In this mode, 65 tones spread out over 175Hz are sent and differs from other digital modes in that is
not a transmit-at-will or send-whatever-you-want mode.
It is highly structured and because the maximum number of characters you can send in free-form is 13 (that’s right, thirteen) messages sent contain just the necessary relevant information for the contact.
The sequence of a contact also depends on the computer clock being very accurate, just 2
seconds out is barely acceptable.
1 second out is usually OK but in practice it should be less than 0.5 seconds out.
JT65 is done in 60 second segments broken down into a 48 second transmission followed by 12 seconds of silence.
During the 12 seconds of silence, the receiving station’s software uses this period to decode the received signals.
The software is capable of decoding multiple signals received at the same time.