Why does FM sound better than AM?
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.