The difficult part of any digital mode software is the decoding, and to a lesser extend the coding, of the audio signal. Luckily the WSJT-X suite contains 2 executables that perform coding and decoding for the JT9 mode. But this executable can not (directly) be used to decode JT9-2 or JT9-5. Rather than adapting the existing decoding software for JT9-2 and JT9-5, the unmodified JT9(-1) decoder is used and the incoming audio signal is modified before it is send to the decoding software.
This modification is done by averaging and "speeding up" the incoming audio signal by a facor of 2.22 (for JT9-2), 5.93 (for JT9-5) and 12 for JT9-10. That way they look like true JT9(-1) signals to the decoder. Apart from this audio conversion SlowJT9 mainly acts as a user interface, while the hard work is still be done by the original JT9 decoder.
At first glance it might seem foolish to convert JT9-2, JT9-5 or JT9-10 signals back to JT9(-1), but this conversion process will also reduce the noise level and thus improve the S/N.
Although this way of working simplifies the software a lot it also causes some restrictions:
- It will not take the full S/N advantage of the slower JT9 modes. In theory JT9-2 should have a 3.5 dB S/N advantage over JT9(-1), while some tests showed a 3.0 dB improvement. For JT9-5 and JT9-10 no tests have been done yet, but in theory the improvement over JT9(-1) is 7.7 dB for JT9-5 and 10.8 dB for JT9-10. In the real world these values will be lower due to frequency and propagation instabilities (ionospheric doppler shift, QSB; ...)
- Speeding up the audio by a factor X results in a identical increase of the audio frequencies. This means that an original 600 Hz audio tone will be at 1333 Hz for JT9-2, 3556 Hz for JT9-5 and 7200 HZ for JT9-10. As that maximum audio
frequency that can be handled by the JT9 decoder is 5000 Hz this means that the maximum usable audio frequency is 2250 Hz for JT9-2, 843 Hz for JT9-5 and 416 Hz for JT9-10.
But SlowJT9 handles all these frequency conversions, so apart from the fact that the maximum audio frequency is limited the users don't have to take care of this (and won't even notice it). - Longer transmission cycles inevitably lead to a longer QSO duration. While a QSO takes at least 5 minutes in JT9(-1), it will take at least 10 minutes in JT9-2, 25 minutes in JT9-5 and 50 minutes in JT9-10.