How it works
Next
Previous

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: