Why does FM sound better than AM?
FM radio is widely perceived as sounding clearer than AM because it encodes audio in frequency changes rather than amplitude, making it more resilient to many common noise sources and enabling features like the “capture effect,” where receivers favor the strongest signal. Commenters note that FM broadcasting also benefits from much wider allocated bandwidth and higher carrier frequencies, allowing transmission of a broader audio range (up to ~15 kHz) compared with heavily bandwidth-limited AM, which makes AM sound more muffled. There is debate over how much of FM’s advantage comes from modulation physics versus regulatory and engineering choices, with some pointing out that well-engineered, wideband AM can sound surprisingly good and that both systems ultimately trade off spectrum efficiency, complexity, and noise tolerance.
Core reasons proposed for FM sounding better
- Two main explanations recur:
- FM’s information is in frequency deviations, so random amplitude noise is less audible after limiting and demodulation.
- FM broadcast is given far more RF and audio bandwidth than AM, so it can carry higher‑fidelity audio (wider frequency response, stereo).
- Several commenters argue bandwidth and engineering choices (filters, processing) are the dominant reasons, not the modulation type alone.
Flashlight–through–trees analogy
- Popular analogy: AM = changing flashlight brightness through moving leaves; FM = constant brightness, changing color.
- Many find it intuitive for distinguishing amplitude vs frequency modulation and why amplitude noise hurts AM.
- Others point out limits:
- Human color perception is not a simple frequency detector, unlike an FM receiver with a PLL.
- Real EM waves interact with materials in frequency‑dependent ways; the “just scaled light” framing is called an oversimplified analogy, not an identity.
- Long subthread debates whether this is “literally the same physics” or “still an analogy.”
Noise, interference, and capture effect
- Lightning and man‑made RF are cited as primarily amplitude‑modulated noise, very audible on AM but much less on FM.
- FM receivers often use limiters and PLLs, which strip amplitude variations and “lock” to a carrier, giving:
- Better rejection of AM noise.
- Capture effect: when two FM signals overlap, the stronger dominates instead of mixing.
- Critics stress that phase/frequency noise also exists; the article’s “noise is mostly AM” line is called oversimplified or “nonsense” by some.
Bandwidth and audio fidelity
- AM broadcast channels: ~9–10 kHz spacing, typically ~5 kHz audio bandwidth (double sideband makes ~10 kHz RF).
- FM broadcast: ~200 kHz RF channels, ~15 kHz audio, plus stereo and data subcarriers.
- Several argue FM’s wider audio band (and less aggressive low‑pass filtering) explains much of the perceived clarity; AM sounds like “telephone” partly due to deliberate narrowing (sometimes to 5 kHz) for noise control.
- Information‑theory arguments (Shannon–Hartley) are invoked: more bandwidth allows more reliable information transfer for a given noise level; others respond that real AM/FM are far from theoretical limits, so modulation details still matter.
Receiver design and modulation details
- Discussion of:
- Superheterodyne architectures and intermediate frequencies.
- FM demodulators: discriminators, quadrature detectors, PLLs, “polar discriminator” methods.
- Sidebands, single‑sideband (SSB), AM stereo schemes, and why SSB isn’t used for broadcast music.
- Some note AM can sound very good with wide filters, good antennas, and careful engineering; poor real‑world AM is often due to narrow IF filters and cheap receivers, not inherent limits.
Other practical and meta points
- Aviation keeps AM so simultaneous transmissions are audibly noticeable, unlike FM’s capture effect.
- Some listeners prefer AM’s aesthetic or ability to hear multiple stations and noise.
- A few lament that the article and many comments mix correct points with misconceptions; links to more rigorous explanations on Q&A sites are shared.
- One commenter notes the thread shows how even in mature “hard” tech, public explanations can conflict and confuse.