How it works
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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 the decoding executable can only decode JT9(-1), not JT9-2, JT9-5 or JT9-10. Rather than adapting the existing decoding software for JT9-2 JT9-5 or JT9-10 the unmodified decoder is used and the incoming audio signal is modified before it is send to the decoding software.

This modification is done by resampling ("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 JT9(-1) signals to the decoder.
SlowJT9 does the resampling and provides a user interface, while the decoding 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 comes with a restrictions:
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).