ring. Again, the ringing is inherent and due to the steepness
> of the EQ change and will happen no matter how the filter is
> implemented. See the Gibbs phenomenon.

The obvious question is exactly how much ringing does it take to be audible.

> So I think what I'm getting
> at is that rapid steep freq response changes produce ringing and
> sound bad no matter how they are created.

It's an easy test. You take some impulsive music recorded at 24/96 and brick
wall it variouis ways at various frequencies, and come up with a usable way
to compare the brick-walled versions with the originals. A bunch of this was
done with the audio samples posted at

http://www.pcabx.com/technical/sample_rates/index.htm


>So if you have a 256 band
> EQ and you create a steep change in the freq response, it will
> probably sound bad regardless of how it is implemented.

You're just speculating, here. Why not let your ears be your guide?

> If you
> need to apply EQ and want it to sound good, the response changes
> should be done gradually.

Now you're making a global rule for everybody based on suppositions with
zero practical experience of your own to back it up.

> So maybe it's not the GD variation from EQ that sounds bad but simply
> the ringing cause by rapid frequency response changes.

Here's a hint - problems arise due to the frequency that the steep cutoff
are implmented, and to a lesser degree how it is implemented. The only
remaining question of interest is what frequencies are good, and what
implementations are good.

> This may also relate to why 44.1 kHz sampling may sound bad to some
> people.

Here we go again. A gobal supposition based on what evidence? People's
opinons of the CD format can be very political. Lots of people have axes to
grind - people invested in analog or otherwise favor it, people who are
invested in something they want to be better sounding that 44.1 or otherwise
favor it, etc. At some point the political madness could be stopped, and
some reliable listening tests could be used to extablish what works and what
does not.

>There is always a steep low pass filter at around 20 kHz.

Not at all. Do your recording at 24/96 or 24/192 and you can move the corner
frequency way up. Given the price of even 24/192 hardware, anybody who has
about $200 to play with can find out for themseveles/

> I know __I__ can't hear any ringing around 20 kHz, but I suppose with
> the right source material and the right speakers someone might be
> able to hear ringing at 20 kHz.

I've recently heard some claims that may be along this line, but I'm not
familiar with the means used to do the listening tests, or I know that they
are not good test procedures.


> And a final comment for anyone wondering about how FIR filters can
> have pre-ringing. How does the filter predict the future?.

The filter implements a delay. If you compare the input to the output, the
whole output is delayed, but the pre-ringing is delayed less.