212 Chapter 7
Because of this uncertainty, the designer of equipment in which price is not a major
consideration will normally seek to attain standards substantially in excess of those that
he supposes to be necessary, simply in order not to fall short. This means that the reason
for the small residual differences in the sound quality, as between high-quality units, is
the existence of malfunctions of types that are not currently known or measured.
7.16 Audibility of Distortion .........................................................................................
7.16.1 Harmonic and Intermodulation Distortion
Because of the small dissipations that are normally involved, almost all discrete
component voltage amplifi er circuitry will operate in class ‘ A ’ (that condition in which
the bias applied to the amplifying device is such as to make it operate in the middle of
the linear region of its input/output transfer characteristic), and the residual harmonic
components are likely to be mainly either second or third order, which are audibly much
more tolerable than higher order distortion components.
Experiments in the late 1940s suggested that the level of audibility for second and third
harmonics was of the order of 0.6 and 0.25%, respectively, which led to the setting of a
target value, within the audio spectrum, of 0.1% THD, as desirable for high-quality audio
equipment.
However, recent work aimed at discovering the ability of an average listener to detect
the presence of low-order (i.e., second or third) harmonic distortions has drawn the
uncomfortable conclusion that listeners, taken from a cross section of the public, may rate
a signal to which 0.5% second harmonic distortion has been added as “ more musical ”
than, and therefore preferable to, the original undistorted input. This discovery tends to
cast doubt on the value of some subjective testing of equipment.
What is not in dispute is that the intermodulation distortion (IMD), which is associated
with any nonlinearity in the transfer characteristics, leads to a muddling of the sound
picture so that if the listener is asked not which sound he prefers, but which sound seems
to him to be the clearer, he will generally choose that with the lower harmonic content.
The way in which IMD arises is shown in Figure 7.42 , where a composite signal
containing both high-frequency and low-frequency components, fed through a nonlinear