Reduced carrier AM

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The last paragraph on page 74 of the latest manual has me puzzled, particularly, "By reducing the AM Carrier level, the percentage of carrier relative to total PEP can be reduced to increase talk power in the AM sidebands". Isn't this 'overmodulation'? I cannot find much information on "Reduced Carrier AM" other than that any 'missing' carrier power has to be supplied to the signal by the receiving station to allow demodulation.

If Reduced Carrier AM is such a such a Good ThingTM, why use regular AM at all?

Mni tnx and hope everybody is recovering from their Thanksgiving meal.
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DrTeeth

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  • chipper.

Posted 3 years ago

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Jim Gilliam

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I find your question very interesting. I have wondered what the envelope of AM looks like on the Flex. Is it 100 percent modulation? If so, the instantaneous peak power can be as high as four times the carrier power. Due to this restriction the final amplifier can run no more than 25 Watts of average power to attain full 100 percent modulation. It will be interesting to see how this thread unfolds.


Jim, K6QE

(Edited)
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K0UNX

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To see a commercial AM envelope, go to General and tune the AM band.  If you see some that look VERY different, those are HDAM signals.  We have quite a few HDAM sigs here in the Denver area.
Jim
K0UNX
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K2CM

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If you reduce the carrier to zero, you would have double sideband. If you reduce the carrier to the point where you have to add carrier on the receiving end, you have SSB with carrier leaking thru or AM with insufficient carrier. I am not sure what you gain by transmitting in AM.
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Jim Gilliam

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There is no theoretical advantage to using AM. It is more of a legacy mode and many hams make a "career" of restoring vintage radios and reliving the days of yesteryear. It is fun to join in as many of the AM users are very knowledgeable technicians and engineers and enjoy making their radios sound like they are of broadcast quality. I have heard several users using the Flex 5000 with linear amplifiers and they sound magnificent.


Jim, K6QE

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Rick Hadley - W0FG

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Once upon a time, when SSB was first gaining presence, and when most receivers did not have product detectors or selectable sideband, one of the common complaints among old-timers was that SSB was too hard to tune.  Reduced carrier AM, or Resdidual Carrier SSB was an attempt to resolve the problem by providing a pilot carrier while still gaining the benefit of increased power in the sidebands.  I haven't though of it for 50 years, until this thread tickled my dormant synapses.
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Jim Gilliam

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When I was in the Navy, Double Sideband with carrier was used in sending data. It was almost impossible to tune the signal without a residual carrier present to "zero beat" in order to ascertain tuning of the signal. That was back in 1958.



(Edited)
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Stan - VA7NF

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What has not been said about reduced carrier is that the carrier for 100% modulation is full carrier, and this is what you typically see on POP music broadcast.  Listen to some talk radio and you will see the carrier drop during quiet portions and will also hear the background noise rise due to your AGC action. 

The carrier for (averaged) lower modulation percentages is reduced but still sufficient to produce clean detection.  Note:  This is also the definition of compression if the AGC is very fast but for normal or slow AGC then the audio levels will be a factor of the peak modulation power for signals that go into high % modulation; slightly exaggerated compression for signals that peak out at lower than 100% modulations.

Re-reading this I see the mud rising to cover the whole explanation.

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Martin Ewing AA6E

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At lower modulation levels, you only need enough carrier so that the negative modulation peaks just go to zero power - there won't be any extra distortion.  If you reduce the carrier level like that (during quieter times), you reduce power dissipation and make less QRM on the band.  On the other hand, the receiver AGC may "pump" and bring up the noise when the carrier is reduced, as Stan suggests. So you probably don't want to reduce the carrier too much.  That's my understanding, but I've never tried to use this feature.
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DrTeeth

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Thanks to all those answering here, even to those dissing the mode. The reason I initially asked was that I could not see the difference between too much audio and too little carrier as to my mind they were the same condition (viz over-modulation) even though I had heard of the reduced-carrier mode. I found FRS' reference to 'increased talk power'  confusing.
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Tim - W4TME, Customer Experience Manager

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Talk power is a function of the total energy being transmitted vs the intelligibility of the signal on the receiving end.  Less carrier put more energy into the modulation envelope, hence more talk power.
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WA2SQQ

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Tim, as I understand my 6500's AM operation there is usually a 1:4 ratio, so 25W resting carrier would produce ~ 100W. If we were to further reduce the resting carrier, in theory it could increase the intelligibility, and, could lessen the load on an external amplifier. The question is, how low can you go before some older legacy receivers start having problems receiving the AM reduced carrier signal? Also, do you know how / why the 1:4 ratio was chosen? I run AM on 160 / 75 and I consistently get fabulous audio comments. No complaints from me, just trying to better understand this option.
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DrTeeth

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@ Robert >> The question is, how low can you go before some older legacy receivers start having problems receiving the AM reduced carrier signal? <<

This sums up the gap in my understanding very nicely. It makes one wonder why classic AM is used for transmitting at all and that double sideband suppressed carrier is not used out of the box.

@ Tim - How far can one take this >>Less carrier put more energy into the modulation envelope, hence more talk power<<? Pls look at my response to Robert's post above
(Edited)
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Jim Gilliam

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The one to 4 ratio is due to the nature of 100 percent modulation. Assume one is looking at the carrier on an oscilloscope and you have a peak to peak reading of one volt. If you inject a sinusoidal signal into the modulator and are modulating 100 percent you will see a peak to peak reading of two volts. The power is proportional to the square of the voltage so the power would increase by a factor of 4. Since the amplifier of the Flex is capable only of 100 Watts, on cannot exceed a resting carrier of 1/4 of that amount or 25 Watts.
(Edited)
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Jim Gilliam

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To really understand modulation is not easy. At the college level it is taught both in the time domain and in the frequency domain. Both domains shed light on the processes involved.
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Kevin WB4AIO

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People with synchronous detectors won't hear any distortion, even with 300 per cent. modulation. People with conventional AM detectors will hear it. Some old "boat anchor" receivers generate considerable distortion at just 80 or 90 per cent. modulation.

The audibility of distortion is a complex subject, too. If the peak to average ratio of the modulating signal is high, clipping distortion won't seem quite as annoying. And if the modulating audio has boosted treble, clipping distortion will seem less objectionable for most listeners.

One of the great things about double-sideband transmission is that it is actually dual-frequency diversity SSB. This gives the guy on the receiving end the ability to rapidly switch between USB and LSB reception for best signal-to-noise ratio in a rapidly-changing QRM situation, without the guy who is transmitting having to change anything. There is even a way -- using a Costas or W3DUQ detector -- to reject the interference from one sideband or the other while still getting the desired signal from both sidebands. This kind of detector is now available on some SDR software platforms.
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Ken - NM9P, Elmer

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There are commercial audio processors out there that will allow up to 150% modulation or more (usually 125%) on the positive swing, but limit the negative cycle to that it doesn't cross over and cause distortion and splatter.  They are used to make an AM station sound "Louder" than others.  But there is a limit, because going too high will cause the sort of distortion because there is not enough carrier to demodulate it.

The adjustable carrier on the 6000 will allow something like this.  but use it sparingly.

Ken - NM9P
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WA2SQQ

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I decided to introduce this discussion into the AMFONE forums. The guys who hang out there are primarily AM operators who use both SDR radios, Class E and legacy “boat anchors”. Some very interesting facts have been exchanged, especially the one I quoted below.

Last night I tried an experiment. I currently use my 6500 to drive an ACOM 1000. With the AM Carrier set to max, I adjust the unmodulated AM output of my 6500 to produce 200W out of the ACOM 1000. This produces ~ 800W out on peaks. Next, while applying an unmodulated signal, I reduced the AM carrier setting to decrease the ACOM’s output to 100W. We compared both scenarios. I received several signal reports, none of which noted anything negative, but no improvements with stations where I was weak. While my signal was not any better, I definitely noted a decrease in the ACOM’s air exhaust temperature. If for nothing else, AM Reduced Carrier can probably be used to reduce power consumption and amplifier wear and tear.  If AM operation is of interest to you, please check out the discussion below.

“(AM Reduced Carrier)  will not increase talk power  or any other kind of power. The Flex has a maximum PEP capability. You can utilize this PEP capability by running the carrier at about 1/4 the max PEP and modulating 100% or you can run less carrier and modulate more the 100% on positive peaks. In any case, you will achieve the same exact output on the receiver at the far end. There is no free lunch.”

http://amfone.net/Amforum/index.php?topic=40995.0

Not sure how much more we can gain from this discussion, but I do think it was a worthwhile learning experience.