Chapter Five: Digital Audio
5. Sampling rates
In reproducing high-quality audio, there are two solutions to the problem of aliasing caused by trying to represent frequencies above the Nyquist frequency. 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.
An anti-aliasing filter is placed in front of an analog-to-digital converter to prevent frequencies above the Nyquist frequency from ever being sampled. So far, existing technology has not afforded us a true "brick wall" filter that completely 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 44.1 kHz (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 its steepness (or 'Q'). The higher the sampling rate used, the less noticeable is the impact of the filter rolloff on audible frequencies, because 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 sampling rates higher than 44.1 kHz (e.g., 96 kHz, 192 kHz). 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:
|32K||older DATs, voice quality|
|44.1K||CD, DAT, digital recording software/hardware|
|48K||DAT, DVD-Video, 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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