[net.audio] CD phase shift question

pmr@drufl.UUCP (Rastocny) (04/11/84)

In the April issue of "Audio" magazine, I read that 11th order filters
(the kind that some CD players use in their anti-aliasing filters) can
alter phase more than 360 degrees.  My question is:

IS THIS AN APRIL FOOLS PRANK OR DO SOME CD PLAYER'S ANTI-ALIASING
FILTERS ACTUALLY ALTER PHASE THIS MUCH?

		Yours for higher fidelity,
		Phil Rastocny
		AT&T-ISL
		..!drufl!pmr

rfg@hound.UUCP (R.GRANTGES) (04/11/84)

Consider it an April Fool's prank, Phil, and forget it.
hound!rfg

karn@allegra.UUCP (Phil Karn) (04/11/84)

Sure, filters can shift phase by as many degrees as they like.
Filters which do this as a linear function of frequency with
zero phase shift at zero frequency are called "linear phase"
and pass all signal components with equal delay related to the
slope of this phase shift characteristic.  If the filter also
passes all frequencies with equal amplitude, then it is called
a "delay line".  Whatever waveform goes in comes out exactly
the same sometime later.  As long as this is done equally in
both channels you cannot call it "distortion", as the same effect
is achieved by hitting the "play" button on your CD a few microseconds
later...

One of the characteristics of high-order filters is that they
introduce more and more delay. This is mathematically necessary
in order to more closely approximate an ideal filter, which
of course would be "non causal", i.e., respond before it saw
an input, which of course is impossible in the real world
(net.misc excepted.)  Bessel filters are just much more careful
about making this delay constant over all frequencies than
other filters, and in fact Bessel filters improve with increasing
order while most other filters get worse.
Phil

spoo@utcsrgv.UUCP (Suk Lee) (04/12/84)

Sure they do, if they're 11'th order or more.  Part of the
problem is how you view "phase difference".  I would prefer
to talk of the group delay of the filter, which reflects how
different frequencies are *DELAYED* (which is what most people
care about), rather than the phase of the filter.  
Since all the nasty phase-shifting occurs at high frequencies, 
I still say, "who cares?"

say thoug
-- 

From the pooped paws of:
Suk Lee
..!{decvax,linus,allegra,ihnp4}!utcsrgv!spoo