Signal Distortions Overview

Distortions

• Distortions are summarized under different types of signal change
• Two main types of distortion are: linear and nonlinear

Linear Distortion

• Linear distortion refers mainly to irregularities in the frequency response
• The listening range of 20Hz to 20kHz is the observation basis
• Almost all real systems exhibit linear distortions of the amplitude-frequency response
• Edge areas of the amplitude response are often affected by these disturbances
• The amplitude response usually falls slightly at the upper and lower end of the frequency range
• An exception may be tape machines, where the response increases towards higher frequencies
• Deviations of the phase-frequency response are also considered linear distortions

Nonlinear Distortion

• Nonlinear distortion involves distortions of the transmitted waveform
• It manifests as a 'deformation' of the waveform at the input
• It arises when a transmission element cannot precisely follow the voltage curve
• Metrologically, this change is indicated by additional overtones in the waveform

Overtone Series

• Distortion of a sinusoidal oscillation with frequency f1 produces new oscillations at frequencies 2f1, 3f1, 4f1, etc.
• These oscillations are known as harmonics or overtones
• Harmonics are categorized into even (e.g., 2f1, 4f1) and odd (e.g., 3f1, 5f1) harmonics
• Even harmonics often sound more pleasing than odd harmonics, especially in tube circuits
• Nonlinear distortions are challenging to assess due to complex overtone series already present in natural sounds
• Distortion can be a stylistic tool, used for brilliant, fuller, or more direct sound effects
• Techniques like tube distortion, tape machine distortion, overdrive, and fuzz are popular

Distortion Compression

• Strong overdriving of amplifier components (e.g., tubes, transistors) results in harmonic overtones and strong signal compression
• Compression effect is more pronounced in distortion devices and less significant in other overdrives
• Signal is gradually guided into the amplifier component's boundary region, increasing compression effects
• Saturation occurs when a component approaches its maximum capacity, causing a flattened upward transfer characteristic

Total Harmonic Distortion (THD)

• THD measures the proportion of harmonic distortion components in a complete signal
• The calculation uses root-mean-square (rms) voltage values instead of peak values
• Practical measurements often limit the calculation to the first two harmonics (k2 and k3) to simplify the value for overall THD
• The specifications regarding distortion (partial or overall) should be clarified in the practical measurement to avoid misinterpretation

Intermodulation Distortion

• Intermodulation distortion occurs when more than one frequency is present in a system
• It creates additional frequencies as sums and differences of input tones
• This type of distortion is sensitive to the human ear, especially in speakers and power amplifiers, where minimal proportions are desirable
• It is often difficult to measure, and measurement results are not commonly reported by manufacturers
• The less intermodulation distortion, the better the audio quality; practically, professional active speakers have less than 2% and high-quality speakers less than 0.5%