ALESIS EC-2 - Manual (Page 11)

appendix A • EC-2 96kHz A/D/D/A upgrade
EC-2 Manual A-9
aliasing, the steeper the filter, the better. Some
converters boasted 10
th -order (-60 dB per octave)
filters. For comparison, most loudspeaker
crossovers have 3
rd or 4 th -order filters (-18 or –24 dB
per octave).
The problem is similar on the output side—the 44.1
or 48 kHz sampling frequency itself has to be
filtered out of the analog output from the D/A
converter, or it will send ultrasonic signals into
amplifiers and tweeters, making toast of them even
if the speakers don’t have response that high. This
called for a steep reconstruction or output filter,
between the D/A and the analog output.
Such steep filters keep ultra-high frequencies from
turning into aliasing noise, but they have their own
negative side effects. Like any equalizer, filters
have phase effects below the frequencies they
directly affect. So, although the first digital
recorders and CD players had flat frequency
response, they did not have flat phase response—in
the top octave from 10 to 20 kHz, the sound would
start going through a small time delay as it
approached the cutoff point of the antialiasing filter.
In the opinion of critical listeners, these filters gave
digital audio a harsh and unnatural high end. Since
this phase response was often the only significant
measurable difference between the input and
output signals, designers focused on eliminating it
(although it was never proven to be audible).
Digital oversampling filters
Throughout the 1980s and 1990s, engineers made
quantum improvements in the design of A/D and
D/A converters. Key among these was the
development of digital oversampling filters. To
vastly oversimplify, an oversampling filter sets its
sampling frequency at a high multiple (originally 8
times, now usually 64 or 128 times) of the final
sampling frequency. Then, most of the filtering
takes place digitally, by throwing out the “extra”
samples. A digital recorder or player with
oversampling filters on its converters still records
and plays back at the standard 44.1 or 48 kHz rate,
but the analog antialiasing and reconstruction filters
don’t need to be “brick wall”: a 12 dB per octave
filter is just fine, since the sampling is taking place
much higher than the audible range. Therefore,
today’s CD players and digital recorders have
almost perfectly flat phase response within the
audible frequency range. (For more detail on this,
we recommend The Art of Digital Audio, by John
Watkinson.)
However, a digital filter with perfectly flat phase
response still filters out frequencies above 20 or 22
kHz. Good analog tape recorders are capable of
recording beyond 30 kHz. And there are those who
believe that higher frequencies, while perhaps not
audible in themselves, may have an effect on the
quality of the audio taken as a whole.
Can you hear it?
You now have in your hands a tool that can let you
hear the ultimate in recording for yourself. By
making comparison recordings at 48 kHz and 96
kHz, you can judge what type of program material
should be recorded at the higher rate, and what
effect that has. But for an accurate comparison,
make sure that everything else in the
record/playback signal path has flat response (see
“Extending the frequency range...” on the next
page).