FM receive limiter patch for Thetis 2.9.0.6 x64
Posted: 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.
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:
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.
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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: