Standard Amplitude-Modulation Broadcast Systems

The microphones and studio facilities used were discussed in Chapters 2 and 4. The broadcast stations are often connected to network programs transmitted over open-wire lines and cables (Chapters 6 and 7), The circuits usually employed pass a band from about 100 to 5000 cycles, although network circuits are available that will provide transmission from approximately 50 to 8000 cycles. In the future most programs will be transmitted by telephone carrier systems.

Three classes of broadcast channels have been established;¹⁵ they are clear channels, regional channels, and local channels. The classification of standard broadcast stations¹⁵ is summarized as follows: Class I stations that are dominant stations and have from 10 to 50 kilowatts output; Class II stations that are secondary stations and radiate from 0.25 to 50 kilowatts; Class III stations that are regional stations and have power outputs from 0.5 to 5 kilowatts; and Class IV stations that are local stations and operate from 0.1 to 0.5 kilowatt. Each of the stations enumerated has other important distinguishing characteristics.¹⁵ The types of services rendered by broadcast stations have been defined¹⁵ as primary service, which is characterized by steady ground-wave propagation and has negligible fading and interference from other stations (page 572), secondary service, characterized both by ground-wave and sky-wave reception and by fading and some interference from other stations; and intermittent service, which is rendered by the ground wave and extends from the limit of the primary service area outward until it has no service value.¹⁵