ALC setting of FT8 ― 2020年06月24日 10時10分33秒
It is often said that ALC should be used with caution when operating FT8 and JT65. Is it really necessary?
Do you care about ALC when operating CW? No. Because CW is a single tone. Both FT8 and JT65 are single tones when viewed in unit time. The monotone mode such as JT65 and FT8 is the same single tone as CW if attention is paid to the short time, and it is devised by software so that the joint is smooth even during frequency shift.
Therefore, it goes without saying that ALC swing does not matter, but there should be no problem even if the high-frequency amplification stage is a class C amplification.
The erroneous recognition that "ALC should not be swinged" was widespread. Because the SSB transmitter was used to generate these digital mode radio waves of PSK31, SSTV, etc. And also you may be using the same digital mode interface as the "ALC should not be swinged" digital mode such as PSK31, SSTV.
Another reason may be that columns such as prestigious magazines made a false statement in JT65 and FT8 that "ALC should not be swinged."
The following is a comment from the WSJT-X development group.
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The WSJT-X Tx audio synthesizer does not change frequency at zero-crossings, it happens at exactly the time the protocol expects the shift. The phase is continuous as far as it possible but you are correct in that the shift is between two samples so there will be a small wideband component at switching of symbols. Since there is only one tone at any time there will be no IMD components.
The start of Tx is at full amplitude but the samples are synthesized starting from the zero-crossing. The tail end of audio is ramped down to zero amplitude over a few milliseconds.
73
Bill
G4WJS.
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I think it is important not to distort the AF signal output from the computer.
In the description of AFSK operation of IC-7300, "Do not swing ALC" is not written.
I will introduce a page that will be helpful.
de JP1LRT Yoshi
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