Guard band has several meanings.
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Radio and electronic signalling

In radio, a guard band is an unused part of the radio spectrum between radio bands, for the purpose of preventing interference.

It is a narrow frequency range used to separate two wider frequency ranges to ensure that both can transmit simultaneously without interfering each other. It is used in FDM/Frequency division multiple access. It may be used in both wired or wireless communications, so that adjacent frequency bands on the same media can avoid interference.

Recording

In sound, video, or data recording, it refers to the unrecorded space between tracks on magnetic tape or other magnetic media which prevents the signals on adjacent tracks from interfering with each other.

Quality assurance

In the realm of product quality control, it is the amount that minimum product specifications are increased to ensure that even with measurement uncertainty the product meets the minimum specifications with a specified level of confidence.

Computer graphics

In computer graphics, the guard band is a virtual area of the screen bordering the real screen to which any rendering is ignored, used to simplify clipping. This technique is called guard band clipping.