PSK31
From Wikiham, the ham radio free encyclopedia
PSK31 is a narrowband digital mode for live keyboard conversations, developed by Peter Martinez, G3PLX. Its best-known variant uses differential binary phase-shift keying at 31.25 symbols per second. A computer typically generates and decodes audio passed through an SSB transceiver.
Modulation and text
In BPSK31, the information is represented by changes in carrier phase. The waveform is amplitude-shaped around phase reversals to limit unwanted spectral spreading. A linear transmit path is therefore important.
Symbol duration = 1 / 31.25 = 0.032 s = 32 ms
Text is encoded using Varicode, a variable-length character code. Common characters can use fewer bits than less common ones, and a pair of zero bits separates characters. The keyboard text rate consequently depends on the characters sent; 31.25 baud is not 31.25 characters per second.
BPSK31 does not provide the error-correcting scheme of the QPSK variant. Noise, interference or rapid phase changes can corrupt the recovered text.
Station setup
A waterfall display lets the operator select a narrow signal within the receiver's audio passband. Frequency stability matters because the wanted signal is narrow. Audio drive should preserve the shaped waveform: clipping, overdrive and unsuitable speech processing can create wide traces and interfere with nearby stations.
Typical exchanges contain free text, callsigns, signal reports and station details. Macros can help with repeated information, but excessive automatic text can make a conversation difficult to follow.
Comparison with other modes
RTTY commonly uses two frequency states and a fixed-length teleprinter alphabet. FT8 uses tightly structured messages and error correction in timed exchanges. PSK31 is designed for conversational text rather than FT8-style contact sequencing or packet delivery.