A circuit called an analog-to-digital converter turns music into digital data. The audio signal is a rapidly changing electrical wave that passes into the ADC. A control signal called a clock pulse triggers the ADC, causing it to take a “snapshot” of the signal’s voltage value at that instant in time; the ADC turns the value into a number. A rapid series of clock pulses generates a stream of numbers that mirror the incoming audio wave. As the wave rises and falls, the numbers increase and decrease.
For an ADC to accurately convert a signal, the trigger pulses must be at least twice the frequency of the highest frequency in the audio. For example, if the audio has a maximum frequency of 20 kHz, the pulses must be 40 kHz or greater. If the pulses have a lower frequency, the ADC generates phantom tones that do not exist in the original audio material. Audio engineers call the lower limit on sampling frequency the Nyquist limit. The pulses must be greater than this limit to produce a decent recording. As long as the sampling frequency is high enough, the ADC avoids aliasing.
When the ADC receives a trigger pulse, it measures the incoming analog signal. When the pulse is “high,” the ADC measures and locks in the value of the analog wave. The ADC’s measurement process is not instantaneous, however; it takes a small amount of time. Because the wave changes continually, the ADC misses out on some of this change, resulting in an error called aperture distortion. This causes a digital-audio system to lose fidelity in the high frequencies. The faster the ADC, the lower its aperture distortion, though no ADC has zero aperture distortion.
Aliasing occurs anytime an ADC attempts to process a signal with a higher frequency than its trigger pulse. Though inaudible, the harmonic overtones of some instruments can be greater than 20 kHz, producing audible aliasing. A good digital-audio system prevents these signals from passing into the ADC. To eliminate aliasing, the audio system has a sharp cutoff low-pass filter just before the ADC. This filter removes any signals higher than 20 kHz. This effectively prevents aliasing. No similar remedy exists for aperture distortion.