Any AM Pros?

K6OZY

Is the Carrier slider in SSDR a % of your output power?  I've been reading excellent post on various forums for PowerSDR and was wondering if they would apply to SSDR too.

http://amfone.net/Amforum/index.php?topic=27012.msg205463#msg205463

Should we be setting our AM carrier level to 50?   I think mine is defaulting to 100.

W4YXU

AM carrier is normally 25% of PEP.  At peak output both voltage and current double therefore 4 times the "idle" power.

If my memory is correct the early versions of SSDR defaulted to a 25 setting on power with a further internal reduction of 25%.!

It has been a few weeks since I used AM, but the last time I did the output with no audio input was the proper 25 watts and the power slider was at 100.

Just let the idle carrier be 25w and set the audio as normal and you should get 100w PEP

Jim
W4YXU

K6OZY

So to summarize,  I should leave the slider at 100 because it is internally being cut to 25% of output power? Seems confusing.. LOL

W4YXU

Well, that is what my scope showed! 

I just went and looked at the link in your post.  I confess I didn't see it back when I had the f5k or I might have played with it more.

When I switch the 6500 to AM I get a 25w carrier and 100w PEP with the slide set to 100.

It makes sense to me since the 6500 is designed/rated for 100w PEP that the slider would be set to 100 with the signal generation taking care to let that be 199w PEP.

I really should check in with some AM guys I haven't heard from in a while!

Back in the late 50's and early 60's I played with some fairly large AM transmitters!  ( around 1 Mhz +/- about 1/2 Mhz! ).

Jim
W4yxu

W4YXU

OOOOOPS! 199 should be 100 ( Typing with elbows is bad!).

K6OZY

Thank you, and if you do find out anything, please post it here as I know NOTHING about proper AM use.   Some of the guys on 14.330 today sounded great.  They were a full 20Khz wide with killer broadcast quality audio.

Ken - NM9P

The way someone once explained it to me is that the "Carrier" slider should be left at 100 (100% of "normal") unless you want to experiment with "reduced carrier" transmission, which has upward modulation above 100% but keeps the downward modulation above the 0% baseline to avoid clipping.  Some operators say they get more "talk power" that way.  Some broadcast stations apparently use 125% positive modulation, but use special processors to limit the negative "peaks."  

K6OZY

Do you have any info on what "positive modulation" and "negative peaks" regarding AM?  It's these details that I am interested in learning about.

Bob N7ZO

The human voice is naturally asymmetric.  So it is possible to run less than a 25% power (50% voltage) AM carrier without introducing distortion (assuming the phase of the voice is preserved).

Here is some background:  http://www.qsl.net/wa5bxo/asyam/aam3.html

Setting the carrier slider to about 75% in SSDR will produce about 12.5% power in the carrier which should not produce much distortion and will allow more power to be put into the information-carrying sidebands.

Flex, being the clever people they are, may be doing some additional processing.  Hopefully the creaters will comment.

Ken - NM9P

Let me take a stab at it...
What I know came from the ARRL handbook years ago.  I can't say that I am much of an expert in old AM, but have run it a few times just to see if I could.  You can't do a lot of talking with only 25 Watts on 160-40 AM!  hi hi.  

synopsis...(in very simple terms, because I am a simple man...)  
Picture an RF carrier modulated by a pure sine wave audio signal (pure single tone) .
As the carrier is modulated by your audio signal, it causes the strength of the carrier to increase and decrease in proportion to the modulating signal.  The "positive peaks" of your audio signal cause the carrier to increase up to 100% of the unmodulated carrier level.  (This is so called "full" modulation or 100%)  

A totally linear modulating signal will also decrease the carrier on the negative swing of the audio signal down to zero, essentially pinching off the carrier totally.

If the modulator is not adjusted properly, or overdriven then it will try to swing the carrier above 100% on the positive peaks and below zero on the negative "peaks" or "valleys" of the sine wave.  Since this cannot happen in real life, the over-crossing of the axis chops up the carrier and generates distortion (essentially key clicks on the AM carrier) which leads to splatter and all manner of disgusting things on the band.  Just look at 11 meters on you flex panadapter and you will see how bad things can get.  (You can also find it on some poorly adjusted SW broadcast stations)

Eventually, in order to give more "guts" to their broadcast sound, broadcasters and hams discovered that you can limit the negative swing of the modulating signal so that it never crosses the "zero" line on negative going parts of the cycle, and at the same time boost the positive swing of the modulating signal.  The result sounds louder, has a little more talk power.  It isn't totally linear, it adds some distortion, but not the splatter that typical overmodulation generates.

There are limits, however, to how much extra boost you can add.  If you swing too far above 100%, then there is not enough carrier in the signal for the typical AM detector to demodulate it properly, making the signal sound like an SSB signal on an AM receiver.

Some broadcasters arrived at a practical standard of 125% positive modulation, while others, and some hams, try to push it as far as 150%, especially when the other station is using SSB receivers or SAM (Synchronous AM) detectors to decode it.  

I hope this helps, and isn't too far from the purists understanding.  I tried to simplify it without getting too far of base with the details.

The main thing is to have fun and listen to yourself in another good receiver and tweak your audio and bandwidth so that you will be proud of your signal!

73 

Ken - NM9P

Edit...BTW.  I understand that reducing the AM "Carrier" control drops the level of the carrier, but the peak signals on positive modulation peaks will remain at the 100 Watt level, while limiting the negative swing to the Zero figure mentioned before.    So you can achieve greater than 100% positive modulation.  (although it is till limited to 100 watts peak power.)

Ken - NM9P

Agreed, Bob... I think the operative words is "much" distortion, or "objectionable" distortion.  All speech processing, including equalization, is a form of distortion of the original waveform.  And some of it, including Flex's new processor, can dramatically increase communications power and intelligibility.  The key is to limit the type and scope of the "distortion" to a level and type that will increase intelligibility without increasing spurious signals outside the transmitted passband.

Bob N7ZO

Hi Ken,

Right you are.  But why risk introducing the distortion?  By eliminating the carrier and the redundant sideband, we have much better power efficiency.  :-)

The Flex radios are quite good at ESSB if you want the wide AM broadcast-like sound - which, I have to admit, is fun sometimes.

Bob N7ZO

Ken,  this raises an interesting question:  Has anyone tried the the DX processor on AM?

Bob N7ZO

Another way to address the crest factor and asymmetry of the human voice is to smear the phase (ala Leonard Kahn and Bob Orban).  I have not heard much of this technique lately.  (Perhaps for good reason?)

A fascinating subject.