The PSK31 specification by ARRL specifies:

There is a preamble at the start of each transmission and a postamble at the end. The preamble is an idle signal of continuous zeroes, corresponding to continuous phase reversals at the symbol rate of 31.25 reversals/second. The postamble is just continuous unmodulated carrier, representing a series of logical ones. This makes it possible to use the presence or absence of the reversals to squelch the decoder so that the screen doesnt fill with noise when there is no signal.

How long are the preamble and postamble? If there's no specification of length, what's common practice?

I am sorry that I cannot provide a concrete reference but I seem to recall that it is around 80 based on some receiving software requirements.

  • 80 symbols? That would be a little more than 2.5 seconds, which seems too long. – Phil Frost - W8II Jul 17 '17 at 19:27
  • I will try to find the source for that reference but I haven't done any PSK31 programming for several years now. – Glenn W9IQ Jul 18 '17 at 2:00
  • I'm with you, @Glen. I can't find anywhere that specifies a specific length. But I do find a lot on adjusting the squelch so it will detect preambles. So from that I deduce that the postamble is long enough to signal EOT (end of transmission) and then shuts off, so squelch shuts off the incoming audio and others can use the channel. Then the preambles must be long enough to open up the squelch before data is sent. – SDsolar Jul 18 '17 at 4:17
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    The preamble also needs to be long enough for the receiver to synchronize. – Phil Frost - W8II Jul 18 '17 at 14:35
  • Good point. That is why they are sent at predictable pulse rates. And I presume each kind of receiver would have their own requirement for the number of pulses necessary. I can imagine a software-based receiver program on a heavily-used computer would take longer than a hardware-based receiver, for instance. – SDsolar Jul 19 '17 at 0:40

The official page has two demo audio files, for BPSK and QPSK.

Open in Audacity and one can see the preamble and postamble. There seem slight different on two files.

Preamble is about 3 seconds. Postamble is about 2 seconds.

http://aintel.bi.ehu.es/psk31.html

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