(1) Also called a reference tone or reference frequency. A sine wave used for servo control, such as on a sync tone. See vari-speed.
(2) A sine wave tone at one of a range of standard frequencies (usually 100, 1,000 and 10,000 Hz). It is set to zero-level and is intended to be used for calibration, such as during a line-up procedure. The APRS-specified line-up tones for magnetic tape recording are:
