Poll: Over-Mod Myth

[543_Dallas]

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.

I'm a fan of a big carrier. You can't get most guys to spend the coin on equipment that will do that...for very long.

I believe this has something to do with FM not being so popular. The guys with "big amplifiers" get their feelings hurt by the watt meter or kill their amplifier.

 

[ExitThirteen]

For the MB8719 double conversion AM RX chassis:

Replace R228 with a 1.2-1.5K resistor. Add a 1 uF, non-polarized capacitor from pin 9 of the TA72222(A)P audio IC to the junction of R228/D63/R194. Replace the AM pass regulator (2SC1419 or 2SD1135) to a TIP41C. Set AM carrier to approximately 2W. Put radio on a scope and adjust AMC pot so negative peaks are 80-90%. Enjoy.



~Cheers~

 

[ExitThirteen]

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.

Ohhh myyy... CK would be outraged that you'd put him in the same sentence (nevermind same design team) with a lowly dumb CB'er like me!


~Cheers~

 

[Captain Kilowatt]

Ohhh myyy... CK would be outraged that you'd put him in the same sentence (nevermind same design team) with a lowly dumb CB'er like me!


~Cheers~

Why? Did you forget to shower this week?

 

[Tallman]

I just thought that putting two bright and intelligent minds together would be kickass for a tech project. I had two engineers working for me that were both members of MENSA. Brilliant people, put them in a room together and the tension was palpable and you could tell if there were no other people around a shouting match would start up and maybe a nerd fight. You know what I mean, throwing pocket protectors, slide rules etc.

 

[ExitThirteen]

Why? Did you forget to shower this week?

Don't be silly, I took my shower already this month! Good to go!

I could see the nerd fight now....

"You take that back! Or I'm going to put this slide rule up your left nostril!"
"Oh yeah?? Well you can take that protractor of yours, put it at 90 degrees, and sit on it!!"


Single conversion AM RX chassis mod:

Replace R204 with a 1.5k ohm resistor. Install a 1uF, non-polarized capacitor from pin 9 of the TA7222(A)P audio IC to the junction of R204/D62/R121. Remove R104 from the PCB (located next to TR33, and solder a 20k trimmer pot in its place on the solder side of the board. Set AM carrier to 2W. Put radio on a scope and adjust the 20k trimmer pot (that's your adjustable AMC pot) for 80-90% on the negative peaks. Enjoy.



~Cheers~

 

[unit_399]

First of all, let me say that anyone who is modding their CB
radios for extended bandwidth and loudness so they can
play music on-the-air are total f...ing IDIOTS.
...........................................................................

Secondly ... there is so much bullshit and misinformation on this thread, that I can't believe.
............................................................................................................................


Asymmetrical modulation , by definition, is distorted. If you
take a pure sine wave, and reduce the negative aspect, it
introduces distortion, pure and simple. Asymmetrical
modulation is used by AM broadcasters to make their signals
louder than their competitors. Rf, modulated to 125% on
positive peaks and limited to less than 100% on negative
peaks, will be 1.94db louder than one modulated
100%/100%. Almost twice as loud. BUT... 125% positive
peaks introduces a ton of even-order harmonics that MUST
to be filtered out in order to maintain a (somewhat)
distortion-free, natural-sounding output. Not to mention keeping the neighbors happy !!

Many people say that the NPC mod for CBs makes for a
more “natural-sounding” audio. This is not true. The human
voice is naturally asymmetrical, and putting it into a
“forced” asymmetrical environment with the NPC mod,
changes it beyond belief. AM broadcast stations spend tons
of money on audio processors that produce 125% positive
and less than 100% negative modulation peaks to meet FCC
regulations. In all honesty, can you believe that a simple
diode/resistor mod on a cb radio will produce the same
result ??? If you do ... WAKE UP ... you’re dreaming.
IMO, you’re better off using tried and true processing
techniques like audio compression, hard peak limiting, audio clipping, etc.,
to make your signal loud and clear.

200% positive/ 95% negative looks great on paper, but I'll bet the on-the -air sound has distortion +++. Besides, the unbiased 1N914 ( or equiv.) diodes used as AM detectors in most AM CB receivers can't even come close to handling 110% positive peaks, let alone 200% peaks. Sure, a transmitter can be modded to do a lot of things, but if Average Joe's receiver can't process the signal, what have you accomplished ???

CB radio is for VOICE COMMUNICATION. If you’re looking to
play like you’re a AM broadcast station, get a part 15
transmitter on Ebay and go for it. Just watch out for the
Federales !! But keep the music and other Bullshit off of the
Cb and freeband channels !!!!!!!

- 399

 

[Mustang 131]

"First of all, let me say that anyone who is modding their CB
radios for extended bandwidth and loudness so they can
play music on-the-air are total f...ing IDIOTS."

I guess I'm a total f..ing IDIOT. Merry Christmas

 

[Captain Kilowatt]

399.........you are the first person outside the broadcast community that I have ever heard say that the human voice is asymmetrical. THANK YOU! In addition to all that gear AM station run is a little device that monitors the program content and switches polarity of the balanced audio line feeding the tx to ensure that the more dominant peaks always go to the positive peaks. It is usually part of the compressor/expander chain. Running TRUE assymetical modulation is more involved than most people think.

 

[Robb]

First of all, let me say that anyone who is modding their CB
radios for extended bandwidth and loudness so they can
play music on-the-air are total f...ing IDIOTS.
...........................................................................

Secondly ... there is so much bullshit and misinformation on this thread, that I can't believe.
............................................................................................................................


Asymmetrical modulation , by definition, is distorted. If you
take a pure sine wave, and reduce the negative aspect, it
introduces distortion, pure and simple. Asymmetrical
modulation is used by AM broadcasters to make their signals
louder than their competitors. Rf, modulated to 125% on
positive peaks and limited to less than 100% on negative
peaks, will be 1.94db louder than one modulated
100%/100%. Almost twice as loud. BUT... 125% positive
peaks introduces a ton of even-order harmonics that MUST
to be filtered out in order to maintain a (somewhat)
distortion-free, natural-sounding output. Not to mention keeping the neighbors happy !!

Many people say that the NPC mod for CBs makes for a
more “natural-sounding” audio. This is not true. The human
voice is naturally asymmetrical, and putting it into a
“forced” asymmetrical environment with the NPC mod,
changes it beyond belief. AM broadcast stations spend tons
of money on audio processors that produce 125% positive
and less than 100% negative modulation peaks to meet FCC
regulations. In all honesty, can you believe that a simple
diode/resistor mod on a cb radio will produce the same
result ??? If you do ... WAKE UP ... you’re dreaming.
IMO, you’re better off using tried and true processing
techniques like audio compression, hard peak limiting, audio clipping, etc.,
to make your signal loud and clear.

200% positive/ 95% negative looks great on paper, but I'll bet the on-the -air sound has distortion +++. Besides, the unbiased 1N914 ( or equiv.) diodes used as AM detectors in most AM CB receivers can't even come close to handling 110% positive peaks, let alone 200% peaks. Sure, a transmitter can be modded to do a lot of things, but if Average Joe's receiver can't process the signal, what have you accomplished ???

CB radio is for VOICE COMMUNICATION. If you’re looking to
play like you’re a AM broadcast station, get a part 15
transmitter on Ebay and go for it. Just watch out for the
Federales !! But keep the music and other Bullshit off of the
Cb and freeband channels !!!!!!!

- 399

When Xit13 did the mod to the 8719 chassis on the forum ('Over 130% Modulation?'), I saw the scope pattern myself. There was no distortion, but a solid, consistent sine wave from source to output. Are you saying that it is creating 2nd/3rd harmonics? Xit already tested that issue as well.
Or did I miss your point altogether?
Or are you just worried/disgusted with jammers using this method?
Sorry; just wanted you to clarify.

 

[Robb]

"First of all, let me say that anyone who is modding their CB
radios for extended bandwidth and loudness so they can
play music on-the-air are total f...ing IDIOTS."

I guess I'm a total f..ing IDIOT. Merry Christmas

I don't think you are an idiot - at all.
Just was asking for your thoughts.

 

[ExitThirteen]

I'll have to see if I can get the results off the spectrum analyzer to a thumb drive and post it here. I did a sweep from 27mhz to 100mhz and the worst spot I saw was about -69dB down. What I DID have to do is do some adjusting to the trap coil/slug, but once I got it all adjusted, that was the final results. Also, I am doing this mod in the audio stage, not in the RF stage. I'm not trying to open bandwidth or anything like that.

Besides, I enjoy experimenting. I seldom talk AM, so 95% of the mods I perform on AM are going into a dummy load anyhow.


~Cheers~

 

[Captain Kilowatt]

Harmonics are one thing while IMD is another. I would be curious to know how the IMD figures are. That's where the splatter and in-band interference comes from. Not harmonics.

 

[ExitThirteen]

The datasheet for a 2SC1969 transistor which is used in my Cobra 142GTL is rated as a 16W device. My radio with this mod done modulates right at about 14W PEP. So from an RF standpoint, based on the datasheet of the transistor, I am operating well within spec of the transistor, so IMD from an RF standpoint is not an issue. It's not like I'm driving the ever-living piss out of the transistor. From an audio standpoint, the 1uF capacitor value is critical. The capacitor itself is providing some filtering. This value must not exceed 1uF, as it does cause distortion that can be seen on the scope. The higher the capacitor value, the more audio it passes, which can increase potential for IMD.

I'm not saying this mod is perfect. No one will ever have the perfect mod. But I don't think it's all that bad, either. Besides, it's been a big learning curve for me, and I've learned a lot in the process.


~Cheers~

 

[Robb]

399.........you are the first person outside the broadcast community that I have ever heard say that the human voice is asymmetrical. THANK YOU! In addition to all that gear AM station run is a little device that monitors the program content and switches polarity of the balanced audio line feeding the tx to ensure that the more dominant peaks always go to the positive peaks. It is usually part of the compressor/expander chain. Running TRUE assymetical modulation is more involved than most people think.

Surely, the IMD product should still be seen on a SA - wouldn't they?