PA8W Amateur Radio

Wil, PA8W,  E-mail: PA8W@upcmail.nl           
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Using Spectrogram software to judge (your) audio

I often use spectrogram software on my computer to study and judge the signals of other hams.
The program I use is SpectraLAB, which is one of the software packages I use in my professional life.

But you don't need dedicated ProAudio software to do the trick: there's also a lot of Ham freeware, developed to decode digital modes, that have a spectrogram function to show the digital signals in the received audio band.
Instead of checking out RTTY and CW signals, you could check out (your) SSB signals as well using the same spectrogram!
SpectrumLab is one of those freeware programs, which is quite useful to our SSB application.
  
It really helps me a lot to quickly analize what can be done to improve the audio.
However, the tone control settings of my IC746pro and the low end roll-off of this rig have to be taken into account.
So, for the best picture and honest judgement, I set my DSP MF filter as wide as possible for SSB (3,6kHz) and use neutral tone control on reception.
Then, I take audio out of the accessoire bus of the IC746pro and feed it directly into my computers sound card.
(That is, via a small audio transformer to prevent ground loops and RF coupling)  
With SpectraLAB I study the stations spectrum while I am listening to it simultaneously.
Again, the stations with best audio quality show a very smooth spectrum, with the transmitters energy uniformly and evenly distributed over the complete audio bandwidth, and no excessive, unnecessary low end!

The following examples will illustrate that. 
Note that the yellow spots  in the spectrum are speech energy, with the red spots as the louder peaks in the signal.
Blue areas are zero energy, green is around background noise level.
(note that a lot of yellow may be QRM as well!)

To some examples, I added the original soundtrack, so you can compare sound with graph!


 Let's start with a common one, including soundtrack:



The red dots of a few seconds of speech are concentrated in the range 250-600Hz. This signal lacks the high end.
No surprise that this signal sound muffled, and difficult to copy. Totally useless in poor conditions.
This is really very common on the Ham bands!

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Ok, I will show you a different extreme, with soundtrack:



This signal is clearly far too high pitched. It sounds annoying, strident to the ears, and way more difficult to copy than a well balanced signal.

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Now look at something completely different, with soundtrack:



This signal is again very much concentrated in the low mid area, with almost no energy above 600Hz...

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The next spectrogram is a good example of too much low, with
soundtrack:  


 
The top half of the spectrogram shows the excessive low, concentrated around 100Hz, with a wide and deep notch in the much more important mid section between 300Hz and 2kHz.
Luckily he does have some excited hi-mid around 2,6kHz, which provides his readability.
The lower half of the spectrogram represents the other, much weaker station, using more efficient audio.

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The following spectrogram shows an extreme example: with soundtrack:
You will need a subwoofer to really hear where the power is concentrated...
But also without subwoofer, you can hear that the excessive low end suppresses the received noise by activating the AGC.
Due to that, on a communications loudspeaker the audio seems to be much less loud than any normal signal.
A bit comparable with some strong station tuning at zero beat, pulling down the audio of the wanted station.



The energy is concentrated mainly between 50Hz (!) and 120Hz, with deep notches at 400Hz and 700Hz.
I am pretty sure that this guy has some graphic EQ running with extreme settings: you can recognise the EQ bands as narrow vertical colomns in this spectrogram.
I would be willing to bet that his 400Hz and 700Hz sliders are pulled down fully for maximum suppression.
This is the most dramatic example that I ever recorded.
At least 95% of his power is wasted on low frequencies...


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Now check out my own spectrogram:



As the spectrogram shows, the energy is distributed very evenly over the complete bandwith, only below 250Hz the the energy is reduced, and almost completely zero below 120Hz.
This spectrogram is made AFTER all audio tweaking was finished. I used other methods, measurements and tests to judge and tweak my audio.
Only later-on this spectrogram confirmed that there's hardly any room left for improvement...

Did I prove my point?


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