Two views of audio

There are two views, or domains, in which audio must be viewed to make any sense of it.

Firstly, the time domain. This is the most common way of thinking of audio. Sampled audio is a time domain representation of the sound. Viewing sound as time domain information is to view it as a changing amplitude over time.

Secondly audio can also be viewed in the frequency domain. The idea of the frequency domain is that a signal can be represented as a collection of sine and cosine waves. A frequency domain representation of the audio views it as changing amplitude over the frequency spectrum.

These two views can be switched between by using the Fourier Transform.

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One Response to Two views of audio

  1. Experts in the electronic engineering discipline of signal processing understand the concepts of time and frequency representations, however, here are some examples of their use as applied to audio systems to help those who are not experts in the field.
    I will start with the time domain representation. Time domain is a two-dimensional signal of amplitude and time, so to make an audio signal louder (ie, fade in) or quieter (ie, fade out) you are adjusting the time domain signal. Similarly, repeating a signal after a noticeable delay, as used in digital delay, is also a time domain process, as is the function of a compressor/limiter, which attempts to amplify quiet and attenuate louder sections of music to reduce the dynamic range of the signal and prevent either loss of signal quality within noise or distortion caused by saturation (clipping) and compression.
    The frequency domain is a three-dimensional signal of amplitude and frequency varying over time, however, you could say that it is actually four-dimensional, if you include the phase of each frequency. Frequency domain processes change the amplitude and phase of frequencies within the signal, such as a filter or parametric equaliser (EQ), or shifts the frequency of the signal, such as a pitch shifter or sampler, or an auto-tuner. Some of these processes can vary over the time. For example, Wah Wah peddles, made famous by guitarists such as Jimmy Hendrix, are effectively band pass filters (ie, filters that cuts frequencies lower and higher than the band pass) where the band pass frequency is controlled by the peddle to sweep it across the audio spectrum.

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