JPL's Wireless Communication Reference Website Chapter: Network Concepts and Standards Section: Broadcast Systems

FM Radio Broadcasting

FM radio, when it was first introduced, delivered a substantial improvement in audio quality over AM radio. (See also analog modulation) Despite the availability of digital alternatives, such as DAB, it is still the predominant form of audio broadcasting. Stereo transmission is made possible by creating a mono (left+right) and stereo difference signal (left-right). Before modulation onto and FM carrier the difference signal the stereo (L-R) information is DSB modulated at 38 kHz (= 2 x 19 kHz) and added to the baseband mono signal. Moreover a 19 kHz reference is also added as a pilot tone for synchronization. This composite multiplex signal is then fed into the FM modulator. Photo: On Air control room of Radio France Bourgogne, a local FM radio station in Dyon.

• Analogue transmission is vulnerable to interference and fading
• FM radio requires large interference protection. The CCIR advises more than 36 dB. In practice FM has a capture ratio 10 dB, so large additional margins are recommended and applied.
  Note however, that FM reception looses its capture properties over a rapidly fading channel.
• The stereo signal is significantly more vulnerable to noise and interference than the mono signal
• Bandwidth per transmitter is 200 to 400 MHz
• Bandwidth for national coverage 0.7 + 2.1 M MHz for M programmes. This is needed because of frequency reuse patterns
• 88 - 108 MHz band typically allows 6 or 7 national programs. (5 national programs are available in The Netherlands' FM plan)
• Adjacent channel interference plays a major role in frequency planning (this is less the case in cellular telephony)
• Transmit powers are typically
  • 5 .. 10 watt for highway coverage of a few miles, e.g. Highway Advisory Radio
  • 30 .. 100 watt for community station, and
  • 10 .. 100 kwatt for large area coverage (tens, sometimes hundreds of kilometers)
• FM is a Constant Envelope modulation method:
  • Inexpensive Class C nonlinear amplifiers can be used.
  • Power efficient

Photo (left): Broadcast and radio relay tower at 1500 m elevation to increase coverage. (Dent du Chat, France)	Figure (right): Typical frequency reuse pattern for FM radio stations (CCIR). Channel spacing 100 kHz. Such theoretical network planning has for instance been used for local radio stations in Belgium and The Netherlands.

FM Modulation Processing

	An interview with Richard Sawyer from Orban about the need for having a 'good sound' on the air.
	Some technical aspects of sound processing
	Audio processing for transmissions over the Internet and over digital systems.

 

FM subcarrier systems