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Author Topic: 3D color Spectrum v. time (spectrogram) using Spectrum Lab  (Read 6547 times)
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AB2EZ
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"Season's Greetings" looks okay to me...


« on: January 11, 2011, 10:17:48 AM »

I know that some of you are familiar with the Spectrum Lab application that you can download for free at: http://www.qsl.net/dl4yhf/spectra1.html

Attached is a screen capture from Spectrum Lab ... in use to produce a 3D "spectrogram" (spectrum v. time) of the audio output of one of my receivers*.

*I was using my SDR-1000 set to synchronous AM mode,  with 8kHz of bandwidth (+/-4kHz), and with the received signal centered in the passband; but, of course, it doesn't matter what receiver, or other audio source I use to produce the audio signal whose spectrum is displayed.

The 2D (power spectral density v. audio frequency) spectrum is painted every 300ms... and the total time shown is 140 seconds... where the current time is 0 seconds.

The amplitude is shown in colors.. where I have adjusted the coloring so that the background audio noise level is blue. You can see in the box at  the lower left how the color (as currently adjusted) varies with the relative power spectral density in dB.

As you can see, the +/-4kHz brick wall i.f. filtering of the SDR-1000 is limiting the audio spectrum of the receiver's output to 4kHz. You can also see that the vocal tract of the person whose signal I was listening to has some natural frequencies where the power spectrum tends to peak up.



 


* capt00.jpg (103.92 KB, 1089x536 - viewed 737 times.)
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Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
Steve - K4HX
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« Reply #1 on: January 11, 2011, 01:18:13 PM »

Quote
You can also see that the vocal tract of the person whose signal I was listening to has some natural frequencies where the power spectrum tends to peak up.

These are called formants. All you need to recover intelligible voice is the fundamental and one or two formants. The SNR implications are obvious.
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Rob K2CU
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« Reply #2 on: January 11, 2011, 01:36:43 PM »

Spectrum Lab is one of several powerful audio analysis tools that are free for the taking. It is most powerful at being a "workbench" in which there are filters, mixers and other signal processing functions to experiment with stereo audio input signals. you can even do I/Q demodulation if you like or just plaiin filtering with the speaker output giving you the result.

I found it very useful for checking the warmup and running drift of receivers. Just tune to 14.999MHZ USB and monitor the 1KHZ tone from WWV. Let the receiver warm up for about a half hour or so. then tune to 14.999MHz and  verify theta the audio output tone if 1KHz on the waterfall display. Set the update rate real slow so that you can see what takes place over the course of a half hour or more. Then, turn off the receiver and let cool over night for example. Next day, turn on the receiver and do not change settings of tuning dial. Now watch as the receiver warms up and the tone drifts towards 1KHz. You can see how long it takes and how it wanders. I used this technique to determine that the bulk of the 1500 Hz warmup drift of my TR-7 was due to the dial pilot light. After changing to LEDs, what a difference!

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w3jn
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« Reply #3 on: January 11, 2011, 02:29:36 PM »

I used a HP VSA that had that 3-d waterfall display and I found it very irritating.  The color spectrogram, with knob-variable ranging, was by far the most useful for digging weak signals out of the noise.  Also useful was its ability to display 4 screens simultaneously, with RF, demodulated baseband in f-domain, and time domain in spectrogram or normal displays.  And even better was its ability to capture a signal to memory, analyze it, then play it back thru the arbitrary source, if needed.  This is almost 20-year old technology by now...
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FCC:  "The record is devoid of a demonstrated nexus between Morse code proficiency and on-the-air conduct."
Steve - K4HX
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« Reply #4 on: January 11, 2011, 04:33:31 PM »

Indeed. I've always considered the 3D waterall to be of more use to the sales guys than the end-user. A colored spectrogram displays the same info in a much more useful manner to my eyes.

But that's the beauty of this technology, you can display the data a bunch of different ways, and make a wider range of user happy. The more recent VSAs also allow you to manipulate the color scale in a non-linear manner. A common friend of ours even figured out how to do a rasterized display.



I used a HP VSA that had that 3-d waterfall display and I found it very irritating.  The color spectrogram, with knob-variable ranging, was by far the most useful for digging weak signals out of the noise.  Also useful was its ability to display 4 screens simultaneously, with RF, demodulated baseband in f-domain, and time domain in spectrogram or normal displays.  And even better was its ability to capture a signal to memory, analyze it, then play it back thru the arbitrary source, if needed.  This is almost 20-year old technology by now...
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KF1Z
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Are FETs supposed to glow like that?


« Reply #5 on: January 11, 2011, 05:16:14 PM »

A 3D waterfall is "built-in" on one of the versions of QS1R software I have.

That version has an "actual" oscilloscope display as well, works pretty good for looking at AM sigs...
Howerver they both needed "tweaks" to be completely usefull.
The Lady who was writing that piece of software left the group after being bugged and complained at for features and fixes.
Too bad, because of course she could have kept it all to herself, and not given it out for free to whoever wanted it...
Which is what she is doing now...

A few complainers ruined it for the rest of us   Cry
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Steve - K4HX
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« Reply #6 on: January 11, 2011, 05:21:39 PM »

Just like back in grade school - one kid hits the teacher with a spit ball and you all have to stay in at recess.  Sad
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K1JJ
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"Let's go kayaking, Tommy!" - Yaz


« Reply #7 on: January 11, 2011, 05:48:38 PM »

Pretty cool picture, Stu.

I like how the audio freqs are labeled.

Looks like a bird's eye view of NYC.. :-)

The guy seems to have a smiley curve to his audio where the mids are sucked out.  I'll have to play around with it sometime.  Right now I find the most valuable picture is the pan-waterfall followerd by a standard pan-spectrum.  The waterfall is great for actual on-air operation where you can forget about the screen, then look back to see who you splattered.. Grin

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
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