Why does Tone Spacing decrease with increasing T/R Periods in WSJT-X modes?

Question

In various weak signal modes of WSJT-X, the Tone Spacing decreases with increasing T/R Periods. For instance for FST4 / FST4W (as decribed on page 1 of the Quick-Start Guide) we see:

T/R period (s)	Tone Spacing (Hz)
15	16.67
30	7.14
60	3.09
120	1.46
300	0.56
900	0.180
1800	0.089

Later on, we read that "oscillator drifts and path-induced Doppler shifts must be less than the tone spacing".

Why was this chosen like this? Wouldn't larger tone spacings provide better SNR decodability? At the very least, they should allow for higher Doppler shifts for the longer-period modes, shouldn't they?

Why isn't there an option in WSJT-X to manually increase the spacings for testing? I mean, is it just because there was no demand for it and it simply wasn't implemented, or is it because it inherently doesn't make sense to increase them?

I understand that signals with slower frequency changes generally occupy less bandwidth, but don't such tiny tone spacings also make decoding harder?

Answer 1

The longer T/R periods also have a longer duration for each symbol. The longer each symbol is, the more finely the decoder can resolve the frequency of each symbol. So the slower modes make narrower tone spacing possible. A narrower signal bandwidth means less noise is contained in the same bandwidth, and so it becomes possible to decode weaker signals against that noise; that's what makes narrower tone spacing desirable.

FST4 is specifically meant for the LF and MF bands, where good oscillator stability is easily achieved, and Doppler effects aren't significant, so it deliberately pushes things just about as narrow as they get. There are other modes that make different tradeoffs and have wider spacing, and there are other modes that have "just make it wider" variants — for instance, the JT65 mode that you normally see on HF is actually JT65A, with a tone spacing of 2.7Hz; but on 2m and 70cm people use JT65B, and on 23cm they use JT65C. Those are exactly the same as JT65A, except 2x and 4x as wide (tone spacings of 5.4 and 10.8 Hz) to account for the stability requirements of those higher bands.

Answer 2

The reason why there isn't a way to manually adjust the spacing is because WSJT-X is also a bit of a standard. If there was such an adjustment, it would make communication more difficult.

Instead of just adjusting spacing, you get different modes, each of which has multiple parameters (not just spacing) tweaked to optimize it for a particular purpose. And each such mode has a designated frequency range that WSJT-X suggests they should be used in, reducing confusion and maximizing use of each designated narrow band.

By designating a frequency range for each mode rather than mixing them, it allows more simultaneous contacts per "channel" (through better packing) and a less complicated demodulator. (It's already fairly complicated and cpu intensive, so to make it more so would not be good.)