In adaptive high-frequency (HF) radio automatic link establishment, a broadcast in which (a) the called stations are unspecified, (b) stations receiving the call stop scanning, and (c) each station automatically responds in pseudorandom time slots.

Address of Record

Abbreviation for: 1)anomalous propagation, 2) application points, 3) access point.

Abbreviation for adaptive predictive coding.

Abbreviation for avalanche photodiode.
Note: apdand a. p. d. are also used.

Application Protocol Data Unit

An antenna designed to have an approximately constant input impedance over a wide range of frequencies.
Note: Examples of aperiodic antennas include terminated rhombic antennas and wave antennas. Synonym nonresonant antenna.

1. In a directional antenna, the portion of a plane surface very near the antenna normal to the direction of maximum radiant intensity, through which the major part of the radiation passes.
2. In an acoustic device that launches a sound wave, the passageway, determined by the size of a hole in the inelastic material and the wavelength. [After 2196]

1. In a scanned image system, electrical compensation for the distortion introduced by the limiting size of a scanning aperture. [IEEE 100; After SMPTE]
2. In television technology, restoration of the depth of modulation to the higher (i.e., higher Fourier) frequency components of the video signal, with the objective of achieving a subjective improvement in image quality. [After SMPTE]
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In facsimile, the distortion of the recorded image caused by the shape and finite size of the scanning and recording apertures.
Note: The distortion may occur in one or more attributes of the recorded image, such as in resolution, density, or shape.