Digital Communications : A system of telecommunications, often used in telemetry or signaling, that employs a noncontinuous or pulsed signal varying in frequency and amplitude
Ralph W. Burklin & Robert G. Purington, Fire Terms: A Guide to Their Meaning and Use, National Protection Association, Boston, MA 1980 USA. (Guide: Meaning and Use, Nat Prot Association Fr Srvc - Amer) {457}