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Author Topic: Positive Peaks / Asymmetrical Modulation  (Read 12823 times)
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KD6VXI
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« on: July 04, 2015, 11:46:04 PM »

Working on my modulator and processor this weekend.  Think I have the headroom and processor power I want.

420 percent positive peaks while at 99 percent negative work?

https://youtu.be/cajJYlqSv6Q

My processor will allow for bringing the positive peaks to whatever level you want, up to a maximum of approximately 5 times enhancement (ie, go from total symmetric sinewave at 100 percent to a 500 percent positive).

When I get the RF out of the SDR (getting nasty hum from stray rf), I'll grab some sdr recordings.

--Shane
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« Reply #1 on: July 04, 2015, 11:55:29 PM »

Could you post the processor and modulator schematic please? It would be very beneficial to learn about this!
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« Reply #2 on: July 05, 2015, 07:26:55 AM »

Sounds interesting.  Of course anything over the natural asymmetry of the human voice is distortion, so be aware of that.

Also of course (this has been discussed time and time again here on this forum!), standard detectors (even low distortion varieties) will not be able to demodulate artificially supermodulated signals.  You'd need a sync detector, and most folks aren't running them.

But, it is interesting that the processor can generate that level of asymmetry.  I would be very interested in hearing the results as well.  Demodulated using a standard detector as well as a sync detector.

Regards,  Steve
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« Reply #3 on: July 05, 2015, 10:28:38 AM »

Adding to what Steve said:

There is, of course, a difference between:

A. An AM transmitter that can accommodate a modulating audio waveform that has positive peaks that are 4x as big as the negative peaks.*

B. An AM-modulated RF waveform, with positive peaks that are 4x as big as the negative peaks, that... when demodulated... is pleasant to listen to.

*One could always adjust the carrier level RF voltage to be 20% of the positive peak RF voltage level (carrier output power = 4% of peak output power)... with almost any AM transmitter. Therefore, if the peak output power is limited to some maximum achievable value, accommodating 400% positive peaks implies a significant reduction in carrier level (v. 125% positive modulation peaks).

Stu
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« Reply #4 on: July 05, 2015, 01:47:14 PM »

Hi Shane,

Sounds like fun.

Probably the easiest way for the masses to achieve outrageous, clean AM peaks is to bypass the filter of a ricebox (in my case my FT-1000D)  and turn the carrier way, way down.  Inject the audio directly into the balanced modulator.  Run it through a big linear amp so that you have a few watts of carrier and 1500 w pep output on audio peaks.   The audio is perfectly clean and flat from DC to 20Khz in this case. Many SDR rigs can do this too.

In a sync detector it sounds marvelous!  No complaints except that maybe the QRM heterodynes are bouncing around like controlled carrier.

The downside is that (the majority) the diode detector guys will be complaining that slob buckets have taken over the frequency.

I once ran a system (a few types) that did 400% modulation on AM. I remember one time after a transmission I un-keyed and guys were talking over me asking what the hell was I saying and why was I running sideband on an AM frequency. Bill W3DUQ was the only one with a sync detector at that time in the 80's and he said I sounded FB... Wink  The rest were stunned so I went back to normal 120% modulation.

The bottom line is if you occasionally get on with some folks who run sync detectors, it can be fun. But for normal AM groups, anything over 150% AM modulation will generally get complaints.


* As a side note about audio processing experiments...  The Huzman once designed for me using SPICE, a simple one stage asymmetrical op amp circuit that took a standard audio sine wave and amplified the positive side more than the negative side, depending on how the gain was set. It could take a standard sine wave and make it 400% positive and 100% negative.

I ran my voice though it and gagged at the wonderful looking asymmetrical waveform I now had!  But on the air, I was told I had some crackling to my audio.  The receiver detectors could not handle the false distortion generated. After all, as already said, anything that is not original is distortion - in my case in spades.

T
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« Reply #5 on: July 05, 2015, 03:27:05 PM »

I'm interested in what modulation / RF approach you are proposing.  High level plate / class C is going to have a near impossible task with that sort asymmetry.  Pulse width / class E will, in my opinion, best handle it. I'm sure there are other methods of modulation that would be handle this sort of asymmetry.

It has already been pointed out that excessive positive peaks (greater than 125 - 140% positive) will result in apparent distortion in most AM detectors.  Exalted carrier detectors (be it Sync AM, Product detection etc) will best handle this level of high positive peaks.  I'd like to suggest some decent audio processing using some compression and some negative peak limiting.  High density processing is good under "combat conditions" but becomes tiresome under less challenging conditions. This sort of high level of asymmetry will produce a very noticeable level of "pumping" not unlike controlled carrier - which most AMers find unpleasant to listen to.

Can I introduce an overall approach? The desired purpose of any of these techniques is to have a commanding signal under challenging propagation conditions or the best possible readable signal operating under during DX conditions.  Maximum legal power, a good, specific antenna for the desired distance from one's station - maybe even experimenting with driven arrays for farther out distances and certainly a good NVIS (cloud burner) antenna can greatly enhance local communication - unless, of course, the whole purpose of this experiment is a "just-for-the-heck-of-it" thing.

Al
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« Reply #6 on: July 05, 2015, 03:43:12 PM »

Tom

I suspect that, in actuality, you were (with your FT-1000 set up as you described) operating DSB with a vestigial carrier. SSB with a vestigial carrier would also work well with an appropriately designed synchronous AM detector.

I.e. in DSB , the terms: "positive modulation peaks" and  "negative modulation peaks" don't have the same meaning as they do in AM... at least with respect to what you would see on an oscilloscope looking at the modulated RF envelope. In DSB, with a vestigial carrier, both positive modulation peaks and negative modulation peaks (which you cannot distinguish from each other by looking at the modulated RF envelope) are much bigger than the vestigial carrier level.

Stu
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« Reply #7 on: July 05, 2015, 03:51:14 PM »

Many years ago, I had the opportunity to experiment with very high positive peak modulation on a transmitter and demodulator that could handle it cleanly. It was at 1390 WEAM in the late 1970s.

The transmitter there was an RCA BTA-10U2 10,000-Watt transmitter being run at half power. The demodulator was a Belar AM modulation monitor.

I modified the Gates Solid Statesman peak limiter so that, at the flip of a switch, I could choose between positive peak limiting points of 100%, 120%, or unlimited positive peak modulation. The transmitter could cleanly modulate in excess of 200% positive, and the peak limiter had a phase flipper that would always phase the maximum amplitude side of the waveform to be the positive one. The listening device was the Belar mod monitor audio output feeding a high fidelity amplifier driving an AR-3 speaker, which sounded very nice.

This system did not generate artificial asymmetry, but would take full advantage of whatever natural asymmetry that was present. It worked by simply biasing the diode that rectified the positive peaks to get the control voltage for the limiter -- or, in the case of the "unlimited" setting, simply disconnecting it. It sounded very clean, no matter where the positive peaks were set.

You'd be surprised how much random asymmetry there is in voices and music when you have a system that can detect it; many records (and, needless to say, announcer's voices) would make the positive peak modulation meter slam against the pin again and again when I chose the "unlimited" setting.

But the surprising thing is how little difference this made in the average perceived loudness -- I'd guess less than 3 dB maximum, often less than that. Listening carefully to many different announcers and songs, I'd throw the switch between "unlimited" and "100%" repeatedly -- and hear just a small difference in perceived loudness. Hardly worth the trouble, especially considering how many receivers weren't getting any benefit at all, just extra distortion -- and how much extra voltage tolerance and headroom had to be engineered into the system.

My experience with supermodulation is that:

1) Systems that just utilize natural asymmetry don't give big loudness gains, though they can sound pretty loud and _very nice indeed_ on a good receiver;

2) Systems with significant loudness increases also generate significant distortion on conventional receivers;

3) The "effective" loudness-boosting systems fall into two categories -- A) W3DUQ-style reduced carrier DSB on the one hand, and B) artificial-asymmetry-generating intentional audio distorting schemes on the other;

4) Both A and B types will sound distorted on conventional receivers; with the A (W3DUQ) type, all the distortion disappears when you listen on a synchronous detector; the B type sounds distorted no matter what you do. (The YouTube video showed a B type system.)


73,


Kevin, WB4AIO.
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« Reply #8 on: July 05, 2015, 04:01:33 PM »

Tom

I suspect that, in actuality, you were (with your FT-1000 set up as you described) operating DSB with a vestigial carrier. SSB with a vestigial carrier would also work well with an appropriately designed synchronous AM detector.

I.e. nothing to do with "positive modulation peaks" being much larger than "negative modulation peaks" as those terms are used in AM.

Stu

Yes, the "wavelets" were present when over 100% positive so it should be called DSB using a balanced modulator as you said.

Ya know, you bring up a good point. From all the experiments I've run using upside-down tube rigs and balanced modulators  compared to standard plate modulated rigs, it seems we can get away with a higher positive peak levels using regular AM before the complaints start. I think the little DSB wavelets, which begin anytime above 100% positive cause premature distortion in diode detectors. In contrast, the standard AM signal will not show these wavelets at all and will start showing distortion above ~ 125% depending on the diode detector and AGC used.

This has been a mystery for many years.  Guys would get on with clean, modified rice boxes and people would complain of distortion at a particular level but not hear it on a similarly modulated plate modulated or even screen/ grid modulated rig.

So, I will agree with Al that a class E rig would probably be the best suited for large positive peaks, simply because it has no DSB wavelets AND it can generate 200% or more modulation cleanly compared to most tube rigs..  A sync detector would solve these receiver problems, however.

The joker in the pack is the SDR rigs. I understand that some can generate real AM without the DSB wavelets. In fact I believe the Yaesu Steve modified for Bob had a chip installed that generated real AM.

Kevin - Interesting information and actual experiences. Your summary at the end makes sense  - thanks.

T



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« Reply #9 on: July 05, 2015, 06:47:33 PM »

We do need to make a distinction between DSB / vestigial carrier and a conventional AM, no matter the modulation means.  I have been a proponent of using exalted carrier methods of detection - Sync detection.  That is we are not depending on the carrier being transmitted that keeps intact all the components of AM including the carrier level - everything.

I use the synch detection on my Flex 1500 (and 5000) and also my Sherwood SE3. 

Another potential problem is to rely on the so-called 3 diode limiter to lop off the negative side of the modulation component so that we can boost up the positive peaks and achieve some form of super modulation: "I've got 200% positive peaks!"  I'm sure that's not what the author of this thread is not speaking about

Al
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« Reply #10 on: July 05, 2015, 07:53:25 PM »

Schematic:  Not yet.   Probably have one,  complete,  amongst five to ten pieces of graph paper,  strewn in the shop.

The system in use here is similar to Bob's approach in the am press exchange,  but I'm doing it all with op amps.   The gain of one stage is set by a peak detector that only responds to one polarity of the peak.

I'm running rack gear in front of the modulator.   5 band compressor,  etc.  With this method,  I can set it to 100 pct pos and neg (I have a phase rotator in line as well),  then set my positive peaks wherever I'd like them to fall.

The setup shown  is my pwm coupled to a supposed class D.   I've also built a class e deck but haven't used it much yet.   It's single ended,  so dirty.

time to get dinner on the tab
Time to get dinner on the table.

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« Reply #11 on: July 05, 2015, 08:35:06 PM »

So how did the Flex stuff work with PowerSDR?
You had the carrier control, in the old versions I think it went 0 to 100, 100 being 100% modulation both ways, 50 being 200% modulation, and all the way down to DSB.
You could set it at 10 and have 100 watts pep output with 1 watt of carrier.

Not sure if you got wavelets or not...

I tend to think its MUCH more important to keep the average audio level up (compression) then worry about peak powers.
Many people on the air have VERY low average levels of audio.
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« Reply #12 on: July 06, 2015, 02:37:51 AM »

I'd like to comment on what I've learned from many hours of QRP+dummy load+spectrum analyzer experiments in my workshop. I have not commented about these nebulous experiments, but this is as good a topic as any for it because I think there is a relevant point to be said.

===
Please forgive me for stating the obvious ideas:

From a completely practical or result-oriented standpoint, I really can't say if the low power audio processing methods are better or worse than high level methods like the 3-diode limiter provided both are perfectly done, but I lean towards the low level audio processing method.

Using a high level modulator, either the carrier can be reduced or the modulator can be oversized so that the desired result is available with the power from the modulator. This is required for either method.

It can't be overstated that the modulator must be able to deliver the needed power without distorting the waveform created by the 3-diode or an asymmetric processor. This applies to both positive and negative modulation peaks.

To proceed, pretend the modulator is an almost perfect voltage source and does not get upset about nonlinear loading.

If anyone finds an error or disagrees, please speak up on it.

===

1.) 3-diode:
With questions of adequate low-distortion modulators out of the way, the fine point I can't really resolve when asking whether the three diode or audio processing method gives a better result comes down to the question of distortion during the negative half cycle's peaks, and how much this distortion might affect the amount of out-of-bandwidth 'noise' audio sidebands produced.

When the 3-diode circuit acts, there are small abrupt changes in the waveform when the keep alive supply suddenly delivers current. These small corners or angles, if I see them correctly, can be approximated to a di/dt (or volts per microsecond if it seems easier) situation that is very fast compared to the audio frequency cycle. From that observation, I concluded that at each ON or OFF transition of the keep-alive supply current, a short duration double transient or noise pulse is created and that it is roughly triangular.

It can be seen in the RF envelope as the fall and rise of a negative going trapezoid at the lowest part of the negative half cycle. It shows up in LTSpice in the modulator audio output and also in the RF envelope from the transmitter.

The pulses seemed larger in amplitude than the switching transients of the diodes. The pulse can be outside the desired bandwidth of the AM signal. An example of this noise would be some narrow band of garbage that is always the same distance from the carrier and is repeated twice per cycle during the negative-most excursion of the modulator.

If the impedance of the RF stage is changed (by changing the current or the voltage while keeping the same power input), the 3-diode limiter has to be readjusted, although with a perfect modulator no adjustment is really needed within reason. An exception might be the size of the negative peak load resistor. At the highest positive modulation levels, a great deal of power may be consumed in that resistor during the negative half cycle as the asymmetry is created in a brute force fashion after the modulator output. This depends on the waveform and may be hard to calculate but it seems to be generally true.


2.) asymmetrical audio process:

I could find no real evidence of additional artifacts outside the audio bandwidth when the audio signal was made asymmetrical. I did not use voice but a bottom-squished sine wave for input, but I think the same idea applies.

While ~150% could be taken as an upper limit for compatibility sake, the ability to make very high positive levels like 400% without distorting that portion of the waveform itself gives a good margin for cleanly making anything up to the 150% level. It is also very important not to distort the negative cycle's usually-rounded peaks while limiting it to 100% or slightly less.

The addition of 'yet one more' low level audio circuit can invite troubles due to RF leakage, hum, ground loop, and other issues.

===
conclusion:

Because of the possibility of making even the small out of bandwidth noises, I want to change over to the low level audio method and leave the keep-alive and or other diode schemes in place only as a protection device for the modulation transformer. Whether a 3-diode circuit or a basic clamp is used, the asymmetrical audio method should be adjustable to make the desired modulation levels while not intentionally or frequently triggering any of the 'negative' diodes into conduction.

In defense of the three diode system, it is fair to say that the power contained in the pulse occurs while the carrier is very small, and that the pulse itself is of low amplitude. The question is to whether or not it is sufficiently powerful in a QRO station to cause an annoyance to 'local' stations on nearby frequencies (such as the anecdotally-crowding-in SSB station 3KC away), and as to whether or not the artifact itself is acceptable from a moral or engineering standpoint. The 3-diode circuit has been used by hams for a long time and I have not seen interference reports or heard of complaints by operators a few KC away. Each person has to decide for themselves. I am not going for ultra high positive peaks myself, but the trouble I allege only happens during the negative modulation peaks. On the air with a 300-350W carrier I use the single diode clamp and 1K series resistance (dynamic resistance of the 8020 rectifier tube) to the 10% keep-alive and no one has complained, but that does not mean it is not making noises.

Suggestions or minimizing the unwanted emission? make the keep-alive voltage as low as possible without cutting off the RF so that the energy in the noise pulses is also low. IMHO 10-15% is too high and 5% or lower is better. The rise and fall sections of the 'trapezoid' will become longer and of lower amplitude, and the unwanted audio sidebands decrease in both frequency and amplitude, as the negative peak is clipped closer to the rounded end and farther from the steep sides, of the audio wave.

Op-amps were mentioned. For those who prefer tubes, don't forget the tube op-amps previously discussed in another topic. It only takes two tube sections to make one, but it also requires a positive and negative low current (VT tube) regulated power supply.
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« Reply #13 on: July 06, 2015, 09:59:55 AM »

Another method to bring the positive peaks upwards is to reduce the carrier output voltage (with out modulation).
For example: the positive modulation peaks are 8 div. the carrier voltage is 2 div.
In this example the positive modulation capability is 8-3=6/2=3*100=300%.
When there is modulation, the carrier voltage rice to its normal value, that is the double value of the carrier voltage with out modulation.
The carrier voltage is in this system 6dB down to its normal value.
This give some audio compression and a bit louder audio, personally I like it because it have more 'depth' in the audio.

This method is called DAM; dependent amplitude modulation or DCC; dynamic carrier control.
Its is commonly used by today's broadcasters.

I have a pwm rig, and I control the carrier voltage in a fast opamp.
The carrier voltage is set to 6dB down referring the normal value.
I don't use a negative peak limiter and I have symmetrical modulation.
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« Reply #14 on: July 06, 2015, 04:46:02 PM »

[...]


2.) asymmetrical audio process:

I could find no real evidence of additional artifacts outside the audio bandwidth when the audio signal was made asymmetrical. I did not use voice but a bottom-squished sine wave for input, but I think the same idea applies.

While ~150% could be taken as an upper limit for compatibility sake, the ability to make very high positive levels like 400% without distorting that portion of the waveform itself gives a good margin for cleanly making anything up to the 150% level. It is also very important not to distort the negative cycle's usually-rounded peaks while limiting it to 100% or slightly less.

[...]



Thanks for this. Considering that it's a hard clipper, it's amazing how good some stations with diode limiters sound, as long as they don't lean into them and do use some kind of distortionless peak limiting in front of them to control the amount of clipping allowed. The Dorrough DAP 310, the first successful multiband audio processor, used a simple clipper for final peak control.

As for the "asymmetrical audio process" technique, remember that it creates distortion, too, even though the waveshapes are rounded. It's like purposely unbalancing a balanced audio stage. It will cause quite a bit of harmonic distortion to be generated on every signal that's fed into it, and -- unlike the clipper technique -- this distortion is added to ALL components of the signal, not just high-level peaks. It also generates intermodulation distortion. Feed a pure sine wave in, get a large series of sine waves out. Feed two sine waves in, get every harmonic and sum-difference combination you can think of out. Same with voice. Probably very fatiguing to listen to, and unnatural-sounding too.

Over the years, I have become less and less impressed with schemes to super-modulate AM transmitters. The cost in lost fidelity is too great; the gains range from small to debatable. If we are into being as loud as possible and getting the message through at all costs, why run AM at all?

Better, in my view, to make your transmitter dead flat and super-clean from DC to 10 kHz at 100% modulation, then use good-sounding low-level multiband compression and limiting to be reasonably dense while not introducing too much waveshape distortion or too many artifacts.

73,

Kevin, WB4AIO.
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« Reply #15 on: July 07, 2015, 08:31:37 AM »



Over the years, I have become less and less impressed with schemes to super-modulate AM transmitters. The cost in lost fidelity is too great; the gains range from small to debatable. If we are into being as loud as possible and getting the message through at all costs, why run AM at all?

Better, in my view, to make your transmitter dead flat and super-clean from DC to 10 kHz at 100% modulation, then use good-sounding low-level multiband compression and limiting to be reasonably dense while not introducing too much waveshape distortion or too many artifacts.

73,

Kevin, WB4AIO.
[/quote]

hear hear ... very well put

we don't all need to sound the same, but we do need to be heard under poor band conditions ... please taylor your audio appropriately
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« Reply #16 on: July 12, 2015, 11:52:27 AM »


Over the years, I have become less and less impressed with schemes to super-modulate AM transmitters. The cost in lost fidelity is too great; the gains range from small to debatable. If we are into being as loud as possible and getting the message through at all costs, why run AM at all?

Better, in my view, to make your transmitter dead flat and super-clean from DC to 10 kHz at 100% modulation, then use good-sounding low-level multiband compression and limiting to be reasonably dense while not introducing too much waveshape distortion or too many artifacts.

73,

Kevin, WB4AIO.

I've come to the same conclusion as Beefus and Kevin, especially after running a lot of AM on the upper bands where the moment-to-moment propagation effects are nearly always at play. Stations with clean and natural sounding audio that work at increasing audio density rather than asymmetry always seem to be the most easily and reliably heard at the other end, even when signal strength is near the noise floor. To my ears, the loudness factor from audio with high average content is very apparent and very compatible with any of my diode detector receivers. It's also much more easily attainable (than asymmetry) with basic compression/limiting hardware or software.

My casual experiments with the SDR platform indicate that insane levels of asymmetry can be easily attained by varying the carrier to sideband ratio, and at least with my own voice, if I don't venture much beyond +130% positive peaks it can sound clean with any receiver. However, although asymmetry looks great on my scope, in my opinion it doesn't have as much audio loudness impact as bringing up the audio density. That being said, the guys running class E or class D with enormous positive peaks and clean high density audio, sound spectacular (clean and loud) no matter what receiver I am using on my end.

Shane, what you have demonstrated in your video is definitely impressive, thanks for sharing the results!

Rob W1AEX

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