Side-by-side comparison
| Parameter | AM | DSB-SC |
|---|---|---|
| Carrier component | Large unmodulated carrier transmitted | Carrier suppressed at transmitter; only sidebands sent |
| Bandwidth | 2W_m (same as DSB-SC) | 2W_m (same as full AM) |
| Power efficiency (m=1) | 33% — sidebands carry only 1/3 of total power | 100% — all power in information sidebands |
| Transmitter power saving | Baseline | Saves up to 66.7% transmitter power vs AM at m=1 |
| Modulation index | 0 ≤ m ≤ 1; over-modulation causes distortion | Not defined by m; no carrier to over-modulate |
| Demodulation | Simple envelope detector (diode + RC) | Requires coherent detector (multiplier + LPF + carrier sync) |
| Carrier synchronisation | Not needed — envelope detector is non-coherent | Required — phase and frequency of local carrier must match |
| Frequency spectrum | Carrier spike + USB + LSB | USB + LSB only; no carrier spike |
| Typical application | AM broadcast (540–1600 kHz), aviation NDB | FM stereo pilot subcarrier, colour TV subcarrier, data modems |
| Receiver complexity | Very low — crystal radio to basic superheterodyne | Higher — needs pilot extraction or Costas loop for carrier recovery |
Key differences
At modulation index m = 1, full AM splits power as: carrier = 2/3, two sidebands = 1/3 total. DSB-SC eliminates the carrier, so 100% of transmitted power is in sidebands — a 67% power saving. Bandwidth is identical at 2W_m for both. The price is demodulation complexity: AM works with a simple diode envelope detector, while DSB-SC demands a coherent detector that multiplies the incoming signal by a locally generated carrier synchronised in phase and frequency, typically via a Costas loop or pilot tone. The FM stereo system transmits a 38 kHz DSB-SC subcarrier for the L−R difference signal, with a 19 kHz pilot for receiver sync.
When to use AM
Use full AM when receiver simplicity is the top constraint — broadcasting to millions of low-cost radios where a diode detector must work without any carrier synchronisation hardware.
When to use DSB-SC
Use DSB-SC when transmitter power is limited and a coherent receiver is feasible, such as in FM stereo multiplexing, two-way HF SSB precursor systems, or measurement systems where carrier suppression simplifies signal analysis.
Recommendation
For power-limited communication links with intelligent receivers, choose DSB-SC — the 67% power saving is too large to ignore. Choose full AM only when you are broadcasting to simple consumer receivers that cannot implement coherent detection.
Exam tip: GATE and university examiners always test the power calculation: for AM at m=1 with carrier power P_c, total power = P_c(1+m²/2) = 1.5P_c; sideband power = m²P_c/2 = 0.5P_c; efficiency = 33% — write out all three steps.
Interview tip: Interviewers ask why DSB-SC cannot be demodulated by a simple diode — the correct answer is that the suppressed carrier means the envelope is no longer a replica of the message, so only a coherent multiplier restores the original signal.