Alesis EC-2 owner manual Can you hear it?, Digital oversampling filters

appendix A • EC-2 96kHz A/D/D/A upgrade

aliasing, the steeper the filter, the better. Some converters boasted 10 th-order (-60 dB per octave) filters. For comparison, most loudspeaker crossovers have 3rd or 4th-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).

EC-2 Manual A-9