Introduction to Computer Music: Volume One

There are two solutions to the issue of aliasing from frequencies above the Nyquist frequency in reproducing high-quality audio. The first is to band-limit the frequencies allowed to enter the system. The second is to have a higher sampling rate, thereby increasing the frequency range before aliasing occurs. In reality, both of these have been addressed by current digital audio systems.

Anti-aliasing filters are placed in front of any analog-to-digital converters to eliminate frequencies above the Nyquist frequency from ever being samples. So far, though steep, existing technology has not afforded us a true 'brick wall' filter that suddenly eliminates unwanted frequencies without having any effect on those in the 'legal' range. These is still a rolloff curve that attenuates frequencies closest to the cutoff.

The diagram above indicates that even at a sampling rate of 44.1K (the CD rate), some audible frequencies are attenuated by the filter. How much is attenuated is determined somewhat by the quality of the filter and it steepness (or 'Q'). The higher the sampling rate used, the less noticeable impact of the filter rolloff on audible frequencies become as more and more of the rolloff is above audio rate. This is part of the justification for digital audio workstations such as Pro Tools and Digital Performer giving users the option of everything from 44.1K to 192K. Both computer speeds and storage speed and size have made this possible.

Current standards of full-quality (i.e. not compressed) digital audio rates are:

rate	use(s)
32K	older DATs, voice quality
44.1K	CD, DAT, digital recording software/hardware
48K	DAT, digital recording software/hardware
96K	digital recording software/hardware
192K	digital recording software/hardware

Why did the CD standard settle on 44.1K rather than say 48K? Rumor has it that video equipment already had clocks that ran at 44.1K that could be integrated into the first CD players. I have also heard that Herbert von Karajan complained to Sony that Beethoven's 9th would not fit on the early CD specifications. By lowering the rate to 44.1K, 74 minutes could be recorded onto a CD using 16-bit samples, enough to do the trick.

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