FM receive limiter patch for Thetis 2.9.0.6 x64

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w-u-2-o
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FM receive limiter patch for Thetis 2.9.0.6 x64

Postby w-u-2-o » Sun Jan 29, 2023 9:58 pm

Our ever inventive Warren Pratt, NR0V, author of much of what we enjoy in all the openHPSDR, WDSP-based app's we use every day (Thetis, linHPSDR, piHPSDR, etc.) saw the discussions of FM receive performance and came up with an interesting solution.

The FM demodulator in the WDSP library allows for a great deal of gain and dynamic range. At the same time, the dynamic range of the audio system in Thetis (and Windows) is not nearly as large. The optimum amount of gain for a signal that generates full quieting in the FM demodulator might be far too much gain when the demodulator only sees noise, or some combination of signal and noise. This can make it difficult to adjust the receiver for best performance with a variety of weak, intermediate and strong FM signals. There is also the challenge associated with folks who don't drive their FM modulators hard enough and transmit strong signals but with weak deviation (no surprise, since there doesn't seem to be a single amateur radio with a deviation meter).

To that end, Warren devised a signal limiter on the output of the FM demodulator before it goes into the Thetis audio subsystem. Activating the limiter provides for an overall gain reduction into the audio chain to prevent the clipping possible at high RX1 (and RX2) AF gain levels. It also very effectively compresses the demodulated signals thereby improving their intelligibility. As with any FM signal, you do need at least 9 or 10dB of SNR to make this work, but once over 10dB it seems to work really well.

For example, I enabled the FM receive limiter (in Setup > DSP > FM) for the reception of a strong, full quieting, 10M FM signal. For the first 10 seconds the limiter gain was set to maximum, 30dB. You can see that the audio dynamic range is reduce to only 6 or 8dB. This is commensurate with many of the highly compressed SSB signals we are used to hearing. For the remaining 10 seconds, the limiter remained activated, but the limiter gain was reduced to minimum, 0dB. You can see that the audio dynamic range is almost 30dB under those conditions.

Capture.JPG
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Until such time as this new feature makes it into a full release of Thetis, Warren has been so kind to provide a patch. The patch Warren developed is targeted at Thetis version 2.9.0.6 and is 64 bit (x64) only. It installs by unzipping the patch folder (attached below) and pasting the contents of the unzipped patch folder over the top of the existing files in C:\Program Files\OpenHPSDR\Thetis.

It is strongly recommended that you make backup copies of the following two folders first:

C:\Program Files\OpenHPSDR\Thetis
C:\Users\<your windows username goes here>\AppData\Roaming\OpenHPSDR\Thetis-x64

If something goes wrong, just delete those two folders and replace them with your backup copies.

Patch zip file:
2023-01-29a Thetis FM 64-bit.zip
(11.3 MiB) Downloaded 181 times
N9HLL
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Re: FM receive limiter patch for Thetis 2.9.0.6 x64

Postby N9HLL » Mon Jan 30, 2023 1:55 am

Works wonderful ! I actually ventured up to 10 today and did some FM work... Thank you Warren !!

Rick (N9HLL)
Life is too short for QRP and less than 4kc
3630 eSSB
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Tony EI7BMB
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Re: FM receive limiter patch for Thetis 2.9.0.6 x64

Postby Tony EI7BMB » Mon Jan 30, 2023 12:00 pm

Thanks Warren
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W1AEX
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Re: FM receive limiter patch for Thetis 2.9.0.6 x64

Postby W1AEX » Mon Jan 30, 2023 4:54 pm

Wow, what an improvement! I just made a contact on 29.6 and the station was perfectly clear with a limiter gain setting of 2 notches (4dB). Playing around with the limiter gain it was perfect through quite a range and the minimal setting suggested by Scott was fine. This will be great fun to play around with the way 10 meters is behaving at this time. Thank you Warren!

73, Rob W1AEX
"One thing I am certain of is that there is too much certainty in the world."
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w-u-2-o
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Re: FM receive limiter patch for Thetis 2.9.0.6 x64

Postby w-u-2-o » Mon Jan 30, 2023 5:52 pm

Just to clarify: I wasn't recommending any particular level, only demonstrating the max. and min.

But to make a recommendation, I find it works best for me somewhere between about 10-15dB. This morning I made a couple of EU contacts on 10M FM and with the new limiter they were very clear and easy to copy.

I hadn't done any 10M FM work until just about a week ago. It's an interesting corner case of ham radio. I'm finding it can be difficult to know whether to be in 2.5KHz (narrowband) or 5KHz (wideband) deviation mode. The number of folks under-driving their modulators is legion. And the audio quality coming from a lot of low cost/converted 10M FM rigs is not impressive. On average, well adjusted SSB signals sound better! But, still, with the new solar cycle taking hold, it is quite amazing how far a 10M FM signal will go. Transcontinental and EU contacts are not rare at all. Sadly, there are no 10M repeaters in my neck of the woods to play with.
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Re: FM receive limiter patch for Thetis 2.9.0.6 x64

Postby SM3PZG » Tue Jan 31, 2023 4:38 pm

This is a great patch. You know what the best part is - the patch got rid of all SEQ errors!

After installing P2 firmware I got SEQ errors pretty regularly on TX both CW and SSB. Very annoying. Now I have been TX for about an hour on both SSB and CW and the SEQ error log is empty!

So this patch did wonders, great job!

73/Sam
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w9ac
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Re: FM receive limiter patch for Thetis 2.9.0.6 x64

Postby w9ac » Wed Feb 01, 2023 2:42 pm

Looks like Warren's patch may include an update to Envelope Elimination & Restoration (EER). If an update was included, are any folks in the community experimenting with EER-compliant amplifiers?

Paul, W9AC
Last edited by w9ac on Wed Feb 01, 2023 4:42 pm, edited 1 time in total.
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w-u-2-o
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Re: FM receive limiter patch for Thetis 2.9.0.6 x64

Postby w-u-2-o » Wed Feb 01, 2023 3:08 pm

Paul,

Please don't create that much thread drift. It would be most appreciated if you started a new topic in the amp section if you want to discuss EER.

Thanks,

Scott

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