What is amplitude modulation?
Low-frequency signals such as audio are hard to transmit directly, so they ride on a high-frequency carrier. Amplitude modulation (AM) varies the carrier's amplitude according to the message. After Fessenden's early experiments it became the standard method for radio broadcasting.
Equation and modulation index
For a message m(t)=Amcos(2πfmt) and carrier Accos(2πfct), conventional AM is
s(t) = Ac[1 + μ·cos(2πfmt)]·cos(2πfct), μ = Am/Ac
μ is the modulation index; above 1 the signal is overmodulated and the envelope distorts.
Sidebands and bandwidth
Expanding with trigonometric identities gives three components: the carrier fc, the upper sideband fc+fm and the lower sideband fc−fm. For message bandwidth W, AM bandwidth is 2W.
Power efficiency
Total power is Pc(1+μ²/2), and the fraction in the information-bearing sidebands is μ²/(2+μ²). Even at μ=1 efficiency is only about 33%; the rest goes to the carrier.
Variants
| Scheme | Feature | Bandwidth | Use |
|---|---|---|---|
| DSB-FC (conventional AM) | Carrier included, simple receivers | 2W | AM broadcasting |
| DSB-SC | Carrier suppressed, better power efficiency | 2W | e.g. FM stereo subcarrier |
| SSB | Only one sideband sent | W | Amateur, maritime and military HF |
| VSB | One sideband plus a vestige of the other | Slightly above W | Analog TV video |
Demodulation
- Envelope detection: a diode and RC circuit track the amplitude outline; works only for conventional AM with μ≤1.
- Coherent (synchronous) detection: multiply by a locally generated carrier of matching frequency and phase, then low-pass filter; required for DSB-SC and SSB.
Pros and cons
AM is simple and propagates far, but noise adds directly to amplitude, so audio quality is lower than frequency modulation (FM).
References
- Haykin, S., & Moher, M. (2009). Communication Systems (5th ed.). Wiley.
- Lathi, B. P., & Ding, Z. (2009). Modern Digital and Analog Communication Systems (4th ed.). Oxford University Press.
- Carlson, A. B., & Crilly, P. B. (2010). Communication Systems (5th ed.). McGraw-Hill.
- ITU-R Recommendation BS.560 / Regional Agreement GE75 (LF/MF broadcasting, 9 kHz channel spacing in Regions 1 and 3).