Feature interaction between monitor and AM

AB2EZ
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Feature interaction between monitor and AM

Postby AB2EZ » Tue Jan 30, 2018 10:16 pm

Normally I use an RF sampler + peak detector to monitor the modulation of my AM output signal. For SSB I use the HPSDR "monitor" functionality.

Today, for the first time, I used the HPSDR monitor function to listen to the modulation of my AM output signal.

I noticed the following:

After about 15 seconds of transmitting, the volume of the monitor audio output drops dramatically (more than 20dB)... but the transmitted signal carrier level and modulation level are still the same. A few seconds later, the carrier power output level jumps up from 25% to over 50%... the volume of the monitor audio output jumps up to its original level, and the monitor audio is (as expected with the increased carrier level) distorted.

If I switch off the transmitter, turn off the monitor function, and turn on the external off-air AM monitor... and turn the transmitter back on... then everything works fine.

If I use the HPSDR monitor on SSB, then everything works fine.

The configuration is:

ANAN10E + Elecraft 100W amplifier.
Pure signal feedback is applied from a sample of the amplifier output to the ANAN10E RX1 input.

Stu
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w-u-2-o
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Re: Feature interaction between monitor and AM

Postby w-u-2-o » Wed Jan 31, 2018 2:49 am

Stu,

Have you modified your 10E for proper PureSignal operation?

https://apache-labs.com/community/viewtopic.php?f=20&t=2660

If not, then that may be the root of the problem.

73,

Scott
AB2EZ
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Re: Feature interaction between monitor and AM

Postby AB2EZ » Wed Jan 31, 2018 4:29 am

Scott

My 10E has been modified properly for Pure Signal operation with my external linear amplifier.

The feedback level is correct

Stu
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w-u-2-o
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Re: Feature interaction between monitor and AM

Postby w-u-2-o » Wed Jan 31, 2018 11:11 am

Obviously something is broken, either hardware or software or firmware, because that is not normal. Have you tried the usual dance of reloading firmware and resetting your database (export it first, of course, in case this is not the problem and you want to re-import it)?
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W1AEX
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Re: Feature interaction between monitor and AM

Postby W1AEX » Wed Jan 31, 2018 5:21 pm

Hi Stu,

I'm not sure if what I see while running AM is the same as what you are reporting, but I stopped running Pure Signal on AM because the MON function only returned audio in one side of the headphones, and it was not particularly useful because it was "predistorted" audio. At times, when Pure Signal was "adapting" and "resampling" my signal, the audio would come through both sides of my headphones and then would randomly go back to only one side. I also noted that the transmitted carrier increases by 15 to 20 percent when Pure Signal is active but once it establishes the correct feedback level it does not change further. As I recall, Warren explained to me that Pure Signal looks at asymmetry as distortion and uses the carrier level as one factor it adjusts to help establish symmetry. I might be explaining that incorrectly but I think that Warren was not surprised by the change in the carrier level when Pure Signal was engaged.

Note that the actual transmitted signal as received by others or with a second local receiver on AM with Pure Signal active is flawless and very symmetrical on my bench scope but the monitor behavior is not very pleasant. So... given that the AM signal without Pure Signal is extremely clean, with excellent bandwidth control (no splatter), I don't run predistortion with that mode to allow real-time monitoring with the MON function.

73,

Rob W1AEX
"One thing I am certain of is that there is too much certainty in the world."
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Re: Feature interaction between monitor and AM

Postby w-u-2-o » Wed Jan 31, 2018 11:11 pm

Fwiw: I only use MON to check that VAC is running ok, and I use PureSignal on all my AM contacts. I get brick wall IMD and very complimentary signal reports.
AB2EZ
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Re: Feature interaction between monitor and AM

Postby AB2EZ » Thu Feb 01, 2018 12:29 am

Rob
Scott

I agree with Rob that the interaction I described in my original post involves the combination of PureSignal, MON, and AM operation.

Additional observations

1. The sudden drops and sudden restorations of the monitor volume occur at instants in which PureSignal is undergoing a re-calibration (i.e. the PureSignal feedback indicator lights up green)

2. On those (less frequent) occasions when the carrier level power jumps up... PureSignal becomes temporarily disengaged (no correction). This occurs at the instant of a re-calibration. The carrier level jumps up to the same level it has when I manually turn PureSignal off. The IMD (as seen in the Panadapter display) during these events is the same as the IMD observed when PureSignal is manually turned off.

As per my original post... these aberrant effects do not occur when MON is not on (I can monitor the modulation with my external off air monitor... and I don't hear any changes in the audio level at the output of that external monitor... nor do I observe any Pure Signal drop-outs).

As a further "piece of the puzzle"... both aberrant effects (MON volume output changes and dropouts of PureSignal correction) occur when the microphone input level is adjusted to just produce 100% positive and negative AM modulation peaks. [Note that I am using an external Orban AM processor that keeps the microphone audio input waveform symmetric... with tightly controlled positive and negative peaks]. As a result, the PureSignal re-calibration occurs every 5-10 seconds. However, if I turn up the microphone gain by 2dB (which results in some negative peak clipping and the associated monitor audio output distortion), only one of the aberrant effects (MON volume output changes) occurs. In that case, the PureSignal re-calibration occurs about once per second, and the PureSignal correction does not drop out.

No problems when using MON and PureSignal on SSB.

Stu
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Re: Feature interaction between monitor and AM

Postby Warren_NR0V » Thu Feb 01, 2018 4:16 pm

Greetings All,

There are a few items on which I'd like to comment:

1. Carrier level with PureSignal ON versus PureSignal OFF. As you know, PureSignal causes your analog amplifier chain to respond linearly, i.e., when drive is 25% of max, output is 25% of max, and when drive is 100% of max, output is 100% of max. Now, imagine the situation where you are operating AM and the peaks of your signal are at 100% of max. If you are set for 100% modulation, and if your amplifier chain is perfectly linear, your carrier output will be 25% of that peak/max. HOWEVER, if your amplifier chain is NOT perfectly linear (PureSignal is OFF), your carrier output may be at, e.g., 20% of max or at 30% of max, just depending upon what the gain curve of your amplifier chain actually looks like. Since PureSignal causes the chain to respond linearly, if turned ON, it will make adjustments such that the carrier output is at 25%, the correct value for 100% modulation. So, the carrier level is correct with PureSignal ON and you may see variations in carrier level between conditions where PureSignal is operating and it is not operating.

2. Using the Monitor function with AM and PureSignal. First of all, the MON function in PowerSDR is somewhat lacking ... and no one, including me, has yet at least made it a top priority to change that. The current MON function just pipes the baseband I and Q output of the transmitter to the audio output - there are no 'demodulators' for the various modes. So, what do we get? Well, monitoring FM doesn't work at all since the I-Q signal does not contain amplitude/audio modulation that we can listen to; LSB delivers an audio waveform to the L output and a 90 degree phase shift of all frequency components of that to the R output; USB is like LSB except the phase shift is in the opposite direction; and AM, well, that depends. The required output for an AM signal is ambiguous -- the phase of signals sent to I and Q is the same; however, the magnitude can be equal in I and Q, it can be all in I with zero in Q, it can be all in Q with zero in I, or, it can be anywhere in between. So, if I is piped to the L audio output and Q is piped to the R audio output, the arbitrary balance of I and Q determines what you hear "in each ear". What do you hear? With PureSignal OFF, you are hearing whatever balance of I and Q that the AM Modulator code provides. In this case, I wrote the modulator to provide equal outputs for I and Q. However, if PureSignal is ON, it may change that balance each time it does a calibration -- there is nothing to constrain it from doing so; in fact, it has no idea that the modulation mode is even AM -- it is merely linearizing the output. The result is that your modulated RF signal can be perfect even though you are hearing amplitudes change radically in the L and R outputs of the monitor.

3. PureSignal and Monitoring. Rob, w1AEX, mentioned the fact that the Monitor signal is predistorted. Remember, MON is the output of the transmitter and, when PureSignal is ON, the transmitter software predistorts the I-Q transmitter outputs. So, Rob's assertion is totally correct and this means that you may hear some slight distortion in the MON signal even when your actual RF transmitted signal has been linearized and sounds much better!

73,
Warren NR0V
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Re: Feature interaction between monitor and AM

Postby w-u-2-o » Thu Feb 01, 2018 4:34 pm

Great explanation, Warren, thanks! I definitely learned a few things. I knew MON had issues, but I had no idea it was actually that primitive.

73!

Scott
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Re: Feature interaction between monitor and AM

Postby AB2EZ » Thu Feb 01, 2018 5:43 pm

Warren

Thanks for the detailed explanation of why the level of the monitor audio output can change each time PureSignal recalibrates.

Do you have any idea (perhaps speculative) as to why PureSignal would occasionally stop applying a correction... immediately after the green calibration light flashes... when the audio being delivered to the microphone input by the external audio chain is tightly constrained to produce no more than 100% positive and negative modulation peaks (I.e. the green PS calibration light flashes only once every 5-10 seconds).

Stu
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Re: Feature interaction between monitor and AM

Postby w-u-2-o » Thu Feb 01, 2018 7:11 pm

Stu,

I'm sure you know this, but you've got to be reliably getting the ALC to 0dB. PS will only kick off a measurement when it has data from a 0dB hit.

Having come over from a legacy Flex radio where the ALC in the legacy PowerSDR was essentially a brick wall clipper, and everything would sound like sh--er--junk when it was exceeded. I tightly constrained my output with a DAW, i.e. a similar approach to you using an Orban, and both surely implemented a very high quality, look ahead limiter (at least my DAW implementation did).

When I first started using the openHPSDR version I was terrifically paranoid about exceeding 0dB ALC, and was using the DAW to keep things at -0.5dB. PureSignal would not run at all. So I opened things up a bit and let the ALC float up to about +1dB (above 0dB you have to start looking at the ALC Comp meter, yes it's a pain, no I don't know why it's like this). And, guess what? No nasty artifacts on my transmitted audio.

Warren's ALC uses a very high quality look ahead algorithm. Now I normally let it float up to around +3dB, and, having abandoned the DAW, actually use the ALC as a final compression stage, along with CFC and the leveler. You won't really notice any harshness until you start exceeding about +4dB.

I'm not sure where you are running things, but I'd recommend driving into the ALC a little harder so that you see +1 or +2 peaks on the ALC Comp meter. This should get you more frequent and reliable PS corrections.

If you can't bring yourself to do that, then consider locking in a correction with single calibrate mode and just letting that ride. A lot of people prefer that. I typically do this for digital mode operations but not AM. You can catch a voice peak with single cal, although it may take several tries before you are happy, or you can use two-tone and really nail it.

73,

Scott
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Re: Feature interaction between monitor and AM

Postby AB2EZ » Fri Feb 02, 2018 6:28 pm

Scott

Good suggestions!

I had never used the ALC Comp meter reading... but I have now switched to that... and turned up the microphone gain to obtain about 1dB on peaks.

I was curious about the changing output level behavior of the MON functionality (as explained by Warren)... and I am still curious about the underlying cause of the dropouts of Pure Signal linearization on AM when the MON function is active.

However, when operating AM, I will use my external AM off-air monitor (which has excellent tracking between the amplifier's output RF envelope, and its own audio output waveform). This provides me with higher fidelity AM monitoring (vs the use of MON)... and it avoids the problems I noted in my first post in this thread.



Stu

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