mirror of
https://github.com/saitohirga/WSJT-X.git
synced 2024-11-05 17:01:17 -05:00
140be15cce
git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@6657 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
194 lines
8.9 KiB
Plaintext
194 lines
8.9 KiB
Plaintext
[[JT65PRO]]
|
||
=== JT65
|
||
|
||
JT65 was designed for making minimal QSOs via EME ("`moon-bounce`") on
|
||
the VHF and UHF bands. A detailed description of the protocol and its
|
||
implementation in program _WSJT_ was published in {jt65protocol} for
|
||
September-October, 2005. Briefly stated, JT65 uses 60 s T/R sequences
|
||
and carefully structured messages. Standard messages are compressed so
|
||
that two callsigns and a grid locator can be transmitted in just 71
|
||
information bits. A 72^nd^ bit serves as a flag to indicate that a
|
||
message consists of arbitrary text (up to 13 characters) instead of
|
||
callsigns and a grid locator. Special formats allow other information
|
||
such as add-on callsign prefixes (e.g., ZA/K1ABC) or numerical signal
|
||
reports (in dB) to be substituted for the grid locator. The basic aim
|
||
is to compress the most common messages used for minimally valid QSOs
|
||
into a minimum fixed number of bits. After compression, a Reed Solomon
|
||
(63,12) error-control code converts 72-bit user messages into
|
||
sequences of 63 six-bit channel symbols.
|
||
|
||
JT65 requires tight synchronization of time and frequency between
|
||
transmitting and receiving stations. Each transmission is divided into
|
||
126 contiguous tone intervals or "`symbols`" of length 4096/11025 =
|
||
0.372 s. Within each interval the waveform is a constant-amplitude
|
||
sinusoid at one of 65 pre-defined frequencies. Frequency steps between
|
||
intervals are accomplished in a phase-continuous manner. Half of the
|
||
channel symbols are devoted to a pseudo-random synchronizing vector
|
||
interleaved with the encoded information symbols. The sync vector
|
||
allows calibration of time and frequency offsets between transmitter
|
||
and receiver. A transmission nominally begins at t = 1 s after the
|
||
start of a UTC minute and finishes at t = 47.8 seconds. The
|
||
synchronizing tone is at 11025 × 472/4096 = 1270.46 Hz, and is
|
||
normally sent in each interval having a “1” in the following
|
||
pseudo-random sequence:
|
||
|
||
100110001111110101000101100100011100111101101111000110101011001
|
||
101010100100000011000000011010010110101010011001001000011111111
|
||
|
||
Encoded user information is transmitted during the 63 intervals not
|
||
used for the sync tone. Each channel symbol generates a tone at
|
||
frequency 11025 × 472/4096 + 11025/4096 × (N+2) × m, where N is the
|
||
value of the six-bit symbol, 0 ≤ N ≤ 63, and m is 1, 2, or 4 for JT65
|
||
sub-modes A, B, or C. Sub-mode JT65A is always used at HF.
|
||
|
||
For EME (but, conventionally, not on the HF bands) the signal report
|
||
OOO is sometimes used instead of numerical signal reports. It is
|
||
conveyed by reversing sync and data positions in the transmitted
|
||
sequence. Shorthand messages for RO, RRR, and 73 dispense with the
|
||
sync vector entirely and use time intervals of 16384/11025 = 1.486 s
|
||
for pairs of alternating tones. The lower frequency is always 1270.46
|
||
Hz, the same as that of the sync tone, and the frequency separation is
|
||
110250/4096 = 26.92 Hz multiplied by n × m, with n = 2, 3, 4 for the
|
||
messages RO, RRR, and 73.
|
||
|
||
[[JT4PRO]]
|
||
=== JT4
|
||
|
||
JT4 uses 72-bit structured messages nearly identical to those in
|
||
JT65. Error control coding (ECC) uses a strong convolutional code with
|
||
constraint length K=32, rate r=1/2, and a zero tail, leading to an
|
||
encoded message length of (72+31) x 2 = 206 information-carrying
|
||
bits. Modulation is 4-tone frequency-shift keying at 11025 / 2520 =
|
||
4.375 baud. Each symbol carries one information bit (the most
|
||
significant bit) and ony synchronizing bit (the least signicifant
|
||
bit). The pseudo-random sync vector is the following sequence:
|
||
|
||
000011000110110010100000001100000000000010110110101111101000
|
||
100100111110001010001111011001000110101010101111101010110101
|
||
011100101101111000011011000111011101110010001101100100011111
|
||
10011000011000101101111010
|
||
|
||
[[JT9PRO]]
|
||
=== JT9
|
||
|
||
JT9 is designed for making minimally valid QSOs at LF, MF, and HF. It
|
||
uses 72-bit structured messages nearly identical (at the user level)
|
||
to those in JT65. Error control coding (ECC) uses a strong
|
||
convolutional code with constraint length K=32, rate r=1/2, and a zero
|
||
tail, leading to an encoded message length of (72+31) × 2 = 206
|
||
information-carrying bits. Modulation is nine-tone frequency-shift
|
||
keying, 9-FSK. Eight tones are used for data, one for
|
||
synchronization. Eight data tones means that three data bits are
|
||
conveyed by each transmitted information symbol. Sixteen symbol
|
||
intervals are devoted to synchronization, so a transmission requires a
|
||
total of 206 / 3 + 16 = 85 (rounded up) channel symbols. The sync
|
||
symbols are those numbered 1, 2, 5, 10, 16, 23, 33, 35, 51, 52, 55,
|
||
60, 66, 73, 83, and 85 in the transmitted sequence. Each symbol lasts
|
||
for 6912 sample intervals at 12000 samples per second, or about 0.576
|
||
seconds. Tone spacing of the 9-FSK modulation is 12000/6912 = 1.736
|
||
Hz, the inverse of the symbol duration. The total occupied bandwidth
|
||
is 9 × 1.736 = 15.6 Hz.
|
||
|
||
[[PROTOCOL_SUMMARY]]
|
||
=== Comparison of Slow Modes
|
||
|
||
Frequency spacing between tones, total occupied bandwidth, and
|
||
approximate decoding thresholds are given for the various submodes of
|
||
JT4, JT9, and JT65 in the following table:
|
||
|
||
Submode Spacing BW S/N
|
||
(Hz) (Hz) dB
|
||
----------------------------
|
||
JT4A 4.375 17.5 -23
|
||
JT4B 8.75 35.0 -22
|
||
JT4C 17.5 70.0 -21
|
||
JT4D 39.375 157.5 -20
|
||
JT4E 78.75 315.0 -19
|
||
JT4F 157.5 630.0 -18
|
||
JT4G 315.0 1260.0 -17
|
||
|
||
JT9 1.7361 15.625 -27
|
||
|
||
JT65A 2.6917 177.6 -25
|
||
JT65B 5.3833 355.3 -24
|
||
JT65C 10.767 710.6 -23
|
||
|
||
|
||
Transmissions in all three modes are essentially the same length, and
|
||
all use 72 bits to carry message information. At user level the modes
|
||
support nearly identical message structures.
|
||
|
||
JT4 and JT65 signal reports are constrained to the range –1 to –30
|
||
dB. This range is more than adequate for EME purposes, but not enough
|
||
for optimum use at HF. S/N values displayed by the JT4 and JT65
|
||
decoders are clamped at an upper limit –1 dB, and the S/N scale is
|
||
nonlinear above –10 dB.
|
||
|
||
By comparison, JT9 allows for signal reports in the range –50 to +49
|
||
dB. It manages this by taking over a small portion of "`message
|
||
space`" that would otherwise be used for grid locators within 1 degree
|
||
of the south pole. The S/N scale of the present JT9 decoder is
|
||
reasonably linear (although it's not intended to be a precision
|
||
measurement tool).
|
||
|
||
JT9 is an order of magnitude better than JT65 in spectral
|
||
efficiency. On a busy HF band, the conventional 2-kHz-wide JT65
|
||
sub-band is often filled with overlapping signals. Ten times as many
|
||
JT9 signals can fit into the same frequency range, without collisions.
|
||
|
||
=== ISCAT
|
||
|
||
ISCAT messages are free-form, up to 28 characters in length.
|
||
Modulation is 42-tone frequency-shift keying at 11025 / 512 = 21.533
|
||
baud (ISCAT-A), or 11025 / 256 = 43.066 baud (ISCAT-B). Tone
|
||
frequencies are spaced by an amount in Hz equal to the baud rate. The
|
||
available character set is
|
||
|
||
----
|
||
0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ /.?@-
|
||
----
|
||
|
||
Transmissions consist of sequences of 24 symbols: a synchronizing
|
||
pattern of four symbols at tone numbers 0, 1, 3, and 2, followed by
|
||
two symbols with tone number corresponding to the message length, and
|
||
finally 18 symbols conveying the user's message, sent repeatedly
|
||
character by character. The message always starts with +@+, the
|
||
beginning-of-message symbol, which is not displayed to the user. The
|
||
sync pattern and message-length indicator have a fixed repetition
|
||
period, recurring every 24 symbols. Message information occurs
|
||
periodically within the 18 symbol positions set aside for its use,
|
||
repeating at its own natural length.
|
||
|
||
For example, consider the user message +CQ WA9XYZ+. Including the
|
||
beginning-of-message symbol +@+, the message is 10 characters long.
|
||
Using the character sequence displayed above to indicate tone numbers,
|
||
the transmitted message will therefore start out as shown in the first
|
||
line below:
|
||
|
||
----
|
||
0132AA@CQ WA9XYZ@CQ WA9X0132AAYZ@CQ WA9XYZ@CQ W0132AAA9X ...
|
||
sync## sync## sync##
|
||
----
|
||
|
||
Note that the first six symbols (four for sync, two for message
|
||
length) repeat every 24 symbols. Within the 18 information-carrying
|
||
symbols in each 24, the user message +@CQ WA9XYZ+ repeats at its own
|
||
natural length, 10 characters. The resulting sequence is extended as
|
||
many times as will fit into a Tx sequence.
|
||
|
||
=== JTMSK
|
||
|
||
The letters MS are often used to abbreviate meteor scatter; the three
|
||
letters MSK mean "Minimum Shift Keying", the modulation scheme used in
|
||
JTMSK. This mode uses the same standard message structure as the slow
|
||
modes JT4, JT9, and JT65. User information is "`source encoded`" to
|
||
72 bits; a 15-bit cyclic redundancy check (CRC) is appended, and a
|
||
convolutional code with constraint length K=13 and rate r=1/2 is then
|
||
applied. This procedure makes for a total of (72+15+12)*2 = 198
|
||
information bits for the encoded message. Three copies of the
|
||
"`Barker-11`" code and three even-parity bits are added for
|
||
synchronization, making a total of 198+33+3 = 234 channel symbols.
|
||
Modulation is carried out using a constant-envelope, continuous-phase
|
||
minimum-shift keying (MSK) waveform, with tone frequencies of 1000
|
||
and 2000 Hz.
|