Re: Serious question
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You are right--RST/RSQ are subjective, and S/N and IMD are measured. But the problem is that it's not always clear what is being measured. Both are based on power measurements, but the software can't really tell the difference between true noise power and another signal--they are just measuring noise within a particular bandwidth, and only of the signal as presented as audio to the software. Here are some places where that could be a problem.
- If there is another signal right next to the signal you are copying (like 50 Hz away), your software will measure its power and attribute that to IMD.
- We usually think of S/N in terms of background noise (true QRN), but in a very crowded band the software will measure the composite signal power of all the other signals within its measurement bandwidth.
- Because the software only looks at the audio signal (after the "downconversion" performed by your SSB demod), it can't tell the difference between distortions in the other station's transmitter and ones created in your receiver.
Now in a very quiet band (low noise and few other signals), the software has an easy job, and the numbers are meaningful. The pictures you posted are examples of that. But some folks use macros that automatically extract the S/N and IMD and send it to everyone all the time. Since I can't tell what that station's waterfall looks like, I typically ignore those numbers because I don't have confidence that they are being measured accurately or appropriately.
The bottom line is that sometimes S/N and IMD are significant, but IMO it's usually too hard to figure out whether it's one of those times (unless I know the other station well).
From: John - KC3FL via groups.io <kc3fl@...>
Sent: Mon, Dec 28, 2020 10:33 am
Subject: Re: [070Club] Serious question
One of my favorite subjects. An RSQ is an arbitrary number that the op makes up. Some give a meaningful RSQ but most give the 599. S/N and IMD are real numbers that your software generates and tells all about the signal. The IMD without the S/N is useless because if the S/N is 15 dB and the IMD is -15 dB, that is a great signal, however if the S/N were 30 dB and the IMD was -15 dB, that is being over driven and probably is 2 or 3 hundred Hz wide. The closer the IMD is to the negative value of the S/N, the cleaner the signal because all the harmonics are suppressed. Unfortunately the software has a tough time getting the IMD right as you can tell as the IMD figure usually jumps around.