there is a further analog between time in the input signal and distance
along the medium.
In an analog system, information is conveyed by some infinite
variation of a continuous parameter such as the voltage on a wire or the
strength of flux on a tape. In a recorder, distance along the medium is a
further, continuous, analog of time. It does not matter at what point a
recording is examined along its length, a value will be found for the
recorded signal. That value can itself change with infinite resolution
within the physical limits of the system.
Those characteristics are the main weakness of analog signals. Within
the allowable bandwidth, any waveform is valid. If the speed of the
medium is not constant, one valid waveform is changed into another
valid waveform; a timebase error cannot be detected in an analog system.
In addition, a voltage error simply changes one valid voltage into
another; noise cannot be detected in an analog system. We might suspect
noise, but how is one to know what proportion of the received voltage is
noise and what is the original? If the transfer function of a system is not
linear, distortion results, but the distorted waveforms are still valid; an
analog system cannot detect distortion. Again we might suspect
distortion, but how are we to know how much of the third harmonic
energy received is due to the distortion and how much was actually
present in the original signal?
It is a characteristic of analog systems that degradations cannot be
separated from the original signal, so nothing can be done about them. At
the end of a system a signal carries the sum of all degradations
introduced in the stages through which it passed. This sets a limit to the
number of stages through which a signal can be passed before it is
useless. Alternatively, if many stages are envisaged, each piece of
equipment must be far better than necessary so that the signal is still
acceptable at the end. The equipment will naturally be more expensive.
When setting out to design any audio equipment, it is important to
appreciate that the final arbiter is the human hearing system. If the audio
signal is reproduced less accurately than our senses, these shortcomings
will be audible, whereas if the system is more accurate than our senses, it
will appear perfect even though it is not. Making the system better still is
then a waste of resources. This topic will be explored in more detail in
Chapters 2 and 13.
1.2 What is digital audio?
One of the vital concepts to grasp is that digital audio is simply an
alternative means of carrying audio information. An ideal digital audio
recorder has the same characteristics as an ideal analog recorder: both of
2 The Art of Digital Audio