PA8W Amateur Radio

Wil, PA8W,  E-mail: PA8W@upcmail.nl           
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SSB Audio dynamic- and spectral analysis using a web-SDR



An efficient way to check your audio is using an WEBSDR (see http://www.websdr.org/ )
A Web SDR is a Software Defined Radio which is connected to the internet so that visitors can listen to Ham bands.
The Web SDR of the Technical University Twente in The Netherlands shows lots of Ham bands simultaneously!

As a visitor, you can  put the cursor on the frequency you want to monitor.
You can also listen to your own transmissions  audio, or maybe record it for later analysis.

And, -very usefull- the Web SDR shows a spectrogram of the entire (or a part of- ) the Ham band.

A spectrogram is a graph which shows the energy density on the band, scrolling a few new lines of data into the graph every second.
In this way, you can observe the activity in the band over some seconds of time.
The upper part of the graph is the "oldest" part.

The frequency "footprint" of Ham transmissions can be observed in the spectrogram.
This way, it is quite easy to identify the type of transmissions, and, of great importance to us SSB-users; 
it shows the energy distribution in our own SSB signal!

Now look at the following graph:




The above example shows a very good, balanced frequency response on 3620 kHz. The total bandwith of that signal is filled with about the same signal intensity. It will be no surprise that this signal was sounding excellent, and very easy to copy.

The signal at 7067 kHz in the lower graph has way too much low end. (visible as a concentrated vertical line in the right side of its spectrum, since it is LSB)
Above that, -more left in the its spectrum- it lacks midrange, and only the high end seems to be ok. 
This means way too much energy wasted in the base frequency of the voice, which contains no intelligibility at all!
So, if the low end would be reduced severely, the same station would sound a lot louder and easier to copy.

The nice thing of a WEBSDR is that it is a wide bandwidth application that gives a very honest judgement of your spectrum.
A drawback is that it needs quite a big signal to produce a clean visual presentation. 
(the 3620 kHz signal on the picture was at least +20dB on my own receiver)

Remember that any audio frequency that's considerably louder than the rest of your audio frequency spectrum will activitate your transmitters limiter and compressor prematurely, pulling down all the rest with it!


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