[net.audio] SR article on CD filtering

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