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61 lines
3.3 KiB
Plaintext
61 lines
3.3 KiB
Plaintext
Do not confuse FST4 with FT4, which has a very different purpose!
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FST4 is designed primarily for making weak-signal 2-way QSOs on the
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LF and MF bands. T/R periods from 15 s up to 1800 s are
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available. Longer T/R periods provide better sensitivity only if
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Tx and Rx frequency instability and channel Doppler spread
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are small enough so that received signals
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remain phase coherent over periods spanning several transmitted symbols.
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Generally speaking, Rx and Tx frequency changes
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during the transmission and channel Doppler spread should each be small compared
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to the symbol keying rate shown for each T/R duration in Table 7 within section
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<<PROTOCOL_OVERVIEW,Protocol Specifications>>. For example, the keying rate for
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the 1800 s T/R period is 0.089 Baud, so
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successful operation using this T/R length requires Tx and Rx frequency
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stability better than 0.089 Hz over the duration of the 1800 s transmission in
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addition to channel Doppler spread smaller than 0.089 Hz.
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Operation with FST4 is similar to that with other _WSJT-X_ modes: most
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on-screen controls, auto-sequencing, and other features behave in
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familiar ways. However, operating conventions on the 2200 and 630 m
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bands have made some additional user controls desirable. Spin boxes
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labeled *F Low* and *F High* set lower and upper frequency limits used
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by the FST4 decoder, and these limits are marked by dark green
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angle-bracket symbols *< >* on the Wide Graph frequency scale:
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image::FST4_Decoding_Limits.png[align="center"]
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{empty} +
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image::FST4_center.png[align="center"]
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It's best to keep the decoding range fairly small, since QRM and
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transmissions in other modes or sequence lengths will slow down the
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decoding process (and of course will be undecodable). By checking
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*Single decode* on the *File | Settings | General* tab, you can
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further limit the decoding range to the setting of *F Tol* on
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either side of *Rx Freq*.
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A noise blanker can be enabled by setting the *NB* percentage to a non-zero value.
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This setting determines how many of the largest-amplitude samples will be
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blanked (zeroed) before the data is submitted to the decoder. Most users find
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that settings between 0% (no blanking) and 10% work best. If the noise
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blanker percentage is set to -1%, then the decoder will try 0, 5, 10, 15, and 20 %
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in succession. Similarly, a setting of -2% causes the decoder to loop over
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blanking percentages 0, 2, 4, ... 20 %. To save time, the multiple blanking
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percentages triggered by negative *NB* settings are tried only for signal
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candidates located near (within +/- 20 Hz) of the *Rx* frequency setting.
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.Open a sample Wave File:
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- Select *FST4* on the *Mode* menu. Set *T/R* to 60 s and *Decode | Deep*.
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- Set *NB* (noise blanker) to 0%.
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- Set up the Wide Graph display with settings appropriate for the FST4-60 mode.
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For example, try *Bins/Pixel* 2 and *N Avg* 4. Set the *Start* frequency and the width of
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the Wide Graph to include the frequency range that you want to decode. For this
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example, make sure that *Start* is less than 1000 Hz and that the Wide Graph extends to above 1400 Hz.
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- Set *F Low* 1000, *F High* 1400. These settings define the decoder's frequency search range.
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- Open a sample Wave file using *File | Open* and select the file
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...\save\samples\FST4+FST4W\210115_0058.wav. After _WSJT-X_ has processed the file you should see something similar to the following screen shot:
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image::FST4-1.png[align="left"]
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