IM - Infinite Modulation

[brandon7861]

OK, I just made that up, but I'm serious.

What if we leave the high level modulation as it is and combine it with the idea of direct injection, but instead of injecting audio, we inject silence.

If I had to get a clean 200% modulation out of a radio without stressing my parts, I would look for a way to keep the peaks and troughs where they are when tuned to 100% factory, but find a way to cut the dead key level in half (thereby changing the ratio of dead key to peak audio without ruining the signal, cheating, I know). Forget how for the moment and lets expand on that idea.

What if I sampled the audio at the mic amp with a detector to modulate a jfet that attenuates the carrier into the preamp? When there is no voice, the DC average tells the jfet to shunt the preamp's RF to ground, goodbye dead key until you talk, and that would be controlled by an RC constant so the muting sounds natural instead of a sharp-edged carrier muting. It would be between AM and DSBSC. It's an ethanol-inspired idea here, so go easy on me lol.

 

[unit_399]

Brandon -
I believe your theory has merit. However I think it would only work on a freq completely free from any other rf signals.
In the periods of no voice (and no carrier) the receiver's AM detector would latch on to another carrier, garbling up the audio. Similar to SSB relay chatter in an amp without an am/SSB switch.
- J J 399

 

[Shadetree Mechanic]

How about a radio with an amplifier. The radio generates the frequency and carrier, but the amplifier is modulated with audio directly. Or maybe modulate both the radio and amplifier?

 

[brandon7861]

Been gone, had a funeral to go to. Just got home.

My thought was that since the carrier is present when there is audio, the people receiving it would hear it normally. I don't see any issue with a dozen other carriers coming into their receive, what I want is that needle to have sideband-like swing on AM when my signal goes into their radio. An amp would obviously need to be biased for SSB for that to work. The receive in the modified radio would remain the same.

I have more thinking to do on it, hopefully I will have time to test it this summer some time.

 

[Ridge Rider]

Am I wrong in thinking that what you're trying to achieve is called 'Negative Peak Compression' along with carrier suppression?

 

[brandon7861]

I don't want to compress anything, merely eliminate the carrier during voice minimums. Imagine looking at a signal on a spectrum analyzer. There are two sidebands and a carrier peak. The carrier goes up and down with the voice, but never goes away. My idea was to make it go away during silent periods but bring it back proportional to the intensity of the voice when talking. This would be different from SSB or DSBSC as those have the carrier completely removed (and requires different receivers). I want it there when talking but not when dead keying based on some exponential curve and a short time constant.

Negative peak compression is used to mitigate pinchoff when the modulating transformer swings the supply more than twice the dead key. I don't want to raise the positive peaks beyond what an unmolested radio with 100% modulation would produce, rather dynamically reduce the dead key using an integrator sampling the mic audio over a short time period.

Doing this would have complications, but I think it could be done without negative effects on the receiving side. Using an amp, there would need to be a delay like in SSB since the carrier would be gone, but the receivers would stay the same since the carrier would still be there when the mic audio is present. I think this would pair very nicely with amps like texas star where there is a SSB relay delay but lacks actual SSB bias. On an AM radio, the receiving stations needle should swing like it does in SSB but require no alterations to the circuit to receive. I think it would make stations sound much cleaner without changing the output power.

 

[Ridge Rider]

I like your idea, would help keep an amplifier running cooler when in AM service.

 

[brandon7861]

Too many projects to try it though. Maybe this winter.

 

[nomadradio]

We managed to persuade more than one kind of radio to reduce the carrier nearly to zero and still maintain a proper modulation envelope. The channel 6 guys were obsessed with reducing a radio's carrier to a half Watt or less and still show the normal 20 or 30-plus peak modulated Watts. But this was only because their freight-train amplifier chain had way more gain than was wise and would flatten the voice peaks as a result.

The operators who just wanted way more swing than the next guy discovered that there is such a thing as not enough carrier. When the other end of the conversation is in strong-signal territory, it's not so bad. Anyone far enough away for your carrier to disappear below the noise level will hear "pumping". The noise is constantly swishing in and out between every word and syllable.

What prevents this in a SSB receiver is the automatic gain or AGC circuit. A sideband receiver's AGC will 'hang'. That is, fast attack but slow decay. AM with no carrier would need that feature in the receiver at the other end. Back in the day, some communications receivers would have a selector on the front panel for "slow" or fast AGC. Modern transceivers select this automagically when you change from AM to SSB.

Besides, they can only tell you stopped transmitting when the carrier stops suppressing the channel noise. Or if you have a roger beep.

The variable-carrier setup I posted for the Cobra 2000GTL will do this pretty much.

73

 

[brandon7861]

I didn't consider the pumping thing. I was hoping for a time constant that would carry over between syllables, but not much more than that. Maybe this wouldn't work as well as I imagined. I still want to try though, I do have plenty of junk radios to test things on. Its just a matter of finding the motivation and the time with other higher priority things to do. At least at work I have the time to get on here and hang out for a while.

Thanks for the history lesson, I didn't know it was attempted before to that extreme.

 

[Ridge Rider]

When I was 15 I picked up a Johnson Viking ranger and I would tune that to maximum audio and then roll the drive knob down to next to nothing and put out less than a watt but swing to 55, clear back in 1975.
- Back then very few people had seen anything like that and every other person told me there was something wrong with my radio, I was keying up at 5:00 but going up to 20 lol

 

[brandon7861]

Yea, you guys are starting to convince me it might not be worth it. Sounded good in my head though.

 

[Uncle Ronnie 336]

i think you need to check out the pinned post about a more involved swing mod. it involves feedback sampling to open er up when speaking. cleaner than the mod where you pull the jumper