2.7 The Information Content of an Analog Signal

Any analog signal source can be characterized by a given useful bandwidth and signal-to-noise ratio. Video signals have very wide bandwidth extending over several MHz and require only 50 dB or so S/N ratio whereas audio signals require only 20 kHz but need a much better S/N ratio. If a well-engineered digital channel having a wider bandwidth and a greater signal-to-noise ratio is put in series with such a source, it is only necessary to set the levels correctly and the signal is then subject to no loss of information whatsoever.

Provided the digital clipping level is above the largest input signal, the digital noise floor is below the inherent noise in the input signal and the low- and high-frequency response of the digital channel extends beyond the frequencies in the input signal, then the digital channel is a 'wider window' than the input signal needs and its extremities cannot be explored by that signal. As a result there is no test known which can reliably tell whether or not the digital system was or was not present, unless, of course, it is deficient in some quantifiable way.

In audio the wider-window effect is obvious on certain Compact Discs made from analog master tapes. The CD player faithfully reproduces the tape hiss, dropouts and HF squashing of the analog master, which render the entire CD mastering and reproduction system transparent by comparison.

On the other hand, if an analog source can be found which has a wider window than the digital system, then the presence of the digital system will be evident either due to the reduction in bandwidth or the reduction in dynamic range. This will be evident on listening to DAB broadcasts that use compression and are inferior to the best analog practice.