6.4.12 FM Applications
Despite being an analog modulation technique, FM is still relevant in modern communications.
6.4.12.1 Broadcast Radio
FM broadcasting was introduced to provide significantly improved audio quality and noise performance compared with AM broadcast systems. By employing wideband FM with relatively large frequency deviation and pre-emphasis/de-emphasis, broadcast FM achieves high signal-to-noise ratio and low distortion over the audio range. The combination of wide deviation (typically ±75 kHz in many regions) and audio bandwidth up to 15 kHz enables near–high-fidelity transmission, making FM particularly well suited to music programming.
Stereo FM was standardized in 1961 and was required to be fully backward compatible with existing mono receivers. It was therefore not possible simply to transmit independent left (L) and right (R) channels. Instead, the system transmits the sum signal (L+R) and the difference signal (L–R). The baseband multiplex (MPX) signal extends from 30 Hz to 15 kHz for the (L+R) component, which directly frequency-modulates the main carrier so that a mono receiver demodulates normal audio without modification. The (L–R) signal amplitude-modulates a 38 kHz suppressed subcarrier (DSB-SC), producing sidebands extending from 23 kHz to 53 kHz. A 19 kHz pilot tone (at half the subcarrier frequency) is transmitted to allow stereo receivers to regenerate the 38 kHz subcarrier for synchronous detection.
A stereo receiver recovers both (L+R) and (L–R) and reconstructs the original channels using:
A mono receiver, with audio bandwidth limited to approximately 15 kHz, receives only the (L+R) component and therefore remains fully compatible.
6.4.12.2 Satellite Communications
Peak use of FM occurred during the 1960s to 1980s, when it was dominant in both civilian and military satellite communications. Civil systems employed FM for analog television, telephony, and telemetry, where wideband S/N performance was critical. Military systems adopted FM primarily for tactical UHF voice and command links, valuing its simplicity, resilience, and compatibility with existing radio networks over spectral efficiency.
A key advantage of FM in satellite television distribution was its excellent post-detection signal-to-noise performance at high deviation ratios (large D), which allowed acceptable picture quality even in the presence of moderate uplink and downlink noise. The constant-envelope nature of FM also permitted the use of efficient high-power amplifiers operating near saturation.
Although FM provided robust, noise-resistant analog communication, its high bandwidth requirements and limited spectral efficiency led to its near-complete replacement from the 1990s onward by digital modulation techniques. Digital methods now achieve superior performance within narrower bandwidths and provide additional advantages such as coding gain, encryption, multiplexing, and forward error correction.
Nonetheless, FM has persisted in certain applications. It continues to be used in TT&C, in low-rate telemetry channels, and in amateur satellite communications, where simplicity, noise tolerance, and low-cost receiver designs remain important.
6.4.12.3 Military Communications
In land combat net radio (CNR), FM has been widely adopted for VHF tactical voice networks because it provides good intelligibility in the presence of impulsive noise and co-channel interference, and because receivers can employ limiting to suppress amplitude noise before demodulation. FM also performs well with constant-envelope power amplifiers, which is advantageous for battery-powered manpack and vehicular radios where power efficiency, robustness, and simplicity are critical. Channel spacings such as 25 kHz (and, in some services, 12.5 kHz) accommodate the typical voice bandwidth and frequency deviations used in narrowband FM tactical systems, with peak deviations commonly around ±5 kHz in 25 kHz channels and proportionally smaller deviations in 12.5 kHz channels. This allows many channels to be deployed within a limited allocation while preserving the noise-immunity benefits of FM. The capture effect further assists tactical operation by allowing a receiver to follow the strongest signal on a net in the presence of weaker co-channel transmissions, albeit with the operational caveat that strong interference can also capture the receiver.
6.4.12.4 Two-Way Mobile And Professional Radio
Professional and public-safety radio systems—such as police, fire, ambulance, utilities, transport, and industrial dispatch—have historically used narrowband FM in the VHF and UHF bands. Typical systems employ channel spacings of 12.5–25 kHz with peak frequency deviations of approximately ±2.5 to ±5 kHz. These parameters provide adequate voice intelligibility while limiting occupied bandwidth to fit within congested spectrum allocations.
FM is particularly well suited to mobile environments because of its immunity to impulsive noise from vehicle ignition systems, electrical machinery, and urban interference sources. The use of limiters in FM receivers further suppresses amplitude noise before demodulation. Additionally, the constant-envelope nature of FM allows efficient transmitter operation in battery-powered handheld and vehicular radios.
Although many professional systems are transitioning to digital standards (such as APCO P25, DMR, or TETRA), analog FM remains widely deployed due to interoperability requirements and its predictable, gradual degradation near coverage boundaries.
6.4.12.5 Analog Television Audio
In analog television broadcasting, the video signal was transmitted using vestigial-sideband AM, while the accompanying audio channel was transmitted using FM. The use of FM for audio provided superior noise performance compared with AM, particularly in fringe reception areas where carrier-to-noise ratios were marginal.
In terrestrial television systems, the audio carrier was typically offset by several megahertz from the video carrier (for example, 4.5 MHz in NTSC systems), and the FM deviation was chosen to balance audio quality with channel bandwidth constraints. In satellite television distribution during the 1970s and 1980s, wideband FM was used for both video and audio, exploiting high deviation ratios to achieve acceptable picture and sound quality despite uplink and downlink noise.
With the global transition to digital television standards, analog FM television audio has largely disappeared; however, its historical importance illustrates the strong noise-resistance advantages of angle modulation.
6.4.12.6 Amateur Radio
FM is widely used in amateur VHF and UHF communications, particularly through repeater networks operating in the 2 m (144–148 MHz) and 70 cm (430–440 MHz) bands. Narrowband FM with deviations typically around ±5 kHz (or ±2.5 kHz in more recent narrowband allocations) provides reliable short-range voice communication with relatively simple transmitter and receiver designs.
The capture effect is advantageous in repeater operation, allowing the strongest signal to dominate and reducing background interference. Furthermore, the constant-envelope property of FM simplifies transmitter design in handheld and mobile amateur equipment. While digital voice modes are increasingly popular in amateur radio, analog FM remains the most common local communication mode due to its simplicity, accessibility, and widespread compatibility.
6.4.12.7 FM’s Continued Use
Although analog FM has largely been superseded in high-capacity systems by digital modulation techniques, it remains an important practical modulation method. Its constant-envelope property, strong noise immunity, and compatibility with efficient power amplification ensure its continued relevance in broadcasting, professional radio, and selected satellite applications.
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