Shane and Nigel,

All the SDR platforms do a pretty spectacular job with AM, but I have not used anything that comes close to the algorithms that Warren NR0V has come up with. His implementation of soft limiting to prevent clipping at the DAC and to restrain the negative peaks with AM works really well.

Another way of seeing it is to watch the scope as an SDR running AM is fully modulated with a 1000 cps tone while pre-distortion is toggled on and off, and then, as the AM Carrier Level is run back and forth through values less than 100 and greater than 100. Keep an eye on the positive peaks, which have been driven to the maximum. They don't flinch at all.

https://www.youtube.com/watch?v=069_uUSz3X4

You'll never catch me running AM with an AM Carrier Level less than 100 with OpenHPSDR!

73,

Rob W1AEX

June 28, 2017, at 2PM EDT, big opening from entire Eastern U.S.(and some points West) to Europe and Africa.

I can tell you that in the Baker Oil Fields (Bakersfield) most all SCADA is done via 400 mhz and 900 mhz band.  Point to point, I think the last ones I worked in had 13 element yagi antennas.

I'm sure, at some point, some idiot will think it's a good idea to make them hackable and accessible.  But today, in Chevrons fields, they are point to multipoint.

--Shane
KD6VXI

Somewarr there is a pic of one operating mobile...

KLC

Thanks for posting that Rob, I was trying to find a way so say it politely but you nailed it.  No point in audio negative peak limiting ahead of an SDR, at least an ANAN SDR, because it does it for you.  The carrier ratio control will give whatever positive peak to carrier ratio desired right down to double sideband with no carrier.

73s  Nigel

Rick,

Thanks for the explanation.  That makes total sense.

--Shane
KD6VXI

If it was readable on your device without WiFi authentication, what's to prevent you from passing the saved document to non-members?
[/quote]
The same thing that prevented people from passing along their hard copies to their friends or local libraries back in the pre-Cambrian era.

I've gone digital with several other publications solely for the purpose of being able to archive them digitally in pdf form and not have to build an addition on my house to keep hard copies. Those other publications make it real easy, just DL and save. Digital formats that require anything more than a download into a universal format do me no favors. Anyone remember some years back when the ARRL offered CDs with their back issues? They had some goofy software that eventually became obsolete and I was stuck with two decades of QST mags that I had to manually compile into pdfs so that I could continue to use them. What a PITA. And to boot the quality of the scans was awful. Apart from that, I was completely satisfied.

I think I see a little space available for a generator to make it a functioning mobile unit. 

Jon

Hi Shane,

Actually, predistortion doesn't kill asymmetry as it's implemented in any of the SDR platforms. The reality is that with SDR you are not modulating a carrier in the traditional sense as you do with plate modulation. Instead, the software mathematically generates a DSB signal and the carrier is added in at the end. Some software allows the user to adjust the AM Carrier Level of their signal and when the SDR users reduce the AM Carrier Level to make a signal with "pseudo" asymmetry, they are not increasing the peak power of the signal, all they are doing is changing the ratio of power between the carrier and the DSB signal. There is no increase in the peak power of the DSB signal when that adjustment is made, only the carrier power is reduced and so there is no increase in loudness. It just looks nicely asymmetrical on the scope!

What adaptive predistortion does after it analyzes the RF sample from the back-end of the transmit chain, is remove the illusion of asymmetry, because it looks like distortion to the algorithms involved. However, instead of limiting the positive peaks, APD raises the carrier level, and in the case of OpenHPSDR, you end up with 100% positive peaks (assuming you are driving the audio to that point) and 99% negative peaks (you cannot reach 100% negative with an AM Carrier Level of 100 in OpenHPSDR). There is no reduction in PEP and the positive peaks can be driven right up to just before the clipping point of the DAC, which is the absolute limit and is also the point where the ALC look-ahead algorithm will assert itself and soft limit them. The algorithms then predistort the signal to remove out-of-band IMD products, and as a bonus, they also clean up in-band distortion products, which improves the signal's THD from a nominal 2% or 3% depending on the audio source, to a very clean 0.2%.

If you listen to a station that toggles Pure Signal on and off while running AM you will not hear any difference in the loudness of their signal because the peak power output is unchanged. If you record them and then look at the amplitude of the signal's waveform, you will not see a significant difference in the level of the audio content. I have found when measuring this with my own signal that there is actually a very slight reduction of roughly 0.1dB with predistortion engaged while running AM. I suspect that this is probably due to the reduction in THD but I have no proof of that.

At any rate, it's easy enough to see the effect that predistortion has on "pseudo-asymmetry" when running AM and shortly after I acquired my ANAN-200D I made a screen capture recording with my SDRPlay to get a look at it. I posted the result on YouTube yesterday. The level of the VU meter that is displaying the amplitude of the recovered audio is pretty consistent as predistortion is toggled on and off. Unfortunately, due to the latency of the USB connection used by the SDRPlay the synch between the VU meter and the panadapter is slightly off, but you can still get the idea. Also, keep an eye on the out-of-band IMD products when predistortion is running.

https://www.youtube.com/watch?v=0_92HNhpfnU

To my eyes, as previously mentioned above, the waveform of the audio track from the video shows a very slight reduction of recovered audio with predistortion engaged in some parts of the video. A screenshot of that is shown below.

As you and Rick both mentioned, the magic is in the math! At any rate, the only way to increase loudness on AM with an SDR is to build up the average audio content with density. The software's internal 10-band continuous frequency compressor does that quite nicely!

73,

Rob W1AEX

Good job!