164 Chapter 6
If there is judged to be any signifi cant difference in the perceived sound quality, as
between different pieces of hardware that are apparently identical in their measured
performance, the type and the scope of the electrical tests that have been made must be
considered carefully to see if any likely performance factor has been left unmeasured or
not given adequate weight in the balance of residual imperfections that exist in all real-
life designs.
A further complicating factor arises because some people have been shown to be
surprisingly sensitive to apparently insignifi cant differences in performance or to the
presence of apparently trifl ing electrical defects—not always the same ones—so, because
there are bound to be some residual defects in the performance of any piece of hardware,
each listener is likely to have his or her own opinion of which of these sounds best or
which gives the most accurate reproduction of the original sound—if this comparison is
possible.
The most that the engineer can do, in this respect, is to try to discover where these
performance differences arise or to help decide the best ways of getting the most
generally acceptable performance.
It is simple to specify the electrical performance that should be sought. This means that
for a signal waveform that does not contain any frequency components that fall outside
the audio frequency spectrum, which may be defi ned as 10 Hz to 20 kHz, there should
be no measurable differences, except in amplitude, between the waveform present at the
input to the amplifi er or other circuit layout (which must be identical to the waveform
from the signal source before the amplifi er or other circuit is connected to it) and that
present across the circuit output to the load when the load is connected to it.
In order to achieve this objective, the following requirements must be met.
● The constant amplitude ( 0.5 dB) bandwidth of the circuit, under load and at all
required gain and output amplitude levels, should be at least 20 Hz to 20 kHz.
● The gain- and signal-to-noise ratio of the circuit must be adequate to provide
an output signal of adequate amplitude and the noise or other nonsignal-related
components must be inaudible under all conditions of use.
● Both the harmonic and the intermodulation distortion components present in the
output waveform, when the input signal consists of one or more pure sinusoidal