Page 124 - Master Handbook of Acoustics

• Phase distortion Any introduced phase shifts affect the time relationship between signal
components.

• Dynamic distortion A compressor or expander changes the original dynamic range of a signal.
• Crossover distortion In class-B amplifiers, in which the output devices conduct for only half
of the cycle, any discontinuities near zero output result in crossover distortion.

• Nonlinear distortion If an amplifier is truly linear, there is a one-to-one relationship between
input and output. Amplifier feedback helps control nonlinear tendencies. The human ear is not

linear. When a pure tone is impressed on the ear, harmonics can be heard. If two loud tones are
presented simultaneously, sum and difference tones are generated in the ear itself, and these
tones can be heard as can their harmonics. A cross-modulation test on an amplifier does
essentially the same thing. If the amplifier (or the ear) were perfectly linear, no sum or
difference tones or harmonics would be generated. The production of frequency elements that
were not present in the input signal is the result of nonlinear distortion.

• Transient distortion Strike a bell and it rings. Apply a transient wavefront signal to an
amplifier and its response might ring too. For this reason, signals such as piano notes are
difficult to reproduce. Tone burst test signals analyze the transient response characteristics of
equipment. Transient intermodulation (TIM) distortion, slew induced distortion, and other

measuring techniques evaluate transient forms of distortion.

Harmonic Distortion

The harmonic distortion method of evaluating the effects of circuit nonlinearities is universally
accepted. In this method the device under test is driven with a sine wave of high purity. If the signal

encounters any nonlinearity, the output wave shape is changed; that is, harmonic components appear
that were not in the pure sine wave. A spectral analysis of the output signal measures these harmonic
distortion products.
For example, a wave analyzer may use a constant passband width of 5 Hz, which is swept through

the audio spectrum. Figure 5-18 shows results of such a measurement. The wave analyzer is first
tuned to the fundamental, f = 1 kHz, and the level is set for a convenient 1.00 V. The wave analyzer
0
shows that the 2f second harmonic at 2 kHz measures 0.10 V. The 3f third harmonic at 3 kHz gives a
0
0
reading of 0.30 V, the fourth a reading of 0.05 V, and so on. The data are assembled in Table 5-3.