872 Chapter 30
to make measurements on a completed design for any other reason than simply to confi rm
that the target specifi cation is met. Similarly, it is argued that it is absurd to attempt to
endorse or reject any standard of performance by carrying out listening trials. This is so
since, even if the results of calculations were not adequate to defi ne the performance,
instrumental measurements are so much more sensitive and reproducible than any purely
“ subjective ” assessments that no signifi cant error could escape instrumental detection.
Unfortunately, all these assertions remain a matter of some dispute. With regard to the
fi rst of these points—the need for instrumental measurements—the behavior patterns
of many of the components, both “ passive ” and “ active, ” used in electronic circuit
design are complex, particularly under transient conditions, and it may be diffi cult to
calculate precisely what the fi nal performance of any piece of audio equipment will be
over a comprehensive range of temperatures or of signal and load conditions. However,
appropriate instrumental measurements can usually allow a rapid exploration of the
system behavior over the whole range of interest.
On the second point, the usefulness of subjective testing, the problem is to defi ne just
how important any particular measurable defect in the signal process is likely to prove
in the ear of any given listener. So where there is any doubt, recourse must be had to
carefully staged and statistically valid comparative listening trials to try to determine
some degree of consensus. These trials are expensive to stage, diffi cult to set up, and hard
to purge of any inadvertent bias in the way they are carded out. They are therefore seldom
done, and even when they are, the results are disputed by those whose beliefs are not
upheld.
30.1 Instrument Types ...................................................................................................
An enormous range of instruments is available for use in the test laboratory, among
which, in real-life conditions, the actual choice of equipment is mainly limited by
considerations of cost and of value for money in respect to the usefulness of the
information that it can provide.
Although there is a wide choice of test equipment, much of the necessary data about
the performance of audio gear can be obtained from a relatively restricted range of
instruments, such as an accurately calibrated signal generator, with sinewave and
square-wave outputs, a high input impedance, a wide bandwidth AC voltmeter, and
some instrument for measuring waveform distortion—all of which would be used in