Sound and audio refer, of course, to one and the same thing, but it is worthwhile clearing up a little semantics confusion between sound levels and programme levels. Sound is a generic term more applicable to the acoustic, whereas audio is a signal and often referred to in the UK as programme.

As far as measurement goes, audio has similar constraints to video— namely, headroom and noise. Between the two exists the signal space available for recording and transmission.

Considering the noise limit first. This is where the signal becomes so quiet as to be lost in the electrical clutter that we call noise. Developments over years have steadily pushed the noise floor farther and farther down. Low-noise amplifiers, ...

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