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Tuesday, September 29th, 2015
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Auto-bandwidth processing for SSB communications

This image shows off a new and, I hope, interesting, feature of my free SdrDx software.

The scope capture shows the s-meter level (bottom white trace), the AGC, (blue trace) and audio (top white trace.)

At the bottom, you can see two dashed green lines. These are 3 dB apart, or 1/2 an S-unit.

When the signal strength is below the bottom dashed line, audio output is muted, just like a signal-strength sensitive squelch.

As the signal strengthens past the lower line, it un-mutes and begins being received with very narrow RF bandwidth. There's an "aggression" setting that controls how narrow it goes.

By the time it is as strong as the upper line (1/2 S-unit more as shown here), the bandwidth is fully open (meaning, to the bandwidth you set it to most recently.) So as shown here, if you're set for 3 khz wide, and aggression is high, it'll unmute starting at about 1500 Hz wide, and the smoothly widen to 3 kHz wide after a 3dB gain in signal strength.

The net result is that very weak signals are automatically received with a narrow bandwidth, increasing the signal to noise ratio at the expense of higher voice frequencies. Once a signal exceeds the top dashed line, it is treated normally (again, that means, at the normal reception bandwidth you have chosen.)

You can set the delta between the two lines to anything from 1 dB to 60 dB in 1 db steps, and you can adjust the trigger level for the bottom line (and hence, the level of the top line as well) anywhere you like, and as mentioned, you can adjust the aggression. It's very flexible, and when set up correctly, does wonders for weak signal intelligibility.

In addition, there's an auto-set that allows you to set the bottom line 1 dB under, or 1 dB over, the lowest signal level presently visible on the scope, which saves a lot of manual fooling around with the level.

I'm pretty pleased with the feature; when I thought it up, I wasn't sure just how useful it would actually be, but as it turns out, I use it all the time. It's much, much easier on the ear than either squelch or constantly listening to noise.

#swl #sdrdx

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