A chirp is a signal in which the frequency increases or decreases with time. In some sources, the term chirp is used interchangeably with sweep signal and it is commonly used in sonar and radar, but has other applications, such as in spread-spectrum communications. In spread-spectrum usage, surface acoustic wave devices such as reflective array compressors are used to generate and demodulate the chirped signals. The name is a reference to the sound made by birds. K = f 1 − f 0 T, where f 1 is the final frequency, T is the time it takes to sweep from f 0 to f 1. Thus this is also called quadratic-phase signal. In other words, if two points in the waveform are chosen, t 1 and t 2, and the interval between them t 2 − t 1 is kept constant, the frequency ratio f / f will also be constant. In an exponential chirp, the frequency of the signal varies exponentially as a function of time, f = f 0 k t where f 0 is the starting frequency, and k is the rate of exponential change in frequency. Unlike the linear chirp, which has a constant chirpyness, a chirp has an exponentially increasing frequency rate. A chirp signal can be generated with analog circuitry via an oscillator. It can also be generated digitally by a signal processor and digital to analog converter, using a direct digital synthesizer. It can also be generated by a YIG oscillator, a chirp signal shares the same spectral content with an impulse signal. However, unlike in the signal, spectral components of the chirp signal have different phases, i. e. their power spectra are alike. Dispersion of a propagation medium may result in unintentional conversion of impulse signals into chirps. On the other hand, many applications, such as chirped pulse amplifiers or echolocation systems. Chirp modulation, or linear frequency modulation for digital communication, was patented by Sidney Darlington in 1954 with significant later work performed by Winkler in 1962 and this type of modulation employs sinusoidal waveforms whose instantaneous frequency increases or decreases linearly over time. These waveforms are commonly referred to as linear chirps or simply chirps, hence the rate at which their frequency changes is called the chirp rate. In binary chirp modulation, binary data is transmitted by mapping the bits into chirps of opposite chirp rates, for instance, over one bit period 1 is assigned a chirp with positive rate a and 0 a chirp with negative rate −a
Spectrogram of an exponential chirp. The exponential rate of change of frequency is shown as a function of time, in this case from nearly 0 up to 8 kHz repeating every second. Also visible in this spectrogram is a frequency fallback to 6 kHz after peaking, likely an artifact of the specific method employed to generate the waveform.
Spectrogram of a linear chirp. The spectrogram plot demonstrates the linear rate of change in frequency as a function of time, in this case from 0 to 7 kHz, repeating every 2.3 seconds. The intensity of the plot is proportional to the energy content in the signal at the indicated frequency and time.
An exponential chirp waveform; a sinusoidal wave that increases in frequency exponentially over time
(a) In image processing, direct periodicity seldom occurs, but, rather, periodicity-in-perspective is encountered. (b) Repeating structures like the alternating dark space inside the windows, and light space of the white concrete, "chirp" (increase in frequency) towards the right. (c) Thus the best fit chirp for image processing is often a projective chirp.