An analog signal is literally an electrical analogy of the physical quantity it represents

The word ‘analog’ comes from ‘analogy’: an analog signal is one that bears an analogy to the source quantity. A microphone converts air pressure variations into proportionally varying voltage — the voltage is an analog of the acoustic pressure. This definition is often conflated with ‘continuous’ but the two are distinct: an analog signal can be sampled (discrete in time) while still being analog in the sense of representing its source by proportion. Understanding the etymological root helps clarify why a digital signal transmitted as voltage swings is still ‘analog’ in one sense: the voltage is an analog of the digital state, even though the information is digital.

Examples

Mic signal: voltage proportional to air pressure at each moment — analog of acoustics. Loudspeaker cone displacement: analog of the voltage driving it. AES-EBU digital signal: voltage swings are an analog of the digital states, but the content is digital.

Assessment

A student says ‘all digital signals are not analog.’ Using the definition of ‘analog as analogy,’ explain why a digital audio signal transmitted as voltage fluctuations is also, in one sense, an analog signal.