Poll: Over-Mod Myth

[Captain Kilowatt]

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

Definitely, if you are testing for it. You need to set up a proper two tone test and look close to the carrier frequency and not near the harmonics. There is also a process of increasing the amplitude of the audio tones up to a certain point as well to determine IMD levels. It's not as easy as just looking ftom harmonic spikes on a spectrum analyzer.

 

[543_Dallas]

If you use any device or method to force asymmetry you are creating distortion....by definition. Harmonics and distortion aside I see no reason to run 200% positive modulation. If you want to run X amount of pep power get a transmitter or amplifier that has enough ass to produce the proper carrier.

If you can't do that you need to be more realistic about your equipment is capable of....or go to SSB. How many watts an amplifier will produce isn't what they should produce.

399.........you are the first person outside the broadcast community that I have ever heard say that the human voice is asymmetrical. THANK YOU!

Really? That's been posted on a CB and ham forums numerous times. I eluded to that in post 48. I didn't think it needed further explanation because it's been covered.

 

[Captain Kilowatt]

If you use any device or method to force asymmetry you are creating distortion....by definition.

Weeeeeel yeeeeesss in the sense that it is not an exact reproduction of the original signal however when it comes to THD the distortion may be so low as to be virtually nonexistent. I had tube type AM transmitters with mod transformers and mod reactors and all kinds or iron in the mix and with 125% mod the recovered audio was still less than 1% THD. The newer gear is even better.

Harmonics and distortion aside I see no reason to run 200% positive modulation. If you want to run X amount of pep power get a transmitter or amplifier that has enough ass to produce the proper carrier.

Totally agree.

If you can't do that you need to be more realistic about your equipment is capable of....or go to SSB. How many watts an amplifier will produce isn't what they should produce.

Also totally agree.

Really? That's been posted on a CB and ham forums numerous times. I eluded to that in post 48. I didn't think it needed further explanation because it's been covered.

You are right but I meant THIS forum but either way..... Had a bad night last night. I don't work nights now but due to shift alignments coming into the holidays I had too last night and it has me out of whack. I had a kidney stone removed a couple weeks ago and at the time they found out I am diabetic and working last night after being up all day had me waaay off. Trust me.....that wasn't the only mistake I made last night. Gotta balance those carbs and proteins a little better tonight.

 

[StrangeBrew]

....
If you want to run X amount of pep power get a transmitter or amplifier that has enough ass to produce the proper carrier.
....

This is the way I always understood it, for DX it's the carrier that gets you there.

Now I have nothing against bumping up the positive peaks for a little more punch if done relatively cleanly, you just have to be honest with yourself regarding the reasons for doing so.

Back in the day a couple of the better sounding locals around here told me that with the right mods you could go pretty high on the positive modulation peaks with minimal distortion, but they recommended to keep it at or below a 5:1 peak to dead key ratio for best results. I really don't know how that ratio translates into percentage though.

 

[LeapFrog]

If you use any device or method to force asymmetry you are creating distortion....by definition. Harmonics and distortion aside I see no reason to run 200% positive modulation. If you want to run X amount of pep power get a transmitter or amplifier that has enough ass to produce the proper carrier.

If you can't do that you need to be more realistic about your equipment is capable of....or go to SSB. How many watts an amplifier will produce isn't what they should produce.

Really? That's been posted on a CB and ham forums numerous times. I eluded to that in post 48. I didn't think it needed further explanation because it's been covered.

That's more than I'd ever dream of running as well, and yes forced asymmetry does not come without its own challenges.

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 !!
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.

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

You know quite about CB and HF communications audio quality, I just wanted to share some links with a little knowledge, to help anyone interested in Improving: "audio punch" w/ some quality.

Link To Original Thread

Correct. The 139XLR, TRC 457, 458, etc. is the second generation 858SSB board, and is by far the better of the two. Both had speech clipping circuits, but the AMC on the 139XLR board is light years better. Uniden put the speech clipper in their AM sets too (Cobra 29XLR for instance) but it was a much less sophisticated design and not nearly as effective. This same design was used in the 1st generation 858SSB boards in the Teaberry and Courier rigs, and their audio wasn't even close to that from a 139XLR. There are other subtle differences that improved the gen 2 board's receiver too.

RT307 -
I wouldn't call the 139XLR board "over-engineered." Back then (1977) the CB craze was going full-tilt, and there was a lot of competition between high end manufacturers like SBE, Stoner, CPI, etc. to make the "ultimate" CB. Uniden was the new boy on the block, so to speak, and wanted to strut their stuff. So they updated their existing 858SSB sets which proceeded to kick ass on-the-air, and, at 1/3 of the price of the high end guys' equipment.

The FCC didn't like the direction that the manufacturers were taking, and decided to rein things in. They directed that any new models had to have less expandability and power potential. By this time the CB craze was winding down, and SBE, Stoner, CPI, Teaberry, etal had already closed shop. With CB sales way down, Uniden redesigned their CB line with the emphasis on low cost rather than max performance. So speech processing and other frills were a no-no. This is pretty much where we stand today. To me, the gen2 Uniden 858SSB was the best mass-produced type-accepted cb ever made.

- 399

Sorry this was so long-winded.

Link To Original Thread

CBC SPEECH PROCESSOR

The CBC processor is pretty much a copy of the Uniden speech clipper circuit used in the Cobra "XLR" series of radios as well as others. It is the primary reason that these rigs sounded so loud and clear on the air. The circuits in the AM and SSB models differ in that the SSB units have a low-pass filter following the clipper to clean up objectionable audio harmonics. An excellent explanation of how the circuit works can be found in "Understanding and Repairing CB radios" by Lou Franklin, pages 168-170. The SUPERSCOPE CB640 (also built by Uniden) had this same clipper circuit on a seperate "Mike Amp and ALC board" (Sam's #239 ). I installed one of these boards out of a junk CB640 into my RCI 2950. It REALLY pumped up the audio.

I think that the specs for the voicemax unit selling on Ebay are pretty exaggerated. I just don't see how you can boost the audio power by that many dB without adding tons of distortion. But...the board works, and that's the bottom line. I would really like to see a schematic of the circuit.

- 399

Link To Original Thread

Negative Peak Clipping is easily applied to ANY radio...IF you know what you are doing. Usually it requires two components... both diodes, but they must be selected to work with the particular setup on your radio.

Selecting the components requires a complete understanding of how Amplitude Modulation works, an RF voltmeter, an oscilliscope, and a thorough understanding of basic electronic theory. Unfortunately... from what I've seen over the past 2 years, this eliminates about 90% of the people on this forum.

My RCI 2950 is setup with 22 db of audio clipping using a Uniden speech clipper PCB from a Superscope CB640. The NPC is a 2-diode device of my own design that allows 130% positive modulation peaks while limiting negative modulation peaks to approximately 95%. This results in a signal that is extremely LOUD and CLEAR without generating any harmonics and splatter. BTW... the mod limiter in this radio is not disconnected.
There is no rocket science to this. There are 2 mods: one for direct-coupled modulators, and another for transformer-coupled modulators. The basic theory is the same, but the diode selection process is different.

When my wife (and our digital camera) gets back from the US, I'll post some O'scope pics of my mod and more particulars on how it works. Like they say... a picture is worth a 1000 words.

- 399

..Also
I just got my first Marantz SuperScope AirCommand CB640, & CB340 Radios, Both Have Bad Solder Joints (factory) LOL & I Cant Wait To Try The Audio Board's Clone Into an XLR Cobra or NPC ala' ExitThirteen style MB8719 Chassis Radio.

 

[unit_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.

I believe Xit 13 make his test using a 1khz tone to modulate the radio. Try injecting audio with high frequency content (3khz and up) and look at the IMD products and the harmonics. The second and fourth harmonics will be off the chart. It is the high frequency content that causes modulators running 125% positive to generate "splatter." IMD will run around 10% or so with +125%/ -95% asymmetrical. With 100% symmetrical, IMD seldom exceeds 0.5% on a properly adjusted radio.

- 399

 

[543_Dallas]

This is the way I always understood it, for DX it's the carrier that gets you there.

Now I have nothing against bumping up the positive peaks for a little more punch if done relatively cleanly, you just have to be honest with yourself regarding the reasons for doing so.

Back in the day a couple of the better sounding locals around here told me that with the right mods you could go pretty high on the positive modulation peaks with minimal distortion, but they recommended to keep it at or below a 5:1 peak to dead key ratio for best results. I really don't know how that ratio translates into percentage though.

Proper AM is the same DX, local or into a dummy load. 5x the carrier would be 125% modulation which is a good number to shoot for.

The problem is that without an oscilloscope you can only hope it's right. Switching the watt meter into average mode you should see a slight forward swing...more of a wiggle.

It is possible to drive an amplifier hard enough that the average power with modulation is not much lower than pep. Looks like hell on a scope but for competition purposes it works. Im my experience the radio of choice for this is a cobra 25 or 29. An export will give you lower average power out of the amplifiers, the same pep and sound worse doing it.

The hot rod export guys have to turn the mic gain back so people can understand what they are saying. When they do that the mod limiter (if they haven't disabled it) isn't adding compression to the audio that will give it even more punch. Tinny sounding crap audio that blends in with the noise floor is the result.

These are just my observations. I'm sure plenty of people disagree.

 

[Robb]

I believe Xit 13 make his test using a 1khz tone to modulate the radio. Try injecting audio with high frequency content (3khz and up) and look at the IMD products and the harmonics. The second and fourth harmonics will be off the chart. It is the high frequency content that causes modulators running 125% positive to generate "splatter." IMD will run around 10% or so with +125%/ -95% asymmetrical. With 100% symmetrical, IMD seldom exceeds 0.5% on a properly adjusted radio.

- 399

OK, so the IMD is present. So, what can be done about it? What is the cause? Cheap resistors or transistors? Other over-driven/non linear components? RF leakage? Typical causes?

 

[LeapFrog]

OK, so the IMD is present. So, what can be done about it? What is the cause? Cheap resistors or transistors? Other over-driven/non linear components? RF leakage? Typical causes?

I think it's a lack of filtering?
But i'm just a dumb CB'er so I don't know what this generalized statement really means. (filtering can be applied many places, and for many reasons)

So I should try a negative peak compression modded (export) radio, with NARROW voice communications quality audio and look for the IMD. Too bad I need access to a spectrum analyzer.(looks like it's time to go to school again )

Proper AM is the same DX, local or into a dummy load. 5x the carrier would be 125% modulation which is a good number to shoot for.

The problem is that without an oscilloscope you can only hope it's right. Switching the watt meter into average mode you should see a slight forward swing...more of a wiggle.

It is possible to drive an amplifier hard enough that the average power with modulation is not much lower than pep. Looks like hell on a scope but for competition purposes it works. Im my experience the radio of choice for this is a cobra 25 or 29. An export will give you lower average power out of the amplifiers, the same pep and sound worse doing it.

The hot rod export guys have to turn the mic gain back so people can understand what they are saying. When they do that the mod limiter (if they haven't disabled it) isn't adding compression to the audio that will give it even more punch. Tinny sounding crap audio that blends in with the noise floor is the result.

These are just my observations. I'm sure plenty of people disagree.

I agree 100% on the average reading meter showing a positive carrier shift (wriggle forward) although I have seen some CB's swing downward and still look good on the scope and sound okay too.
The only way to remedy this was to lower the carrier or increase the modulation limit.


I like the idea of a 5:1 peak to carrier ratio, just got to watch the negative peaks.
The Cobra 25/29 can be setup to do 100-110% without looking too bad with voice audio, after that though you have change a few things and the real increase seen @ 125% cannot be always clean in my experience. (Try it with a signal gen, without turning the mic gain way down/up, or lowering the carrier to a ridiculous level ).
In my experience it's kinda hard to get a Cobras (AM only) to even look asymmetric on the 'scope above 2 watts carrier power. And usually looks the best @ 500 milliwatts swinging to less than double that, average power (maybe more)! lmao.

Heck, 1 watt carrier to 5 p.e.p. might become the new fad! CB QRP! CB QRP!
It's all in the antenna anyways.

 

[543_Dallas]

1 watt carrier and 5 pep works. Thats actually very close to what I need from a base radio.

 

[Captain Kilowatt]

OK, so the IMD is present. So, what can be done about it? What is the cause? Cheap resistors or transistors? Other over-driven/non linear components? RF leakage? Typical causes?

Overdriving an amplifier stage is a leading cause of poor IMD performance.

 

[Captain Kilowatt]

https://en.wikipedia.org/wiki/Intermodulation

 

[Mustang 131]

The % of modulation has nothing to do with quality. It can add dynamic range with low amount compression. Hell it fun to run 100% modulation talking and do a 80htz bass drop at 300% to wake up the band. But why run more then 180% when a lot of radios don't receive it correctly. Crank up the carrier and remove the noise. The NAB and FCC have proved this with many studies that 150% and under is the way to go.
AM I wrong in saying a PROPERLY used xmitter sounds good to 125% on any am radio? But super truckers love da swang..
Clean is mean. Run class A,AB,AB2 all the way to the final amp..B or a tuned C will get you home.

 

[LeapFrog]

Related to the thread in some manner.