AM/FM Modulation Simulator

Radio broadcasting: The most familiar application. Medium-wave (MW) AM broadcasting occupies a narrow band per station and reaches far via ionospheric reflection, so it is used for wide-area news, weather and disaster information. FM broadcasting takes a large modulation index for high fidelity and suits music programmes and community radio. Even within "radio", AM emphasizes range and spectral economy while FM emphasizes audio quality and noise immunity — two different design philosophies.

Wireless and land-mobile radio: Analog land-mobile and amateur radio have long used narrow-band FM (NFM) for voice. It resists amplitude noise and fading, and the amplitude can be flattened by a limiter, which suits mobile communication. Aviation radio, however, still uses AM: unlike FM — where the "capture effect" lets only one station survive when several transmit at once — overlapping AM transmissions can be heard together, which is actually desirable for safety.

Instrumentation and sensor signal transmission: The FM idea is also applied to telemetry and sensor signals. Voltage-to-frequency (V-F) conversion turns a voltage into a frequency, which is far less affected by amplitude attenuation and noise over long cabling and can be recovered accurately with a frequency counter. From pulse outputs of rotation sensors to analog transmission over optical fiber, there are many cases where "frequency is more robust than amplitude" pays off.

Audio and electronic instruments: FM modulation is also used to create sounds in synthesizers (FM synthesis). The FM synths of the 1980s frequency-modulated an audible carrier with another audible signal, generating metallic, harmonically rich timbres from the abundant sidebands set by Bessel functions. The spreading spectrum you see when you raise the modulation index β in this simulator is exactly the principle behind FM synthesis timbre changes.

The most common mistake is assuming "the higher the modulation index, the better". In AM, pushing the modulation depth μ toward 1 puts more power into the sidebands and raises transmission efficiency, but the instant μ exceeds 1 the signal is overmodulated, the envelope collapses and the sound distorts. In FM, raising the index β increases the frequency deviation and improves the S/N, but the occupied bandwidth grows without bound according to Carson's rule B=2(β+1)f_m and encroaches on adjacent channels. The modulation index is a trade-off between "efficiency / fidelity" and "bandwidth / interference" — not a parameter to maximize blindly.

Next is believing "in AM the information is in the carrier". In fact the carrier itself carries no information at all. Only the two sidebands carry the information; the carrier is sent merely as a reference for demodulation. This is why the AM power efficiency η=μ²/(2+μ²) tops out at just 33% even at μ=1 — most of the transmitted power is spent on a carrier that conveys nothing. That is exactly why DSB-SC and SSB schemes, which suppress the carrier or one sideband, were developed. This tool handles standard AM, but the low efficiency is an important design starting point.

Finally, the misconception that "FM is better the wider its bandwidth, so we can widen it freely". Wide-band FM does have a better S/N, but once the receiver input S/N drops below a certain threshold, the output degrades abruptly — the "threshold effect" (FM threshold). Widening the bandwidth also lets more noise power into the receiver, which actually worsens the threshold. In a weak field, narrow-band FM is often more robust, so "wide-band FM" does not always mean "high fidelity". Bandwidth must be designed around the S/N of the intended reception environment.