20 Chapter 1
When the frequency discrimination is too wide to distinguish the two tones as shown in
Figure 1.14(c) , the result is that they are registered as a single tone. The amplitude of the
single tone will change from one measurement to the next because the envelope is being
measured. The rate at which the envelope amplitude changes is called beat frequency,
which is not actually present in the input signal. Beats are an artifact of fi nite frequency
resolution transforms. The fact that human hearing produces beats from pairs of tones
proves that it has fi nite resolution.
1.9 Frequency Response and Linearity ..........................................................................
It is a goal in high-quality sound reproduction that the timbre of the original sound shall
not be changed by the reproduction process. There are two ways in which timbre can
inadvertently be changed, as Figure 1.15 shows. In Figure 1.15(a) , the spectrum of
the original shows a particular relationship between harmonics. This signal is passed
through a system [ Figure 1.15 (b)] that has an unequal response at different frequencies.
Figure 1.15 : Why frequency response matters. (a) Original spectrum determines the
timbre of sound. If the original signal is passed through a system with a defi cient frequency
response (b), the timbre will be changed (c).