Neither the noise mitigation part nor the firmware update completely eliminate all background. That is why it is called "mitigation part" and not "elimination part." The noise is generally mitigated to the point where it is virtually inaudible when masked by actual radio traffic. You will not find a unit that has complete elimination.
Thank you so much for this.
What I, and probably others, found worrisome were the reports of "zero" hum while we can still hear it on our scanners. I managed to mitigate the noise quite a bit on mine, probably the maximum possible, and put it in a noisier room to help mask it, but it's not "zero."
Personally, I feel a lot better knowing that we now know we "will not find a unit that has complete elimination" because it puts the question to rest, finally, on whether our units are still defective or not.
I've heard a few of these units now and have been able to hear the "hum" on every single one. I've just about managed to get to a place where I think of it like the high-pitched whine on an old CRT and know if I can hear it, the unit is powered on and working.
I'm now back in the market to probably pick up one of these "newest SDS200s" as a second unit when I'm able, especially since the hum issue seems to be addressed as much as possible on the latest batches. Thanks for the reports and clarification.