Non-linear and time-variant systems generate frequency components absent from the input

Evidence

If a system does not meet the LTI condition of Equation (3.12), it is time-variant if its response properties change as a function of time, or it is non-linear if its transfer properties depend on the signal passing through it. In both cases the system can generate new frequency components that do not exist in the input signal. The analysis, modelling, or synthesis of such systems is typically substantially more difficult than for an LTI system. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 358

Any deviation from an ideal response in audio reproduction can be called distortion. The non-ideal magnitude and phase response is called linear distortion. In a linear (and time-invariant) system, the output contains only the same frequency components as the input, scaled by the magnitude response. In a non-linear system, however, new frequency components appear. Such behaviour is called non-linear distortion. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 512

Categorization

• DEFINE: Contrast pair defining time-variant vs non-linear, both of which produce new spectral content.
• corroborated: 7 independent passages support this claim — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 358; Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 512
• XREF: connects to: Nonlinear vibration generates new frequencies absent from the original stimulus; Pendulum period is only approximately amplitude-independent for small swings — Eric J. Heller, Why You Hear What You Hear- An…, 291; Real signals contain smooth distributions of infinite frequencies (+5 more)

Broader Connections

• Nonlinear distortion adds frequencies that were not in the original signal #corroborates (1.00) — Toole, Floyd E, Sound reproduction- the acoustics and…, 946
• Combination tones arise from the hearing system’s own nonlinearity #exemplifies (0.80) — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 430
• A combined waveform can produce a perceived pitch absent from its actual frequency components #exemplifies (0.75) — Eric J. Heller, Why You Hear What You Hear- An…, 323
• Equalization of linear distortion can amplify non-linear system effects #complicates (0.72)