Field Guide · term

Also known as: demodulation

Demodulation recovers the original modulating information from a carrier — the inverse of modulation.1 For FM/FSK it tracks instantaneous frequency; for PSK it tracks phase; for AM it tracks the envelope. In a digital receiver the demodulator does not output bits directly — it outputs a continuous, noisy waveform whose value at each symbol instant encodes the transmitted symbol.

demod recovered message
Demodulation recovers the original modulating signal from the carrier — the step before decoding bits.

How it works

The demodulator undoes whatever operation the transmitter’s modulator performed. Which quantity it estimates depends on where the information lives:

  • Frequency modulation (FM, FSK, C4FM) — recover the instantaneous frequency, the rate of change of phase. In SDR this is usually a quadrature demodulator: the phase difference between successive IQ samples.
  • Phase modulation (PSK, QPSK, π/4-DQPSK) — recover the carrier phase. This needs a phase reference, typically a Costas loop or a differential comparison against the previous symbol.
  • Amplitude modulation (AM, ASK, and the amplitude axis of QAM) — recover the envelope.

The output is a continuous, noisy stream that contains the symbols; clock recovery then slices it at the right instants into discrete symbols, and decoding turns those symbols into bits. Demodulation handles the waveform; decoding handles the data.

Variants

  • Coherent vs non-coherent. A coherent demodulator reconstructs the carrier’s phase and frequency and mixes against it — optimal at low SNR but requiring a tracking loop. A non-coherent demodulator (an FM discriminator, an envelope detector, a differential PSK slicer) avoids carrier recovery at a modest sensitivity cost. Robustness and simplicity often make the non-coherent path the practical choice for narrowband land-mobile signals.
  • Hard vs soft output. A hard-decision demodulator emits the nearest symbol; a soft-decision demodulator emits a confidence value per bit, which a downstream forward-error-correction decoder can use to recover several dB of coding gain.

In practice

A full digital receive chain is a pipeline: tune and filter to baseband, demodulate to a symbol-bearing waveform, recover symbol timing, correct any residual carrier offset, slice to symbols, find the frame boundary, then FEC decode and de-interleave to bits. Demodulation sits early in that chain, and its output quality sets a ceiling on everything after it: a constellation diagram or eye diagram of the demodulated signal is the standard way to see whether that ceiling is high enough to decode.

Relevance to SDR

Choosing the matching demodulator for a signal’s modulation is the core of recovering it. Every mode GopherTrunk decodes — the C4FM of P25 and DMR, the four-level FSK of NXDN, the GFSK of paging — runs through a demodulator sized to that modulation before symbol and frame recovery. The constellation visualises this stage, and a demodulator mismatched to the signal produces a smear that no amount of downstream decoding can rescue.

Sources

  1. Demodulation — Wikipedia, on recovering the modulating signal as the inverse of modulation. ↩

See also

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