Post by William Sommerwerck Post by Bob Cain
something to the sound that nature cannot do.
Nature is strictly causal. Linear phase filtering is not.
It sounds as if you're talking about an IIR filter, which can be "non-causal"
(ie, the output can "precede" the input). (Strictly speaking, this is
impossible. I'm thinking of "pre-ringing.")
I think there is a lot of confusion here.
Linear phase means that all frequency components are delayed by the
same amount of time. So a linear phase system, assuming no EQ (all
frequencies passed equaly) yields the SAME WAVEFORM output as what
Analog filters (such as EQ) can not provide you with attenuation or
boost without ALSO AT THE SAME TIME altering the phase response. In
fact the "rules of the game" are somewhat stiff - analog circuits work
with poles and zeros. You insert a pole at a frequency f0, and it is
where you will have 3dB less gain. The slop will be 6dB per octave,
and the phase shift 45 degrees. The phase alteration due to that pole
at an ovtave below (f0/2) is near zero. At an octave above it is near
The bottom line is - analog filtering alters phase. It is not phase
The IRR is a digital aproximation of analog filters done in the
digital domain. They are very efficient compute power wise. One must
be doing the design right to avoide art-effects (such as limit
cycles). There is a lot to say here and I will just leave it alone for
One can do a linear phase EQ in the digital domain - the FIR filter.
FIR's is where the pre ringing stuff comes in, and you may hear it
with some particular audio signals if the sample rate is too low. By
the time the sample rate is 60KHz you are certainly away from such
FIR works great for audio with 88.2KHz and 96KHz sampling. It takes a
lot of processing power to do well.
FIR is non causal. I would not call IIR's not causul. Normaly, you do
not get an output out of IIR or an analog circuit before the impulse.
You do get preringing out of FIR's. I think here is too much talk
about preringing this days, it should be quantified. Some low
amplitude ringing at a frequency above your hearing is not the key to
everything, in my view. You may hear it with 44.1KHz but not at
88.2KHz... And it should be weighed against the advatages of FIR's
which are many.
Back to hearing phase - it has been said that if all audio channels
have the same phase curve, the sound arrives at your ears at the same
time for any given frequency, so "all is fine". The claim here is that
linear phase is an overkill and "equal phase response" treatment for
all channels is the key. I do not subscribe to it because changing the
waveform (non linear phase) will be treated differently by the
electronics. For example, I can take a fast rising signal and alter
the phase to lower slope... In any case, I do not know many that do
the same EQ to all channels, especialy in the case of many tracks...