Do not confuse FST4 with FT4, which has a very different purpose!
FST4 is designed primarily for making weak-signal 2-way QSOs on the 
LF and MF bands. T/R periods from 15 s up to 1800 s are 
available. Longer T/R periods provide better sensitivity only if 
Tx and Rx frequency instability and channel Doppler spread
are small enough so that received signals 
remain phase coherent over periods spanning several transmitted symbols.
Generally speaking, Rx and Tx frequency changes 
during the transmission and channel Doppler spread should each be small compared
to the symbol keying rate shown for each T/R duration in Table 7 within section
<<PROTOCOL_OVERVIEW,Protocol Specifications>>. For example, the keying rate for 
the 1800 s T/R period is 0.089 Baud, so
successful operation using this T/R length requires Tx and Rx frequency
stability better than 0.089 Hz over the duration of the 1800 s transmission in 
addition to channel Doppler spread smaller than 0.089 Hz. 

Operation with FST4 is similar to that with other _WSJT-X_ modes: most
on-screen controls, auto-sequencing, and other features behave in
familiar ways.  However, operating conventions on the 2200 and 630 m
bands have made some additional user controls desirable.  Spin boxes
labeled *F Low* and *F High* set lower and upper frequency limits used
by the FST4 decoder, and these limits are marked by dark green
angle-bracket symbols *< >* on the Wide Graph frequency scale:

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

{empty} +

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

It's best to keep the decoding range fairly small, since QRM and
transmissions in other modes or sequence lengths will slow down the
decoding process (and of course will be undecodable).  By checking 
*Single decode* on the *File | Settings | General* tab, you can
further limit the decoding range to the setting of *F Tol* on
either side of *Rx Freq*.

A noise blanker can be enabled by setting the *NB* percentage to a non-zero value. 
This setting determines how many of the largest-amplitude samples will be 
blanked (zeroed) before the data is submitted to the decoder. Most users find
that settings between 0% (no blanking) and 10% work best. If the noise 
blanker percentage is set to -1%, then the decoder will try 0, 5, 10, 15, and 20 %
in succession. Similarly, a setting of -2% causes the decoder to loop over
blanking percentages 0, 2, 4, ... 20 %. To save time, the multiple blanking 
percentages triggered by negative *NB* settings are tried only for signal 
candidates located near (within +/- 20 Hz) of the *Rx* frequency setting.

.Open a sample Wave File:

- Select *FST4* on the *Mode* menu. Set *T/R* to 60 s and *Decode | Deep*.
- Set *NB* (noise blanker) to 0%.
- Set up the Wide Graph display with settings appropriate for the FST4-60 mode. 
For example, try *Bins/Pixel* 2 and *N Avg* 4. Set the *Start* frequency and the width of 
the Wide Graph to include the frequency range that you want to decode. For this
example, make sure that *Start* is less than 1000 Hz and that the Wide Graph extends to above 1400 Hz.
- Set *F Low* 1000, *F High* 1400. These settings define the decoder's frequency search range.
- Open a sample Wave file using *File | Open* and select the file
...\save\samples\FST4+FST4W\210115_0058.wav. After _WSJT-X_ has processed the file you should see something similar to the following screen shot:

image::FST4-1.png[align="left"]
