Positive peaks, asymmetrical limiting and selective fade..

[W2NBC]

Tis the season..

So it's winter time soon.. LOW static, signals from everywhere, propagation patterns changing.. I have had little time to operate but some things I have noticed.. Stations "close in" (60-600 miles) with high asymmetrical positive peaks (over 130%) suffer with intelligibility due to lots of selective fading this time of year, and increased sunspot activity..  Now these are the fine business stations that always have rock signals.. The "A" list! The reason I have noticed this is because I have been turning the R-390A on (when I have free time) using a "standard" detector.. The other receivers here SP-400, SP-600 (also using standard detectors ) all suffer the same severe almost non copyable selective fade products when it occurs.. Other stations with peaks around 100% do NOT get nearly as much selective fade distortion... The flex-1500 I use at another operating position with synchronous detection is MUCH better, and is the great equalizer!

This all leads to these fundamental questions and observations. TO BE ASYMMETRICAL OR NOT TO BE ASYMMETRICAL.. for the HF bands.. That IS the question.. After researching the multitude of Orban pdf's and his theory on phase rotation ( as in all pass filtering), and symmetry etc., and the positive peak vs. loudness wars, I wonder..
I do know that "shortwave" stations have a relatively low modulation index for that purpose.. to limit the effects of selective fade.. It just isn't a fair comparison to reference a Broadcast band local daytime station ground wave to HF stations that exhibit propagation and selective fading when it comes to peaks and selective fade..
Almost all of my transmitters are set for FULL THROTTLE asymmetrical positive peaks (over 130%) with 3 diode peak limiting or low level asymmetrical negative peak limiting..

So, I wonder if just like we narrow our frequency response to be "neighborly" when the band is crowded, or change EQ settings to cut through the noise.. Should we have the ability to fully modulate symmetrically (when needed) when the dreaded selective fade rules the air waves??

Random thoughts..
Jeff

[WA3VJB]

Running reduced carrier DSB never really rang my chimes Jeff, past maybe 125% positive. 

Sure, it sounds more sexy to call it asymmetrical modulation, and to assert being able to hit 150% positive or wherever.

But not only does it aggravate the impact of selective fading and aurora, but most conventional detectors don't like being deprived of carrier.

Put it back !

In my opinion, it's more enjoyable to hear a transmitter where the op takes the time to refine and implement an audio compression protocol, as I hear on your station, along with tailored audio response to suit conditions.

[W1DAN]

Hi Jeff:

My thoughts....

clipping-created asymetrical modulation is a naturally distorted, but we all know this is louder. Many envelope detectors and AGC systems will be negatively affected by asymetrical modulation. Some envelope detectors distort with such strong audio peaks, and some AGC systems will try to follow this modulation.

The distortion on any selective fade is due to one of two reasons: 1 the carrier is reduced by skywave cancellation, and 2, one sideband is modified and attenuated (often at a moving frequency and phase) relative to the other. The symptoms can be reduced with using synchronous detection and listening to one sideband.

CRL made a HF processor called the MBL-100 in the 1980s. The manual has some good theory on selective fading. See the PDF manual Page 93 Appendix C.3 for details.

http://ftp://ftp.orban.com/CRL/

In summary, it is easier for a highly-asymmetrical signal to create distortion in an envelope-detected signal than a symetrical one. To me this is one of "modulation to carrier level detector headroom" at the receiver. And, if you modulate say 50%, the headroom is doubled because the carrier can to be reduced more before detector "overmodulation" occurs.

Hope this helps!

73,
Dan
W1DAN

[W2VW]

Running reduced carrier DSB never really rang my chimes Jeff, past maybe 125% positive. 

Sure, it sounds more sexy to call it asymmetrical modulation, and to assert being able to hit 150% positive or wherever.

But not only does it aggravate the impact of selective fading and aurora, but most conventional detectors don't like being deprived of carrier.

Put it back !

In my opinion, it's more enjoyable to hear a transmitter where the op takes the time to refine and implement an audio compression protocol, as I hear on your station, along with tailored audio response to suit conditions.

How much are you paying this guy Jeff?

[ke7trp]

Add in the fact that those stations have the bass cranked to the max and I cant even understand them here in Arizona.  All the while, a guy a stock valiant comes on and is 100% clear.  Really sad.

Mike Dorrough told me and has suggested many times that its better to be symetrical on the HF bands. I use his DAP processor and have it set for 85 to 90% neg peak limiting and typicaly hit 85 to 95 pos.  I used to run it at 150 % pos and 85 negative (I have a 900 watt modulator) I got way to many complaints from people with various receivers when i did. 

I think its best to have a Good agv action and multi band compression with an symetical pattern.  But thats me.  I have my station setup for Clarity and signal purity in mind. 


C

[WA3VJB]

Mike Dorrough told me and has suggested many times that its better to be symetrical on the HF bands. I use his DAP processor and have it set for 85 to 90% neg peak limiting and typicaly hit 85 to 95 pos. 
C

Aye to the DAP

Wonder how much compression you're dialing in at 95 % Clark ?
You know it's fast acting and can prevent overshoot. Why not take it up to baseline and ease it back a wee bit. I don't believe in wasting modulator power with underuse.

[ke7trp]

Paul, I really do not know how much compression.  I asked Mike one day and he said to run the Dap so the three compression meters where deep into compression and almost pegged. I asked how much that was and he would not tell me.  It is not listed in DB on the meter and I did not go further with it.

I have the version 2 peak limiter card with the offset adjustment. I can set it 100% equal or rotate it either way to chop pos or neg peaks off.  I ran it at 40 neg and 150 pos for fun once.  It sounds the best symetrical.  I keep the peaks down as I am in a neihborhood here and bothering these people is a concern so I really dont crank it up or ever run anything past 1500 pep. 

If simply reach down and turn the audio gain up on my toa BG10 driver, the negative will stay at 85 to 90 max and the pos just keeps going up to 150% or so.  Then, My modulator plate current starts to get up past the limit of where I am comfortable. My iron is 1000 watt rated Tape wound electro eng and will take the swang.

C

[kb3ouk]

Sounds like a good reason to find an old symmetra-peak or build something similar, like this :
http://www.w3am.com/8poleapf.html

[ke7trp]

I got the all pass filter off ebay, along with the power supply, connectors, parts and a chassis.  I never put it togher, I got busy with work and its in a drawer. Maybe i will find time soon to try it out.

C

[WA3VJB]

Sounds like a good reason to find an old symmetra-peak or build something similar, like this :
http://www.w3am.com/8poleapf.html

Yeah I first met W3AM at the Gaithersburg hamfest.  Knows his stuff.  He quotes Kahn's approach as wanting to minimize assymmetry, as I understand it, as a better prerequisite to loudness processing.  That speaks to the idea of compression and peak limiting instead of reducing the carrier power in proportion to sideband energy.

Clark -- sounds like you could be The Dominator in any wartime scenario.  Keep the screwdriver handy if you run into something.

[W1AEX]

Should we have the ability to fully modulate symmetrically (when needed) when the dreaded selective fade rules the air waves??

Random thoughts..
Jeff

As always Jeff, your random thoughts are interesting. After listening to the last mp3 "air check" you sent me, recorded with your R-390 setup, it seemed wise to re-think what I have been doing here with my smug radio. In that recording my positive peaks were floating as high as 150% and I could definitely hear things getting a little buzzy in your "air check" as the R-390 coped with those peaks.

Monitoring my own signal with a scope and an in-line RF pickup and diode detector I used the same 150% transmit profile as a starting point and adjusted audio levels and carrier level settings so that the positive peaks were restrained to a bit over 100%. It sounded clean but seemed a bit anemic as it just didn't sound as loud. Increasing the audio density by using more compression seemed to compensate and bring the loudness back and it still sounded clean with the diode-detector monitor. I'll give this "higher density lower peaks" setup a shot and see how it works out. You and I don't have too much problem with selective fading between us on 75 meters but it might be fun to try out both profiles with some of the stations I work on 40 meters or 10 meters.

73,

Rob W1AEX

[W2NBC]

Dan, Paul, Clark, Rob, Shelby..  yes even Dave, thanks for the replies!

The real reason I have been thinking about symmetry and selective fade was actually because I have built up a power supply modulator for my Johnson 500 using Gates iron from a BC1-T, Heising configured with 813's in the modulator.. The thing is capable of 140% positive with no carrier shift, power supply sag, with minimal low level negative limiting.. so been thinking about it..
Paul, yes it is EASY with carrier insertion and DSB to configure tremendous positive peaks, and as you and Dan have suggested the effect on selective fade combined with non-synchronous envelope detection and detection headroom can get a little funky beyond 125% positive on the receiving end without either a quality low distortion detector or sync AM.. And Rob, it's so cool that with the Flex you can call up the "selective fade" profile and A/B when fading is present..

So what I am going to do is build up an APF (all pass filter) and switch it into a separate audio path when needed.. As we all know, some of our EQ, peak limiting, etc. is geared to maximize positive peaks once phase polarity is established for our voices.. So adding an APF changes lot's of settings!

Fun stuff, and thanks all

[W2VW]

You can get the same result from wearing boy's underwear.

[W2NBC]

Ha!

How's that going?

Maybe its time to get a woman's symmetry on da radio at your end.. OLD man..

signed, (fruit of your loom)

[W2VW]

No self respecting woman would get near this radio.

Wait a minute.....

[W2PFY]

Add in the fact that those stations have the bass cranked to the max and I cant even understand them here in Arizona.

Bass? Around here we call it our East-West filter If we don't want any westerners listening in, we crank up the bass

[W2VW]

Add in the fact that those stations have the bass cranked to the max and I cant even understand them here in Arizona.

Bass? Around here we call it our East West filter If we don't want any westerners listening in, we crank up the bass

Might as well just use ssb.

[ke7trp]

Its the same reason some guys take a perfectly good pick-up truck and jack it up with huge mud tires and suspension.