Modulation is the process of varying a property of a carrier wave — its amplitude, frequency, or phase — in step with a message so that information can travel over radio.1 It is the bridge between a baseband signal (audio, data) and the radio-frequency carrier that can actually be radiated, and the reverse process at the receiver is demodulation. Choosing a modulation scheme is a trade among data rate, bandwidth, power, and robustness.
How it works
There are only three things about a sinusoidal carrier you can change, and every modulation scheme is a way of changing one or more of them:
- Amplitude. AM writes the message into the carrier’s envelope. SSB is a bandwidth- and power-efficient variant that transmits one sideband only.
- Frequency. FM shifts the carrier frequency with the message; its constant envelope makes it resistant to amplitude noise, the reason it sounds cleaner than AM.
- Phase. Phase modulation nudges the carrier’s timing; it is closely related to FM and underlies most digital schemes.
Analog modulation varies a property continuously. Digital modulation instead switches the carrier among a finite set of symbols, each standing for one or more bits: FSK toggles between discrete frequencies, PSK between discrete phases, and QAM among combinations of amplitude and phase. The symbols are naturally drawn on a constellation diagram, the map of the IQ plane the receiver uses to decide which symbol arrived. Packing more bits per symbol (more constellation points) raises spectral efficiency but shrinks the spacing between points, so it needs more signal-to-noise to keep them distinct — the fundamental power-versus-rate trade that Shannon capacity bounds.
In practice
Modulation is chosen to fit the channel and the job. Low, robust orders survive weak or fading links: land-mobile digital voice uses C4FM / four-level FSK (DMR, NXDN, P25 Phase 1’s alternate) and π/4-DQPSK precisely because they hold up at modest SNR. Pulse shaping with a root-raised-cosine filter keeps each symbol from smearing into its neighbours and contains the occupied bandwidth. High-throughput systems (LTE, Wi-Fi, DVB) climb to 16-, 64-, or 256-QAM over OFDM subcarriers when the channel is good, and adapt downward when it degrades.
Relevance to SDR
Recognising a signal’s modulation and applying the matching demodulator is the heart of decoding. GopherTrunk’s chain identifies the trunking waveform, then runs the appropriate symbol recovery: four-level FSK slicing for DMR/NXDN, π/4-DQPSK carrier-and-symbol tracking for P25. The three carrier properties reappear as the axes of the IQ plane, so once the samples are in software, demodulation is a matter of measuring amplitude, frequency, and phase and mapping them back to symbols and bits.
Sources
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Modulation — Wikipedia, overview of analog and digital modulation methods. ↩