Main page Phase shifting a carrier in known steps to convey digital data.
This is a modulation method that is rarely heard on the air at high symbol rates because it is so difficult to detect: signals just sound like noise and is easy to miss. However, PSK modulation is widely used in commercial applications for data transmissions and occurs on HF and satellite.
Phase modulation is achieved by shifting the phase of the RF carrier in steps which represent symbols. This contrasts with FSK in which the frequency of the carrier is shifted, however it is possible to achieve much higher data rates with PSK modulation because of its efficiency.
Normally, PSK signals are transmitted by generating an audio signal which is sent using SSB modulation. It is also possible to directly phase-key a radio frequency carrier.
The technology used for receiving PSK signals is extremely complex with compensation required for spreading of the signal energy between symbols (inter-symbol interference) and for removing the effects of multipath in the HF environment.
This is very much flavour of the moment in the amateur radio community at present. This an experimental mode used by radio amateurs as a teleprinter signal using extremely narrow bandwidths. This has the advantage that the mode is resistant to noise and requires comparatively low power levels for successful communications. The standard baud rate is 31.25 baud, which means that only 31 Hz of spectrum is occupied compared with 300 - 500 Hz with an equivalent conventional frequency shift keying (FSK).
A coding scheme that uses short unique symbol sequences for common characters and longer ones for less common characters is used (in the same way that Morse does). This permits message speeds up to 50 wpm in practice.
PSK31 has two operating modes: bi-phase shift keying and four-phase shift keying using a convolutional encoder to provide some resilience to burst noise interference (but which introduces a 640ms latency into the communication link). Raised cosine filtering on the symbols to be transmitted.
Try listening around the following amateur radio frequencies: 3579, 7034, 10139, 14069, 21079 or 28079 kHz.