Appendix A – The JT65 Protocol
JT65 is a tightly structured protocol with a severely limited set of structured messages and a
limited 'free text' message type limited to 13 characters in length.
In any one minute period you may send any one of the structured message types, 13
characters of text or any one of the 3 shorthand messages. Nothing more, nothing less (yes,
you can send a CW ID at the end of certain message, so there is that as well).
JT65 is not a free form data transmission protocol like most other modes we use. Each
transmitted 'frame' is of a fixed length, starting and ending at a predefined time. By using this
restrictive timing arrangement and a fixed frame length it becomes possible to achieve some
'signal gain' by knowing when and where to look for signals.
It is much easier to find a weak signal when you know when it should begin and end
and that each transmission will have a fixed format within its bit space.
The JT65 protocol states that a transmission will begin at precisely 1 second into a new
minute and end 46.811 seconds later. The protocol further defines that a frame will convey 72
bits of information with an additional 306 bits of error correction data (FEC) for a total of 378
bits. It is the structure of JT65, its timing, modulation method and heavy FEC that allows it to
work so well at very low signal levels.
Within that frame of 72 data bits you may contain any one of the set of structured messages,
a 13 character free form text message or (while different in the actual details of transmission,
but close enough for now) any one of the 3 shorthand message types.
Most of the sequences will follow the structured form with a precisely limited vocabulary.
Structured messages are as follows.
CQ CALLSIGN GRID
CALLSIGN CALLSIGN GRID
CALLSIGN CALLSIGN -##
CALLSIGN CALLSIGN R-##
CALLSIGN CALLSIGN RRR
CALLSIGN CALLSIGN 73
With structured messages the text is not sent character by character, but, instead, the
software uses a mathematical formula to 'translate' a call sign or grid into a unique numerical
value that takes less bit space to convey than a character by character transmission format.
This is why structured messages can appear to exceed the 13 character limit.
A structured message actually consists of 3 'fields' with the lengths of 28 bits, 28 bits and 15
bits for a total of 71 bits. The 72
nd
bit is used as a marker and when set (1) indicates the
message is 13 character text or, if unset (0) the 71 bits contain a structured message.
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