Carrier 42 HQX Manual de usuario descargar pdf (Pagina 32)

Taken from the JT65 protocol definition is the following;

“An amateur call sign consists of a one or two character prefix, at least one of which must be

a letter, followed by a digit and a suffix of one to three letters. Within these rules, the number

of possible call signs is equal to 37×36×10×27×27×27, or somewhat over 262 million. (The

numbers 27 and 37 arise because in the first and last three positions a character may be

absent, or a letter, or perhaps a digit.) Since 2^28 is more than 268 million, 28 bits are enough

to encode any standard call sign uniquely. Similarly, the number of 4 digit Maidenhead grid

locators on Earth is 180x180 = 32,400, which is less than 2^15=32,768; so a grid locator

requires 15 bits in a message.”

This implies that any pair of call signs [that fits the encoding rule] and a grid can be sent in

28+28+15=71 bits.

Take specific note of the idea of a call sign that fits the encoding rule. While most amateur call signs do indeed

fit the rule some perfectly valid call signs do not. For example, some special event call signs (often seen in

Europe) like DL2008WC [a made up example] will not resolve to a 28 bit computed call sign value thus can not,

in practical terms, be used with JT65 no matter the program offering the implementation.

Sent character by character the exchange KC4NGO KG6CQZ CM87 would be 18 characters

(count the spaces) and in free text format you simply could not send the text in the available

bit space. So, structured messages use a sort of compression to fit more information into a

restricted number of bits. Notice that under the rules of encoding (compressing) a call sign or

gird that you have more values available than actually needed.

For call signs you have 262,177,560 possible unique values but in a 28 bit space you can

represent 268,435,456 unique values leaving an 'extra' 6,257,896 values.

For the grid you have 32,400 actual grid values but in a 15 bit space you can represent

32,768 unique values leaving an 'extra' 368 values.

These 'extra' values are not wasted in JT65. In fact they're used to convey information that

would not otherwise fit into a structured message frame and this further explains the rigidity of

what JT65 allows/disallows.

For instance, take the structured message “CQ KC4NGO EL98”. From the explanation of the

rules you can be sure that KC4NGO is converted to a 28 bit value and EL98 is converted to a

15 bit value, but what about CQ? It simply uses one of the 'surplus' 28 bit values left above

the last possible call sign number. So “CQ KC4NGO EL98” still uses a pair of 28 bit numbers

plus a 15 bit number for 71 bits.

CQ can in fact contain an extra offset value in hertz which the caller expects a responder to apply when

answering. This is seldom, if ever, seen on HF and is not implemented in JT65-HF.

Now we come to the structured message “KC4NGO KG6CQZ DM06”. This is more obvious

in that it is a pair of 28 bit encoded call signs plus a 15 bit encoded grid for, again, 71 bits.

Now things change a bit. “KG6CQZ KC4NGO -10”. Here you have the pair of 28 bit call

signs but the grid has been replaced by a signal report. Again, as in CQ, JT65 uses some of

the excess values, but this time in the 15 bit 'grid space' to convey the signal report. The 15

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Carrier 42 HQX Manual de usuario Pagina 32