What Is Anti Aliasing Filter

Aliasing is ane of the more circuitous concepts of digital audio.

Luckily, most audio engineers can spend their days being creative rather than having to worry nearly it.

Nonetheless, understanding aliasing actually helps explicate a lot about how digital works.

Aliasing 101

When a signal is sampled, it is inherently band-limited in frequency.

In other words, when a indicate is sampled by a finite number of points, information technology cannot represent an infinite range of frequencies.

A conventional D-to-A converter for audio volition only create signals within a specific frequency range that is determined by the sampling rate.

If there are any recorded frequencies outside of this range, they are interpreted by the converter and mapped to frequencies within this range. This is aliasing — when 1 frequency is coded equally a different frequency.

Nyquist Frequency

The sampling rate determines this frequency range because it sets the Nyquist frequency.

The Nyquist frequency is the maximum frequency that can be recorded by a specific sampling rate. The Nyquist frequency is half of the sampling rate.

When it comes to audio recording, if the sampling rate is 48,000 samples per second, the Nyquist frequency is 24,000 Hz. If the sampling rate is 44,100 samples per second, the Nyquist frequency is 22,050 Hz.

If a signal contains any frequencies greater than the Nyquist frequency, they are interpreted by the converter and mapped to frequencies less than the Nyquist frequency.

Anti-Aliasing Filter

Aliasing would be a big trouble for digital audio, because it is usually not desired for frequencies to alter in a signal. The good matter is that in that location is a defended component to prevent aliasing as part of the analog-to-digital and digital-to-analog conversion process. This component is called an anti-aliasing filter.

Conceptually, the anti-aliasing filter blocks frequencies above the Nyquist frequency from existence converted. This is going to prevent any signals from changing frequency during the conversion process.

Visual Example

Many people take observed aliasing without even realizing it.

A common example of aliasing is in the visual domain, rather than the auditory domain.

In many automobile commercials, it appears every bit though the wheels on the auto are rotating backwards while the car is traveling forwards. In reality, the automobile and the wheels are both traveling (or rotating) frontwards.

The "camera trick" in the video that causes this illusion has a conceptual similarity with aliasing.

The "snapshots" of the wheel's position are non sufficiently fast to capture wheel's true rotation. If the photographic camera captures one frame while the wheel rotates 359°, the video will appear as though the wheel has traveled 1° in the contrary direction. Find that a very fast rotation would really be captured equally a very ho-hum rotation by the camera.

Therefore, in lodge for the camera to capture the actual rotational direction of the wheel, it must take a "snapshot" before the wheel reaches 180°.

A like concept occurs in digital audio.

Many audio signals are oscillatory in nature, meaning they repeat in cycles. Unless a proper number of samples are recorded per bike, the indicate that represents with path of vibrating air molecules is not sufficiently represented.

Aliasing in Audio

Cheque out the video beneath to learn more about aliasing equally it pertains to audio signals:

Eric Tarr

Eric Tarr is a musician, audio engineer, and producer based in Nashville, TN. Currently, he is a Professor of Audio Technology Technology at Belmont University.