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