Poll: Over-Mod Myth

[LeapFrog]

My Cobra 142GTL I have runs about 220% positive and 90% negative on the scope with my Asymmetric mod done to it. On the spectrum analyzer, 3rd order harmonics checked at -69dB down. Everything else was lower. Acceptable levels is -40 or -45dB down, if memory serves. You CAN make it clean and be loud.

~Cheers~

Yes Sir.
Asymmetrical modulation is the ticket for loud, clean & clear AM audio.


I wonder what double sideband suppressed carrier modulation is also known as?


I like to keep the negatives between 95-100%, and the positive peaks anywhere between 125-175%
The receiving station must be capable of cleanly demodulating such a signal though, as we know.

Configuring the asymmetry is made easier with use of a variable resistor, also something to note:
Different diodes can give different characteristics! This makes the exercise much more interesting i.m.o.

You get to swap diode types out and record the different affect!

 

[StrangeBrew]

Not a tech so take this with a grain of salt but to my understanding if we are talking about different modulation levels on identical carriers then louder modulation will get you out. But in reality most of us will be limited by the headroom of our station, this means that we will most likely have to sacrifice carrier in order to get that high percent of modulation. This changes things.

So are we talking identical carriers with different modulation percentages or identical stations set as best they can be with different modulation percentages?

Nice question LeapFrog, Clear as mud

 

[543_Dallas]

Asymmetry is a good thing as long as you don't get carried away with it. Even on 11 meters I'd stay under 150% positive peaks. With the right bandwidth and eq setting most men can get >100% positive peaks and not baseline the carrier.

If you use amplifiers they have to be linear and ran so that there is plenty of headroom. Your best bet it to build a good bias circuit for a transistor amplifier or run a tube biased class AB or higher. If you really want quality you have ever to forget about efficiency and big watts from a small amplifier.

The d104 is a fine microphone. Too many bone heads use them and give them a bad reputation. It doesn't help that many of those old mic elements being used are no good.

 

[543_Dallas]

As for the ham rigs. Yes I can get some asymmetry out of my yaesu 1200 with just a mic plugged into the front. Around 115 percent if the carrier is set very low. The bandwidth is so narrow it does not have the sound you're after.

Radios can be modified but out of the box the Apache Labs anan can sound as good as anything you've heard out there.

 

[LeapFrog]

Not a tech so take this with a grain of salt but to my understanding if we are talking about different modulation levels on identical carriers then louder modulation will get you out. But in reality most of us will be limited by the headroom of our station, this means that we will most likely have to sacrifice carrier in order to get that high percent of modulation. This changes things.

So are we talking identical carriers with different modulation percentages or identical stations set as best they can be with different modulation percentages?

Nice question LeapFrog, Clear as mud

Identical stations, different carrier levels.
Might just be a fault on my part for not trying hard enough because it just seems a lot easier to reduce the carrier then it is to increase the "headroom".

 

[Captain Kilowatt]

I remember back in the day around here the formula for DX was a big ass carrier modulated at a 4:1 peak to dead key ratio, the swing mods were mostly used to achieve a 4:1 ratio on the output of an amp that was less than linear.

My vote is good antenna and big carrier for getting out there, modulation should be set to sound good once there.

The BIG international shortwave broadcasters used to run high power but lower modulation. The idea was that the high carrier power was needed to overcome interference and noise (just like when we DX) but they ran modulation levels typically in the range of 85-90%. The reason was that there was very little difference in audio levels however the modulator tubes lasted much longer. When you are talking about pushing quarter of a million watts or more and needing an additional 150-200Kw of audio to modulate that RF you are talking some seriously expensive tubes. Somehow I think if the likes of VOA, BBC, Radio Deutsche Welle etc. figured that was the way to go then I believe the key is in the RF power and not so much in ridiculously stupid modulation levels. It doesn't matter how LOUD you are if you can't push the S-meter higher than the other guy.

 

[LeapFrog]

As for the ham rigs. Yes I can get some asymmetry out of my yaesu 1200 with just a mic plugged into the front. Around 115 percent if the carrier is set very low. The bandwidth is so narrow it does not have the sound you're after.

Radios can be modified but out of the box the Apache Labs anan can sound as good as anything you've heard out there.

Increasing the tx audio bandwidth has likely been covered for the Yaesu 1200 before (I'm going to search).

Thanks for the Apache Labs anan suggestion, what a beast of an SDR! I would sooner undertake the project of rolling my own for that price tag though, I love this hobby because you can usually find more than one way to get to the same place.

 

[Tallman]

My Cobra 142GTL I have runs about 220% positive and 90% negative on the scope with my Asymmetric mod done to it. On the spectrum analyzer, 3rd order harmonics checked at -69dB down. Everything else was lower. Acceptable levels is -40 or -45dB down, if memory serves. You CAN make it clean and be loud.

~Cheers~

Bravo! The truth is here and it has been spoken. I don't know ExitThirteen personally but I do follow his work and I find his work to be exemplary and always technically spot on. Captain Kilowatt and ExitThirteen would make an AWESOME design team.

 

[543_Dallas]

Increasing the tx audio bandwidth has likely been covered for the Yaesu 1200 before (I'm going to search).

Thanks for the Apache Labs anan suggestion, what a beast of an SDR! I would sooner undertake the project of rolling my own for that price tag though, I love this hobby because you can usually find more than one way to get to the same place.

You can hold down a couple of buttons to get 4k ssb. That doesn't really do much if anything for AM bandwidth. Even after making adjustments to the internal eq settings it doesn't sound as good as a stock CB. Maybe a different mic would add more high end.

Another option with the ham rigs is to pipe external audio into the back door. Bob heil covered that in a Ham nation episode 36 that can be found on YouTube. Should give you a little over 4k bandwidth. I haven't tried it with the Yaesu. My Kenmore ts440 with external gear into the balanced modulator works too good.

 

[ExitThirteen]

Lol The master hasn't spoken.... I'm just a dumb CB'er posting my results.


~Cheers~

 

[Robb]

Lol The master hasn't spoken.... I'm just a dumb CB'er posting my results.


~Cheers~

Sure would like to see the mods you did and the corrections you made to make the asymmetric mod work. Last thing I recall about it, you were working on doing it to a 148 GTL.

 

[ExitThirteen]

The mod works on both the single conversion AM RX chassis and the double conversion AM RX boards on the MB8719 chassis. So it's good for Cobra 140/142GTL, Uniden Washington, President McKinley, Realistic TRC-450, Uniden/President Madison, Cobra 2000/148GTL, President/Uniden Grant, and other radios using the MB8719 PLL.


~Cheers~

 

[LeapFrog]

Sure would like to see the mods you did and the corrections you made to make the asymmetric mod work. Last thing I recall about it, you were working on doing it to a 148 GTL.

Not to speak for Exit13, but Robb I think the info is in the "Over 130% Modulation" thread.

 

[ExitThirteen]

Yes, that is correct, look in that thread, pick it up about page 6 or 7. Don't mind the stupid Marvin the Martian "Space Modulator" crap, tho.... was a waste of space on that thread. You'll have to sift thru the garbage to get the good stuff.

~Cheers~

 

[Robb]

Not to speak for Exit13, but Robb I think the info is in the "Over 130% Modulation" thread.

Yes, I know that.
Just was hoping Xit would cut loose with his corrections and solutions.