karn@petrus.UUCP (06/30/85)
The July issue of Stereo Review has a very good (for an audio magazine) article comparing digital and analog filtering techniques. It provides an overview of the two methods and how digital filtering eases the task of constructing the (inevitable) final analog filter. The conclusion? The author prefers digital filters -- not due to any audible differences in phase response, but because digital filters ought to be more reliable and may also provide a flatter amplitude response. The following excerpt is also of interest. Can anybody provide a more complete citation to the JAES article? "The upshot of all this is that it has yet to be conclusively demonstrated that any of the differences in high-frequency phase response between analog and digital output filters are audible, audiophile opinions notwithstanding. Carefully conducted tests reported in the Journal of the Audio Engineering Society show that listeners cannot detect the operation of *either* type of filter when impulses are being reproduced, even when several of the filters are connected in series! Earlier experiments showed that listeners could not detect the presence of steep-cutoff analog filters when the cutoff frequency is as high as 20,000 Hz, as in CD players." Phil
sjc@angband.UUCP (Steve Correll) (07/03/85)
> The July issue of Stereo Review has a very good (for an audio magazine) > article comparing digital and analog filtering techniques...Can anybody > provide a more complete citation to the JAES article? Perhaps they are referring to the November 1984 issue, Vol 32 No 11, "Perception of Phase Distortion in Anti-Alias Filters" by Preis and Bloom. The experiment used broadband clicks, not speech or music, and concluded that the ear is "significantly more sensitive in the middle of the audio band (4 kHz) than at the upper edge of the band (15 kHz) to group-delay distortion". Listeners weren't able to discriminate between unfiltered and filtered sounds with 67% reliability at 15 kHz until the experimenters cascaded 8 pairs of seventh-order elliptical anti-aliasing filters. At 4 kHz the listeners scored better than 67% with only a single pair of elliptical filters. Butterworth filters made discrimination much harder at 4 kHz, but weren't tried at 15 kHz. Whether you agree that the experiment proves that such filtering is inaudible depends, among other factors, on whether you accept the 67% threshold and whether a test involving clicks is more or less severe than a test involving speech or music. -- --Steve Correll sjc@s1-b.ARPA, ...!decvax!decwrl!mordor!sjc, or ...!ucbvax!dual!mordor!sjc