Halliday & Matthiessen (2014: 85):
It is the segmental kind of structure, with clearly separated constituent parts organised into a whole, that has traditionally been taken as the norm in descriptions of grammar; the very concept of ‘structure’, in language, has been defined in constituency terms. This is partly because of the kind of meaning that is expressed in this way: experiential meaning has been much more fully described than meaning of the other kinds. But there is also another reason, which is that constituency is the simplest kind of structure, from which the other, more complex kinds can be derived; it is the natural one to take as prototypical — in the same way as digital systems are taken as the norm from which analogue systems can be derived, rather than the other way round.
In analogue technology, a wave is recorded or used in its original form. So, for example, in an analogue tape recorder, a signal is taken straight from the microphone and laid onto tape. The wave from the microphone is an analogue wave, and therefore the wave on the tape is analogue as well. That wave on the tape can be read, amplified and sent to a speaker to produce the sound.
In digital technology, the analogue wave is sampled at some interval, and then turned into numbers that are stored in the digital device. On a CD, the sampling rate is 44,000 samples per second. So on a CD, there are 44,000 numbers stored per second of music. To hear the music, the numbers are turned into a voltage wave that approximates the original wave.