JT65 Notes by W6SZ ( 1st run )
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Sources:
Users guide and Reference Manual August 2006
QEX Sept/Oct 2005
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Tones:
Mode SSB, USB, assumed virtual SSB carrier at 0 Hz
TXDF = 0 Hz
Lowest frequency audio tone being generated and transmitted is is 1270.7 Hz
This is also the audio freqency of the sync pulse.
Half ot the transmit cycle is used to transmit sync tones.
126 contigenous time intervals each of 0.372 secs.
Transmission starts 1 sec after UTC integer minute, 46.8 secs of information, total 47.8 secs.
IF TXDF = +/- FREQ Hz, the lowest tone frequency being generated and transmitted is 1270.7 +/- FREQ Hz
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Define type of Messages:
--- Shorthand messages: [ BLUE ]
RO
RRR
73
--- Type 1 messages: [ YELLOW ]
2 Legal callsigns, optional grid locator, optional signal report OOO
KC2QII W6SZ DM14 OOO
CQ or QRZ can be substituted for first legal callsign.
Substitutions for the grid locater:
An add-on prefix followed by / sign W6SZ/7
A suffix proceded by / sign DL/W6SZ
A signal report -04 or R-14 - sign required!
Message fragments RO, RRR , 73
--- Other messages: [ RED ]
A maximum of 13 Characters of arbitrary text will be encoded and sent.
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Define windows
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Notes on Decoding and Averaging issues:
Central to the process is the detection of the so called sync pulse(s).
Search window in Spectrum View, ( SpecJT ) is 1200 Hz wide.
The Time line in SpecJT is approx. 53 secs long.
Time for the acquisition is the first 48 secs of the 1 minute Transmit/Receive period.
Center Frequency is set by the user by:
A left mouse click in waterfall on Sync. markers of desired signal block.
A double left mouse click on Red Line in spectrum window ( Frequency line )
Freeze: Locks center frequency for sync pulse detection.
Sync: Sets minimum level of sync detection threshold ( default: 1 sync pulse detected ).
Tol: Search window in Hz for sync pulse detection centered around center frequency.
Flatten Spectra: Allows to correct amplitude variations due to Receiver IF pass band.
Dsec: Allows for matching of local receiver computer time to transmitter local time.
DT is Delta Time in secs : Time scale from - 2 to 10 secs ( Transmission max 48 secs )
DF is Delta Frequency in Hz : Frequency scale from - 600 Hz to + 600 Hz
W width in Hz of sync pulse : ( range 1 - 5 Hz )
In addition to values for DT, DF and decoded TEXT lines, information is generated and provided on:
--- Sync a number indicating the relative strength of the synchronization, sync pulses detected.
--- dB the average Signal to Noise Ratio in dB, relative to noise power in 2500 Hz Bandwidth
( -16 dB difference )
--- W, the measured frequency width of the synchronization pulse in Hz.
--- A symbol following the value of W, indicating that an adequate level of synchronisation has
been achieved:
"*" will be displayed for a normal message
"#" will be displayed for a message that includes the signal report "OOO"
[ sse page 14 of user manual ]
2 numbers appear on the end of each TEXT line:
The first number indicates whether the soft decision Reed Solomon decoder failed or succeeded.
0 means failing
1 means suceeding
The second number indicates a relative confidence level on a 0 to 10 scale for results
produced by the Deep Search Decoder.
Short hand messages do not produce these added numbers at the end to the TEXT Lines.
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If a JT65 transmission SYNCHRONIZES CORRECTLY ( indication is the appearance of a "*"
or a "#" symbol )
see indicators of "properly synchronized"
the spectral information of this particular message is placed in an accumulation array
or accumulation buffer.
Subsequent transmissions, if correctly synchronized, are added to this accumulation buffer.
The accumulated valid spectra in this buffer may make it possible to decode even if the
individual spectra, though correctly synchronized, could not be decoded.
The results of this particular decoding process are displayed in the AVG TEXT box.
Example of correctly sunchronized message with no text line decoded
021800 2 -4 0.2 -579 3 *
022000 3 -3 -0.3 -573 3 * CQ K4BQH EM78 1 0 ( decoded text line )
Include/Exclude: When avaraging is being used it is suggested to analyze each acquired spectrum for
correct synchronization.
If synchronization is not correct, the acquired spectrum can be "excluded" from the
accumulation process.
If the signal level of an acquired spectrum is larger than - 33 dB and even if
Sync number is less then the stated threshold, this spectrum may be "included"
in teh accumulation of spectra.
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Decoding options selected by user:
Decode EME calls only: Only a subset of call signs in the data base that include an "EME"
flag will be used in the Deep Search routine.
No Shorthands if TX1: If you wish to suppress shorthand decoding when you are still
transmitting TX1
Quick decode:
Deep search decoding options selected by user:
No Deep Search: Deep Search is disabled.
Normal Deep Search: Deep Search is ON bur suppresses Text Lines with confidence levels
less than 3.
Include Average in Deep Search routine applied to the accumlated spectra as well as
Aggressive Deep Search: to the most recent received spectrum.
Sked: Use sked if you want to see decoded data only from your sked partner
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Questions: ( waiting for answers )
--- Are all messages with sync > 0 and a "#" or "*" accumulated in the accumulation buffer?
----What about messages such as 020400 0 -30 9.4 30 4 #
020500 0 -28 0.1 30 4 *
Sync = 0
W = 4 secs
Could they be included?
Are they included? ( the carry added "*" "#" marker )
It appears that results are generated with a "#" and "*" when sync = 0 and db is ?
larger than - 33 dB. ?
?
Why in one case a "*" and in another case "#" ? ?
see from real results:
064500 0 -28 3.3 -3 2 #
064600 0 -29 -0.8 -3 3 *
064700 0 -28 0.3 -3 3 *
???????
--- What is it that the sync number really represent?
Is it the number of sync pulses detected during an acquisition period?
Have observed numbers as high as 16!
--- Inclusion and exclusion needs to be done by the user in real time? Within 50 or so seconds
before next generated data set is appears?
--- What is "correctly synchronized"?
Proper Synchronization is indicated by a detected sharp upward spike on the red freq. curve ( DF )
and another broader peak on the blue time curve ( DT )
--- What to do, if anything, when monitoring 2 stations and one is weak so that averaging
would be useful. Does the procedure know which spectra data to put in the accumulator?
--- Is is possible to fill the buffer before a Text line has been decoded ?
--- What is causing a decoded Text line to be displayed in the AVG window?
--- What happens if Text lines with added "*" and "#" are properly decoded right away?
Are the source spectra of these messages still accumulated?
--- What is indicated in the left side of the AVG window?
--- What is happening if no TEXT Line shows in the AVG spot , after for instance a series of
weak transmitted cq's?
--- What is the Shift Function?
8/11/07