FST4 is is designed for making 2-way QSOs on the LF and MF bands.  Do
not confuse it with FT4, which has a very different purpose!  Most
on-screen controls, auto-sequencing, and other features behave in FST4
as in other modes.  However, operating conventions on the 2200 and 630
m bands make it desirable to have additional user controls that set
the active frequency range for decoding.  Spin boxes labeled *F Low*
and *F High* set lower and upper frequency limits for the FST4
decoder.  

image::FST4_center.png[align="center"]

Decoding limits are marked by dark green angle-bracket symbols *< >* on
the Wide Graph frequency scale:

image::FST4_Decoding_Limits.png[align="center"]

In general the specified range should be no larger than you need, since
detected transmissions in modes other than the selected FST4 sequence
length will be undecodable and will slow down the decoding process.

If *Single decode* on the the *File | Settings | General* tab is
checked, the decoding range is further limited to the *F Tol* range
around *Rx Freq*.