BONUS MATERIAL 261
DC transmission A form of transmission in which the DC com-
ponent of the video signal is transmitted.
Decibel dB A measure of the voltage or power ratio of two
signals. In system use, a measure of the voltage or power ratio of
two signals provided they are measured across the same value of
impedance. Decibel gain or loss is 20 times log base 10 of the
voltage or current ratio (Voutput/Vinput), and 10 times log base
10 of the power ratio (Poutput/Pinput).
Decoder The circuitry in a receiver that transforms the detected
signal into a form suitable to extract the original modulation or
intelligence.
De-Compression The process of taking a compressed video
signal and returning it to its original (or near original) form it had
before compression.
Defi nition The delity of a television system with respect to the
original scene.
Delay distortion Distortion resulting from the nonuniform speed of
transmission of the various frequency components of a signal, caused
when various frequency components of the signal have different times
of travel (delay) between the input and the output of a circuit.
Delay line A continuous or periodic structure designed to delay
the arrival of an electrical or acoustical signal by a predetermined
amount.
Density A measure of the light-transmitting or refl ecting proper-
ties of an optical material. It is expressed by the common logarithm
of the ratio of incident to transmitted light fl ux. A material having
a density of 1 transmits 10% of the light, of 2 transmits 1%, of 3
transmits 0.1%, etc. See Neutral density fi lter.
Depth of fi eld The area between the nearest and the farthest
objects in focus. For a lens, the area along the line of sight in which

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