Reduced Carrier AM

[WA2SQQ]

In the Flex Community forum (link below), someone started a discussion about Reduced Carrier AM which is mentioned in the 6500’s manual. It states, “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”. I’ve read through the discussion and I just can’t grasp what’s going on. Is Flex saying that by slightly reducing the resting carrier level (which is normally a 4:1 ratio) you can increase the modulated signal in cases where it’s not achieving 100%? I’m not embarrassed to admit that I don’t understand this concept.
 
https://community.flexradio.com/flexradio/topics/reduced-carrier-am?utm_source=notification&utm_medium=email&utm_campaign=new_topic&utm_content=topic_link

[M0VRF]

As far as I am aware the carrier can be run at ANY level and modulated up to the maximum voltage of the system.

However with simple 'Diode' demodulation going beyond a certain % (150%?) will cause distortion on reception whereas using a synchronous demodulator would not.

This is my humble understanding, I'm sure someone can provide more detailed knowledge.

JB.

[W7NGA]

I used it on my Flex 6300 as the gain was too low on the Accessory port to fully modulate at 100% on peaks. Reducing the carrier maintained the sideband energy available and afforded full modulation. My new Flex 6500 is a bit more refined in the audio chain and I have a balanced input with plenty of gain to easily over-modulate if I so desire. It's algorithmic in that the processing is done before the PA stage, so reducing the output level does not have the same effect.

W7NGA  dan
San Juan Island, Wa.

[N2DTS]

You can not always get to 4x peak power and that depends on your voice and phasing as well.
The carrier control allows you to adjust the carrier level separate from the peak power.
Not sure how it works with various software that is out now, it used to be in the flex world that 100 on the carrier was full carrier power, 1/4 pep.
Then they changed it to 50 for full power carrier or something like that.
The ANAN software has it back to 100, but my radio will never hit 4x carrier power at 100, it does so around 70 (on most bands).

If you put the radios into an amplifier, you can make it a bit easier on the amp to have say a 200 watt carrier and 1200 watts pep. 1200 watts would normally be about a 300 watt carrier.

I can do the same thing somewhat with my screen modulators, turn the carrier down but keep the screen voltage swing and get 1400 watts pep with a 200 watt carrier.
In the case of the screen mod, that would over modulate in the negative direction, an sdr prevents that.
Not to say 200% modulation is going to sound good in most receivers...

[flintstone mop]

WBCQ uses reduced carrier AM on their 9330 and 5110 and another 17 mhz TX and it is a challenge for some receivers to demodulate the carrier. I do not know the technical advantage of this process.
WBCQ, 7490 is still a full carrier shortwave service of 50KW.

[K1JJ]

Here's a short article from the AMfone archives about the subject.

http://amfone.net/ECSound/k1jj22.html

The percent of modulation you run your AM transmitter at will depend much on your own preference and also on what kind of receiver detector the receiving station is using.  

I would guess that 1/4 to 1/3  of listeners are using an SDR sync detector so that we can run any modulation level we wish, even 300% positive and still sound clean to them.

But the majority use older diode detectors which limits distortion-free listening to maybe 120% modulation at most.

I personally adjust my carrier level either through plate loading and/or drive to a linear amp so that I can achieve a natural 140% positive peak level that lets my own asymmetrical voice fill it out where it wants to go.  It's all a compromise. If too many people complain about us sounding like an ssb station, especially during fades, then our modulation level is set too high.

BTW, our voices can change asymmetry levels (and phase at certain frequencies) from day to day or year to year, especially from early morning to night.  It all depends on what our vocal chords are doing. They can be sore or relaxed from use, a cold, or whatever.  So don't panic if you set all the audio gear perfectly and the next day things look funky.  I've seen my own voice change from as low as 110% positive to as high as 140% positive over a week's time. Waking up with some mucus congestion in the throat can get us reaching for the EQ low cut in a hurry.  Lows can sometimes dominate the asymmetrical effect and are much affected by the present condition of vocal chords.

T

[N2DTS]

There are people that sound good (mellow and relaxing) with a strong carrier and 60% modulation, and people who sound loud with a lot of audio on the carrier.

Its nice to be able to set it however you want.
With the new ANAN radios, the compressors work well and even include look ahead so you do not spike amps and hit the baseline in the negative direction.
Unlike the old software, it seems to work very well.

As an operator, its nice to not have to pay any attention to the audio level, set and forget and you will not be too low or over modulate over a wide input range.
What will they think of next!

[w1vtp]

At some point we need to differentiate between "normally generated" AM and that where the carrier is, due to the process, eliminated.  An example of the latter is during the use of a double balanced modulator the carrier is eliminated and only the sidebands appear - in a perfect world or a SDR radio.  The carrier can be reinserted by one way or another so that the individuals at the other end can properly demodulate the DSB modulation / reinserted carrier.  Assuming the DSB generation  occurs without distortion products resulting from several possibilities during the modulation: mixing & amplification process, there is no longer an assumed carrier to sideband relationship (ie: the fabled 4:1 SB / carrier relationship).

If one is to be received with little or no "distortion," it is important to establish a proper relationship between the DSB component and the reinserted carrier.  Otherwise, be prepared to receive "overmodulation" or "distortion" reports.  This problem can be dealt with using various enhanced carrier detection (ECD) methods at the receive end.  The most obvious ECD method is the use of the product detector that is normally used during SSB detection - a less than desirable method.  Another method - the one most satisfactory to all of us is to use Sync  AM detection.  This approach supplies a phase coherent enhanced carrier that is used to detect an AM signal. Yet another method is to route the received carrier, filter it, amplify it and then reinsert it into the detector.  Most of us are not familiar with this method - it has its limitations (notably not being phase coherent with the modulation & preexistent carrier components) and will probably never be a good solution for this form of "super modulation" where the carrier is below what would be the expected carrier level that can be used for older AM detectors.

I think JJ has the best philosophical approach: be considerate of the folks at the other end and limit your positive peaks to around (Tom's value) of 140%.  My class E system produces more than that positive modulation peak value but that's my voice.  I think I have received around 2 "distortion" reports while I have put the class E transmitter on the air.

Conclusion: be considerate while adjusting your DSB / carrier reinserted rig and try to keep the positive peaks reasonable so that those of us who use the old gear so that we won't want to twist the frequency dial.  Please do this with some sort of test equipment so that a QSO won't turn into an "audio clinic" where the poor guy at the receive end has to listen to a bunch of "fives, yayyys or yayllowws" while we dial it in at the transmit end.

Cheers, Al

[flintstone mop]

The understanding I have from Timtron on WBCQ is that they reduce the carrier by 6dB.
Using an SDR in the SAM mode loses sync trying to receive their reduced carrier stations. An R390 didn't seem bothered by the reduced carrier.
Supposedly reduces operating costs of making a full carrier station and putting more modulation into the sidebands. I understand that part of the equation.
Supressed Carrier is for SSB.
What has been mentioned here so far from ops reducing their carrier from their Anan or other SDR Transceiver is not the same as a reduced carrier AM transmitter used in commercial services. Maybe Timmy might chime in and give a more sensible response than mine.
Fred

[w1vtp]

<snip>  What has been mentioned here so far from ops reducing their carrier from their Anan or other SDR Transceiver is not the same as a reduced carrier AM transmitter used in commercial services. Maybe Timmy might chime in and give a more sensible response than mine.
Fred

The basic difference between the two methods described above is the refinement of the process in the case of commercial broadcasting.  We can implement the same refinement - we have the technology

Al

[N2DTS]

I think it is the same, just done in a different way, but a signal that has the carrier reduced by 6db is a signal that has its carrier reduced by 6 db no matter how its done I would think.
In the sdr's, I think, if you turn the carrier all they down it just runs DSB.

What has been mentioned here so far from ops reducing their carrier from their Anan or other SDR Transceiver is not the same as a reduced carrier AM transmitter used in commercial services. Maybe Timmy might chime in and give a more sensible response than mine.
Fred

[KD6VXI]

Apples and oranges,  to a point.

Big broadcash and shortwave use controlled carrier,  up to 6 db. I read somewhere that is the limit that doesn't mess with diode detectors.

My assymetry generator just let's me dial in positive peak stretching.   Ie,  all the way down,  I'm at 100 pct pos and neg.   Turn it all the way up,  still 100 pct neg,  but can hit 420 pct pos peaks.

My assymetry generator only increases the pos peaks.   

That doesn't mean the series or pwm (use both,  depending on which TX I'm using)  don't sit generate a DC offset.   But,  looking at the output of the peak stretcher,  there is no DC offset generated there.   

--Shane
KD6VXI

[Steve - K4HX]

It will not increase talk power(1) 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.

(1)  Always be wary when you hear someone use a term like talk power. It's nonsense.

In the Flex Community forum (link below), someone started a discussion about Reduced Carrier AM which is mentioned in the 6500’s manual. It states, “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”. I’ve read through the discussion and I just can’t grasp what’s going on. Is Flex saying that by slightly reducing the resting carrier level (which is normally a 4:1 ratio) you can increase the modulated signal in cases where it’s not achieving 100%? I’m not embarrassed to admit that I don’t understand this concept.
 
https://community.flexradio.com/flexradio/topics/reduced-carrier-am?utm_source=notification&utm_medium=email&utm_campaign=new_topic&utm_content=topic_link

[N2DTS]

What does full power with only downward modulation sound like?
Rough on the final tube I suppose...

[kb3ouk]

The understanding I have from Timtron on WBCQ is that they reduce the carrier by 6dB.
Using an SDR in the SAM mode loses sync trying to receive their reduced carrier stations. An R390 didn't seem bothered by the reduced carrier.
Supposedly reduces operating costs of making a full carrier station and putting more modulation into the sidebands. I understand that part of the equation.
Supressed Carrier is for SSB.
What has been mentioned here so far from ops reducing their carrier from their Anan or other SDR Transceiver is not the same as a reduced carrier AM transmitter used in commercial services. Maybe Timmy might chime in and give a more sensible response than mine.
Fred

I believe that its 6db below the peak power, which is 50 kw, and since -6db would be 1/4, that's the same 4 to 1 ratio as 100% modulation of a full AM signal, but those transmitters at WBCQ are also only running the upper sideband. Lower sideband is suppressed, you can receive it decently on a receiver on USB, when I do this I switch to LSB to zero beat the carrier (no sound so it is easier to hear when you zero beat), then switch to USB to receive the audio.


I can also do something like that with my icom 718 (dsb AM, not single sideband), using the external alc controller. With the radio's menu power set to the highest, use the controller to set the carrier, I can turn the thing all the way down to like 10 watts of carrier and be pushing almost 150 watts pep. The techincal term for it would be double sideband reduced carrier, I guess.

[kb3ouk]

What does full power with only downward modulation sound like?
Rough on the final tube I suppose...

Some of the old tube datasheets always had notes about using "modulation essentially negative" or something like that. I tried it once with my icom briefly into a dummy load, it didn't sound bad. Actually, I think it would actually be easier on the finals (in something like a linear) than normal AM would. You would be running the full output at a resting carrier, but at something more like 60-80% efficiency, then with modulation, the tube would probably start running slightly cooler due to the actual power it is putting out being somewhat lower.

[flintstone mop]

Downward modulation in a linear amp is from improper tuning of the amp to achieve full power PEP in the CW mode from the exciter. When properly loaded and tuned the exciter is set for an AM carrier set for 25% of the PEP carrier carrier and the magic happens. Downward modulation in a linear is severe flat topping of the audio peaks and obvious distortion on the air.
Downward modulation in a table top AM  TX is usually too much or too little grid drive from the RF driver to the control grid of the finals. There seemed to be a fix discovered for the venerable DX100 which had a little too much drive causing non-linear response using the  X-Y inputs of a scope viewing linearity of the final by observing the trapezoid pattern on the scope. Perfect straight lines of the modulation. The bright tip of the trapezoid is the negative peak the opening to the ends of the pattern are the pos peaks

Fred

[KD6VXI]

6db of carrier compression means between words,  the transmitter drops to a quarter of the peak power (100 pct Symmetrical mod) AND the carrier drops another 6 db.

So a 50 kw transmitter would have a 50k carrier with no carrier reduction scheme.   With carrier reduction dialed in,  the carrier would drop another 6db, to 12.5 kw.

This is done in the digital transmitters....   So now your talking maybe 18kw input  at carrier,  vs nearly 80 kw on the old style xmitter and 60 kw for a typical digital...

Some others have digital algorithms to change the carrier compression relative to the input.   IE,  if you have a 50 pct mod signal,  the  the carrier compression dials itself down,  actually increasing the carrier power to ensure max coverage.   I believe that's a brown boveri (bbc)  design,  but I could be wrong.

So,  to sum it up,  the difference between carrier and full power modulation can be 12 db.   At 100 pct modulation.

--Shane
KD6VXI

[N2DTS]

I would think, if you wanted (mostly) downward modulation, you would need to run the rf deck as a linear to reproduce the modulation cleanly. If only in the downward direction.

I have seen people on the air who had very little or no positive modulation, and sometimes they do not sound too bad.
That was surely because something was wrong, but if you WANTED to modulate that way, you might be able to clean it up some.
My 32V transmitters did not like to modulate a lot in the positive direction till I did the screen voltage mod.  
Phase it wrong and you could have little positive modulation with plenty on negative modulation...

[N2DTS]

Modern versions of the old controlled carrier?

[kb3ouk]

I would think, if you wanted (mostly) downward modulation, you would need to run the rf deck as a linear to reproduce the modulation cleanly. If only in the downward direction.

I have seen people on the air who had very little or no positive modulation, and sometimes they do not sound too bad.
That was surely because something was wrong, but if you WANTED to modulate that way, you might be able to clean it up some.
My 32V transmitters did not like to modulate a lot in the positive direction till I did the screen voltage mod.  
Phase it wrong and you could have little positive modulation with plenty on negative modulation...

I know for a fact that a lot of stock riceboxes do that, they have no trouble making 100% negative but barely do 80% positive, even with the carrier set to 1/4 the pep. Now if you turn it up the whole way, you'll be putting out full pep power and not have any headroom for positive peaks, only negatives. I tried this as an experiment with my 718 once, one time at full power and a second time with the carrier at 1/2 the pep of the transmitter (so that it only did about 50% positive peaks). The audio wasn't too bad, there did seem to be a slight amount of distortion when running 100% negative and no positive modulation, but it wasn't nearly as bad as what a highly modulated signal sounds like on a diode detector at like 150-200% positive. if you had a linear tuned up to put out full power and fed a signal like that into it, it would put out its full pep at resting carrier and then should modulate 100% negative very nicely. The output components would be under a lot of stress with the full power carrier on them, but the tube would probably be running a lot more efficiently. if I had a rig that could handle running like that I'd give it a try, but I don't think there is sone unless you built it yourself. What I would really be curious about is the different in received signal between say a 375 watt carrier modulated 100% to make 1500 watts pep, and a 1500 watt carrier modulated 100% negative only (no postive modulation).

[KD6VXI]

Brett,

Pretty much,  yes.

The BBC method is slick.   Under a certain pct of mod,  they actually increase the carrier.   This gives more range.   As the density goes up,   amount of carrier being rolled back between peaks changes.

There method actually increases range while decreasing cost.

The normal method is DX 60 style controlled carrier.

--Shane
KD6VXI

[N2DTS]

I think you could play with circuit values or even make it adjustable.
I have heard people running the controlled carrier and it did not sound too bad, and its easy on the amplifier if you use one.
I think it gets bad when you have almost no carrier when not speaking, but at something like 50% you almost need a scope to tell.

[WA2SQQ]

It will not increase talk power(1) 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.

This is kind of what I've been thinking, but due to my lack of a clear understanding I decided to introduce the topic and use it as a learning experience.

[WA2SQQ]

So last night I tried an experiment.
I currently use my 6500 to drive an ACOM 1000. With the AM Carrier set to the default 1:4 ratio 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.I made a few contacts and nothing negative was noted. To stations where I was weak, no differences were noted when comparing normal operation to the reduced carrier. HOWEVER, 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. I'm going to do more testing to see how much I can reduce it. I don't have a mod scope, so I am very curious how this will all effect the modulation. Audibly, it sounded fine.