466 Chapter 15
When such a fader is moved, any music signal being processed at the time is subjected
to stepwise changes in level. Although small, the steps will result in audible interference
unless the changes that they represent are themselves subjected to the addition of dither.
Thus although the addition of a dither signal reduces the correlation of the error signal to
the program signal, it must, naturally, add to the noise of the signal. This reinforces the
need to ensure that the digitized audio signal remains within the processing circuitry with
(d)
Figure 15.14(d) : The sinusoidal wave form can be described by the simple equation:
xt()Asin(2 )πft , wherex ( t ) is the value of the sinusoidal wave at time t , A is the peak
amplitude of the waveform, f is the frequency in Hz, and t is the time in seconds and its
probability density function is as shown here.(e)
Figure 15.14(e) : A useful test signal is created when two sinusoidal waveforms of the same
amplitude but unrelated in frequency are added together. The resulting signal can be used
to check amplifi er and system nonlinearity over a wide range of frequencies. The test signal
will comprise two signals to stimulate the audio system (for testing at the edge of the band,
19 and 20 kHz can be used) while the output spectrum is analyzed and the amplitude of the
sum and difference frequency signals is measured. This form of test is considerably more
useful than a THD test.