Version   Installation file (1)   ZIP file (2)   Executable only ZIP file (3)   Users 
 Changes      download read
  • Timestamp of preview decodes fixed (will always show the start time of the decoded transmission).
  • Late start / message change button added (instead of right-click on the "Enable TX" button).
  • Minor layout change.      download read      download read
  • Beacon mode enabled for all JT9 submodes
  • Online users guide added (replaces the help function)
  • Fatal bug fixed that caused the program to hang when clicking on certain buttons during decoding
  • Non fatal bug failing to display non Latin characters properly fixed    download    
  • Sample rate of the recording device can be selected between 12000 Hz and 48000 Hz (default)
  • Preview decoding added:
    If "preview decoding" is selected (Settings → Mode → check "Allow preview decoding") SlowJT9 will try to decode incoming signals before the end of the cycle. This will be done after:
    • 30 and 40 seconds for JT9-1
    • 60 and 90 seconds for JT9-2
    • 150, 180, 240 and 270 seconds for JT9-5
    • 300, 360, 420, 480, 540 and 570 seconds for JT9-10
    For each received message only the first decode will be shown, in case of a early (preview) decode the shown SNR might be less than the final SNR.
    Preview decoding will be performed as a single decode (no multiple decoding). The final decoding however will be performed as multiple decoding, if this option is checked.
  • Late TX start added:
    If transmitting is enabled a right click on the "Enable TX" button will initiate a "late TX start":
    • In RX mode or idle mode SlowJT9 will switch to TX instantly and the remaining part of the message will be transmitted
    • In TX mode the transmitted message will be changed and the remaining part of the new message will be transmitted
    It is obvious that a late TX start can have a negative effect of the SNR or the message may be not be decoded at all.      download  
  • Non fatal bug in UPD broadcasting fixed.
  • Non fatal bug in the watchdog timer trip fixed.
  • UDP hartbeat message changed (included the actual JT9 submode)
  • Beacon mode added:
    • RX: In JT9-5 and JT9-10 submodes received messages in the format callsign+space(s)+WW-locator will be recognized as beacon and spotted to
    • TX: a beacon mode is added for JT9-5 and JT9-10.
      Settings → Beacon:
      • TX ratio: Enter the TX ratio (1/1 = every cycle, 1/2 = every 2 cycles, 1/3 = every 3 cycles ... with a maximum of 1/99).
      • Transmit in beacon mode: check this to enable beacon mode, uncheck to return to QSO mode.
    Beacon mode is delberately restricted to the JT9-5 and JT9-10 submodes as WSPR / WSPRNET is available for shorter cycle beaconing. For JT9-5 and JT9-10 PSK Reporter spotting of beacons can be used as a WSPRNET alternative.      download  
  • Bug in decoding of JT9-5 and JT9-10 fixed. For the use of these modes upgrading from versions - to is required.      download  
  • "RR73" option added.
    The "RRR" generated message can be replaced by "RR73" (Settings → Mode → check "Use RR73 instead of RRR").
  • Right-clicking in the "Band Activity" or "RX Frequency" windows will copy the window content to the clipboard (control+C equivalent).
  • Fatal bug fixed that caused the program to crash once in a while during the samplerate conversion.
  • Fatal bug fixed that caused the program to hang when transmitting in JT9-5 and JT9-10 modes.
  • Upgrading from to is strongly recommended!  download download    
  • Soundcard sample rate changed from 12000 Hz to 48000 Hz.
  • Alert function added (including UDP broadcasting).
    This function allows to generate an alert when a received message meets certain criteria. One or more of following criteria can be selected:
    • Your call is received.
    • A new call is received.
    • A new DXCC is received.
    • A new US state is received.
    • A DX station (more than xxx km) is received.
    • A new DX station (more than xxx km) is received.
    • A call from the included list is received.
    In case of an alert:
    • The DTR or RTS pin of a serial port can be set high. This can be used to trigger an alarm.
    • A UDP message can be broadcasted. UDP messages can be copied on any device (computer, tablet, smartphone) within the same LAN.
    SlowJT9_Monitor can be used to receive and display the UDP messages on another computer.
  • A Front End Mixer added:
    The resampling and decoding process limits the maximum RX audio frequency to 4980 Hz for JT9-1, 2240 Hz for JT9-2, 840 Hz for JT9-5 and 415 Hz for JT9-10. This can be inconvenient if a higher RX audio frequency is needed to fit the signal in the passband of a narrow filter.
    This can be fixed by using the Front End Mixer that downconverts the incoming audio signal (fOUT = fIN - fLO).
    E.g.: if you want to downconvert the 1200 .. 1400 Hz incoming audio to 400 .. 600 Hz the local oscillator frequency must be set to 800 Hz.      download  
  • "Multiple decoding" option added (Settings → Mode → check "Allow multiple decoding").
    If "Multiple decoding" is enabled the received audio will first be decoded the audio as it is (unshifted) and then it will be shifted +/- 1 second in time steps of 0.2 seconds and decoded again for each step.
    Thus there will 11 decoding steps in total.
    However decoding always will stopped at 1 second before the end of the cycle at latest, so the next cycle will not be corrupted.
    Decoding can also be stopped by clicking on the Band activity or RX frequency windows. After each decoding step the new (non-duplicate) decoded messages will be shown.
    So even during the decoding process any already decoded message can be selected by triple-clicking on it (a single click for stopping the decoding followed by a double click for selecting the message).
    Be aware that each decoding time can take several seconds on a slow computer, in that case only a limited number of decoding steps will be performed.
    The DT parameter in the Band activity or RX frequency windows will show the time shift of the decoded file. "0.00" means that the message is retrieved from the unshifted (original) audio, any other value means it is retrieved from shifted audio (and thus would be missed without multiple decoding).
    Read more!  download download    
  • spotting added.
  • Flexible frequency boundaries for the spectrum display added (100 - 5000 Hz in 100 Hz steps).
  • Help file added.      download  
  • Spectrum display adjust changed:
    1. Gain: click here to adjust the visual gain (active when background is green).
    2. Offs: click here to adjust the spectrum display offset (active when background is green).
    3. AGC: click here to enable/disable the Automatic Gain Control (enabled when background is green).
    4. "Black level": click on the spectrum display to set the "black level". Any signal below this level will appear as black on the waterfall display. The small red line right of the spectrum display indicates the "black level".  download download    
  • JT9-10 mode added.
  • Waterfall display speed will adapt to selected JT9 submode.
  • Any (reasonable) value can be entered for tone spacing (allowing flexible frequency dividing of the modulated signal).
  • Lower frequency limit dropped to 200 Hz (was 400 Hz).
  • Option to discard the audio frequency limits added (Settings → Mode).
    In that case the TX boundaries will be 100 - 4980 Hz regardless of the JT9 submode.
    The RX boundaries however will still be limited by the resampling process:
    JT9-1: 100 - 4980 Hz
    JT9-2: 100 - 2240 Hz
    JT9-5: 100 - 840 Hz
    JT9-10: 100 - 415 Hz
  • Non fatal bug fixed that skipped PTT handling in the first TX cycle after 'Halt TX' was pressed.       download
  • JT9-5 mode added. download      
  • Option added to select double or quadruple tone tone spacing (Settings → Mode)
  • Option added to display the own transmission in the RX frequency window (Settings → Mode).
  • Fatal bug fixed that sometimes caused the application to crash at the end of a transmission. download      
  • Reporting to debug.txt is been extended (in particular during TX).
  • JT9-5 and JT9-10 modes are temporarily disabled, my first goal is to have a stable version before tackling new features.
  • Fatal bug fixed that caused a array bound violation once in a while.

(1) Installation file:

Download and run the installation file.
Advantages: SlowJT9 will be installed in Windows and a desktop shortcut will be created. SlowJT9 can be uninstalled by Windows.
Disadvantages: Any version of SlowJT9 can be installed only once and it is not possible to run multiple instances of the installed version.
It is strongly recommended to install SlowJT9 outside the Program Files folders.

(2) ZIP file:

Download and unzip to a folder of your choice.
Advantages: SlowJT9 can be 'installed' multiple times in different folders. This way multiple instances of SlowJT9 can run simultaneously (each installation can run once).
Disadvantages: You need to know what you are doing, eventual desktop shortcuts have to be created manually.
It is strongly recommended to 'install' SlowJT9 outside the Program Files folders.
It is possible to combine 1 real installation (1) with one or more 'ZIP installations' (2).

(3) Executable only ZIP file:

First you need to install the previous 'full version' ((1) or (2)), if not already done.
Next download and unzip the executable.
Replace the original SlowJT9.exe by this one.

What you might want to know about SlowJT9

About SlowJT9

SlowJT9 was born out of the idea to to revive the slower JT9 submodes that initially were supported by WSJT-X, but abandoned at an early stage. On the 2200 m and 630 m bands these submodes can be useful for weak signal (DX) communication.

Rather than developing a new decoder, the existing JT9 decoder of the WSJT-X suite (JT9.exe) was used. As this decoder will only decode JT9-1 (1 minute cycle JT9), the slower submodes are resampled to JT9-1 and then decoded. SlowJT9 takes care of this resampling and provides a WSJT-X like user interface.

Although the resampling process is a convenient tool to be able to use the existing JT9 decoder it also has some limitations, mainly in regarding with the available frequency range (see "Audio frequency range").

As per v0.9.14.0 following JT9 submodes are available:

Keep in mind that the frequency stability (drift) requirements increase as you go slower (see "Frequency stability").

Audio frequency range

Up to v0.9.12.5 SlowJT9 will handle audio signals within following boundaries:
JT9-1: 400 - 1400 Hz
JT9-2: 400 - 1400 Hz
JT9-5: 400 - 840 Hz

In v0.9.14.0 the default lower boundary was reduced to 200 Hz and the frequency boundaries can discarded for TX and RX (Settings → Mode).
In that case the TX boundaries will be 100 - 4980 Hz regardless of the JT9 submode.
The RX boundaries however will still be limited by the resampling process:
JT9-1: 100 - 4980 Hz
JT9-2: 100 - 2240 Hz
JT9-5: 100 - 840 Hz
JT9-10: 100 - 415 Hz

In v0.9.20.0 a Front End Mixer was added that allows to receive JT9 signals at frequencies beyond the limits of the resampling process.

Frequency stability

The slower the JT9 submode, the stricter the requirements for frequency stability.

As no information about the required JT9 frequency stability was found, some tests were done that showed that for JT9-1 and frequency drifts up to 5Hz/min the SNR (dB) drops by the square of the drift (Hz/min). The measured SNR losses fit well to the function ΔSNRdB = 0.1×(ΔfHz/Δtmin.)2.
For the slower JT9 submodes the frequency stability requirements (in Hz/min) will raise with the square of cycle duration.

To keep SNR loss less than 1dB it is recommended to keep the drift at TX and RX below

Keep in mind that even if the TX and RX frequency stability is sufficient, ionospheric instabilities can thwart successful decoding. So far JT9-5 and JT9-10 have proved to be reliable for long distance ionospheric communication on 2200 m. On 630 m JT9-5 seems reliable for long distance communications most of the time and JT9-10 had been copied between the US and Europe.

Questions, suggestions, bug reports

Questions, suggestions and bug reports can be addressed to

For bug reports please


73, Rik   ON7YD - OR7T