Analog delay line

An analog delay line is a network of electrical components connected in series, where each individual element creates a time difference or phase change between its input signal and its output signal. It operates on analog signals whose amplitude varies continuously. An example is a bucket-brigade device.

Other types of delay line include acoustic (usually ultrasonic), magnetostrictive, and surface acoustic wave devices. A series of resistor–capacitor circuits (RC circuits) can be cascaded to form a delay. A long transmission line can also provide a delay element. The delay time of an analog delay line may be only a few nanoseconds or several milliseconds, limited by the practical size of the physical medium used to delay the signal and the propagation speed of impulses in the medium.

Analog delay lines are applied in many types of signal processing circuits; for example the PAL television standard uses an analog delay line to store an entire video scanline. Acoustic and electromechanical delay lines are used to provide a "reverberation" effect in musical instrument amplifiers, or to simulate an echo. High-speed oscilloscopes used an analog delay line to allow observation of waveforms just before some triggering event.

With the growing use of digital signal processing techniques, digital forms of delay are practical and eliminate some of the problems with dissipation and noise in analog systems.

Inductor–capacitor ladder networks were used as analog delay lines in the 1920s. For example, Francis Hubbard's sonar direction finder patent filed in 1921. Hubbard referred to this as an artificial transmission line. In 1941, Gerald Tawney of Sperry Gyroscope Company filed for a patent on a compact packaging of an inductor–capacitor ladder network that he explicitly referred to as a time delay line.

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