Modulation: writing information onto a wave
A bare carrier says nothing. Modulating means varying one of its properties to the rhythm of the information. Three candidate properties: amplitude, frequency, phase.
Analogue
- AM (amplitude): the information modulates the carrier's height. Simple, but sensitive to amplitude noise. (AM radio, aviation)
- FM (frequency): the information modulates the frequency. Robust against noise → FM radio quality. In the Le waterfall (cascade temporelle), an FM station is a wide (~200 kHz), stable stripe.
Digital — we transmit symbols (bits):
- ASK / OOK: switch the carrier on/off. Many 433 MHz remote controls.
- FSK: hop between two (or more) frequencies. Many ISM sensors; LoRa is a cousin (chirps).
- PSK: encode the information in the phase (which is why I/Q samples, which measure phase, matter).
- OFDM: hundreds of subcarriers in parallel → high throughput, a wideband signal. That's WiFi, and drones' video links.
Recognising by eye: bandwidth, stability, temporal pattern and frequency hopping in the waterfall often tell you what kind of signal you're looking at — before any decoding.
Waterfall identification table
| Signal | Width | Pattern in the Le waterfall (cascade temporelle) |
|---|---|---|
| FM broadcast | ~200 kHz | wide, continuous, stable stripe |
| NFM (handhelds, repeaters) | 12.5 kHz | thin, intermittent line |
| AM aviation | ~8 kHz | thin carrier + voice in bursts |
| CW (Morse) | < 500 Hz | very fine dotted line |
| FT8 (HF digital) | 50 Hz | small tiles every 15 s (14.074 MHz) |
| LoRa | 125 kHz | brief diagonal chirps |
| WiFi/OFDM | 20–40 MHz | wide, grainy block |
| Mode S / ADS-B | ~2 MHz | ultra-short bursts |
What about digital voice?
Modern handhelds (DMR, D-STAR, C4FM) carry voice as bits (4FSK and cousins): in the waterfall, a thin line with a steady rhythm — but nothing audible in FM; you need the right decoder. Detection vs decoding: Détecter vs décoder.
👉 Spot a wideband OFDM signature: Capstone — detect a drone