Also known as: minimum-shift keying, MSK
Minimum-shift keying (MSK) is a form of binary continuous-phase FSK in which the frequency spacing is the smallest that still keeps the two tones orthogonal — a modulation index of exactly h = 0.5.1 Keeping the phase continuous across symbol boundaries gives MSK a constant envelope and a compact spectrum, and it is the direct precursor to GMSK, which simply adds a Gaussian pre-filter.
How it works
In MSK, a data one nudges the instantaneous frequency up and a zero nudges it down, but by just enough that the carrier phase rotates by exactly ±90° over one symbol period. Because the deviation equals one-quarter of the bit rate, the two tones differ by half the bit rate — the minimum separation at which they remain orthogonal and can be distinguished by a matched detector. The phase never jumps; it ramps smoothly, so the signal has a constant amplitude and no abrupt spectral splatter.
There is a second, equivalent way to see MSK: it is identical to offset QPSK (OQPSK) shaped with half-sine symbol pulses. The even and odd bits drive the I and Q channels, staggered by half a symbol, and the half-sinusoid weighting turns the abrupt QPSK phase steps into the smooth ±90° ramps above. This duality lets MSK be built and demodulated with either an FSK-style frequency discriminator or a coherent I/Q PSK receiver.
In practice
MSK’s constant envelope means it can be amplified by cheap, efficient saturated power amplifiers without spectral regrowth — a decisive advantage over ASK or unfiltered PSK. Its main limitation is that the raw spectrum, while compact, still has sidelobes that fall off only moderately fast. Passing the data through a Gaussian filter before the modulator squeezes those sidelobes down further, producing GMSK — the modulation of GSM, AIS, and D-STAR. MSK is thus best understood as the clean, unfiltered baseline that GMSK refines.
Relevance to SDR
MSK and its GMSK descendant are everywhere in software radio: GSM cellular bursts, AIS ship reports, some satellite telemetry, and a range of low-power data links all rest on this continuous-phase family. Recognising a signal as MSK on a waterfall — a narrow, constant-power carrier with no amplitude blinking — tells you to reach for either a frequency-discriminator or a coherent OQPSK-style demodulator.
GopherTrunk decodes the closely related C4FM/4FSK land-mobile modes directly and uses a GMSK demodulator in its AIS path; pure MSK is documented here as the theoretical hinge between the FSK and PSK worlds and as the parent of the GMSK that GopherTrunk actually implements.
Sources
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Minimum-shift keying — Wikipedia, for the h = 0.5 continuous-phase definition, the OQPSK/half-sine equivalence, and the relationship to GMSK. ↩