AM Demodulation Quality Flex-6000 VS Others

  • 1
  • Problem
  • Updated 4 years ago
  • Acknowledged
While using my 6500 to check into the local 75 meter AM net I became frustrated trying to copy the other stations. I experienced this with the 6300 also.
 
AM amateur signals show up on the scope with a huge carrier and tiny modulation and the audio seems the same. Selective fading is so pronounced as to make speech difficult to make out, weak and distorted. No amount of AGC-T, bandwidth or other changes seems to help. No, NR wasn't on!
Using the SAM mode helps some however even it lacks good quality. 

So during the NET I fired up a few other receiver to compare. First my 75A4 then the RCA AR-88, IC-706, R390A and finally the TS-2000. All provided much better demodulation and lacked the severe distortion from selective fading. Of course I was using the same antenna. FYI The TS-2000 did better than the others. The other stations's carrier acted to quieten the receiver noise, as it should, and speech was clear and understandable.  The Flex doesn't seem to respond to the carrier and quieten the band noise as the others do. It's as if the carrier isn't actually there. 

Interestingly, AM stations on the BC band sound pretty good, great in fact at least until you open the bandwidth over 6khz then you begin to get a lot of hiss. I can't really claim that this is any better than my 70 year tube receivers however. 

So my question is....What have others experienced in comparison to other receivers? Is there hope for improvement with SSDR's demodulation ability? 

CW and SSB are just awesome and far better than the others, even my K3 (Which BTW also stunk on AM). Sorry to say, AM just doesn't cut it.
73
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Steve N4LQ

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Posted 5 years ago

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W7NGA

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I'll disagree. I used to design receivers. I have a Collins 75S3C, Collins 75S3, multiple Collins 75A4s, Drake R4Cs, Drake R4, Collins R390A, National HRO50T1, NC183D, INC98, SX24, Icoms, Yaesus ... nothing compares to my Flex 6300 for AM demodulation. And then we also have SAM ...

dan  W7NGA
San Juan Island, Wa.
(Edited)
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Steve N4LQ

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I'm glad to hear that so what am I doing wrong? 
Seems like AM was even better when I had the Flex-1500. 
Any suggestions? I may resort to make a video for demonstration. 
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Ken - NM9P

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Where are you setting your AGC-T?  Even on AM you need to set it on a clear frequency to the "knee" or "Sweet Spot."  Many Flex 6000 receive problems are caused by having the AGC-T set too far to the right so that the quieting effect is not optimized.

Also, I usually use FAST AGC settings so that a quickly fading carrier won't leave the signal in the quiet zone for too long.

However, I have sometimes wondered if on AM the AGC could be set so it doesn't kick in for a few more dB, in effect being linked more to the modulation than the carrier?  That might remedy some of the "low AM audio detection" issues.
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N4HY, Elmer

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Steve:    Sorry to have taken so long to respond.  I am assigned this issue by Flex as I am the architect and original author, back on PSDR, of the original demodulation approaches we have taken to AM, both standard HPF (sqrt(I^2+Q^2)) which is remove DC to low cut off, as well as SAM (which is phase lock to the carrier, but otherwise demodulate as if double side band).  SAM is mostly immune to the overmodulation which occurs when selective fading wipes our the carrier or reduces its power well below that of the sideband power.

I have listened to your fantastic A/B of SSDR and Kenwood TS-2000.  I hear, with my own ears, two things.  One the AGC-T is set much better in the TS-2000 and finally, there is a major difference in audio response between the SSDR and TS-2000. 

I am going to suggest a way to settle this and give us actual measurements.

I would request that you download and install SpectraVue if you do not have it already and attempt to give us as close in time as possible A/B comparisons of the AUDIO OUTPUT of the TS-2000 and the SSDR running through SpectraVue.  If you could do the youtube process again it would be most helpful.   I apologize in advanced for all the extra work and thank you in advance for bringing this issue to us and understaking the measurements.   Some are going to LOVE the nearly flat group delay, amplitude response and other things that are the staple of the Flex system.  Others, such as you,  will prefer that which they are accustomed to.  For Flex, "It's only software" but any such modifications have to folded into the build, test, roll out, deal with unintended consequences of the changes process Flex must go to with its finite software resources.
 
73s, respectfully,
Bob N4HY
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Steve N4LQ

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Hello Bob and thanks for the interest. 

A spectrum analysis would be helpful I suppose. I tried to get SpectraVUE working but got too frustrated with all the hoops.  You probably have another receiver there you can do the A/B testing with and get better results. 

I will say that there is nothing particularly special about this TS-2000. It happened to be handy to make the video. I get the same results when using my WWII AR-88.

Perhaps I could hook up a O'scope and demo this better however take a careful listen to this video I made. BTW: I am using an Android phone for the recording so all you can do is listen to each receiver and form an opinion. The scope can come later.

https://www.youtube.com/watch?v=rab-LwAL34w

 Steve
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Ken - NM9P

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Steve,
I just watched your video.  I agree that you have more phase distortion on your Flex than on the TS-2000.  You have SOME on the Kenwood, but it is not as often and not as deep as on the Flex.  Hmmm.

I would like to hear more, perhaps with the RX EQ flat.
Also, could you test with AGC OFF but adjusted so that it isn't overloaded?
It may reveal whether the distortion is in the actual demod routine or the AGC routine.

Ken - NM9P

UPDATE:  I also noticed that the distortion is more prevalent in the lower registers on the Kenwood, but is more in the higher frequencies on the Flex....
(Edited)
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Steve N4LQ

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Ken
I just made another video demonstrating the clipping in AM mode. Really I should use an o'scope but I think you will be able to hear it. 
Remember the old receivers with diode type "noise limiters"? Well this is about what it sounds like. Perhaps this will offer more insight.
https://www.youtube.com/watch?v=rab-LwAL34w
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Steve N4LQ

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I have a postulation:
The 2 sidebands become out of phase with each other and produce distortion. 
Kenwood demodulates only one sideband. Flex demodulates both. 
By eliminating one sideband we reduce the opportunity for distortion. 
Please view these 2 videos.

Typical AM BC signal with one sideband removed
https://www.youtube.com/watch?v=3G0v9Uftdx8

Ham on 75 meters with one sideband removed
https://www.youtube.com/watch?v=8fbFeVKnd8c
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Bill -VA3WTB

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I wish I knew what is going , I read were many people think Am mode is really good, but not for you
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Steve N4LQ

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Trust then verify.
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Steve N4LQ

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Ken
While I agree with your suggestions, believe me, this has all been tried. Also I would suggest that one should not have to seek a "sweet spot" just to receive such a simple mode but of course, as I said, I have tried all manner of AGC-T settings and fast vs slow agc for the past year but still the old timer receivers provide cleaner AM on ham signals. 
I mention "ham" signals because on the AM broadcast band you can't beat the 6000 but for some reason, hams with their weaker signals and lower modulation percentages seem to be weaker and distorted. Especially bad is the phase distortion from fading. Sure you get some of that on regular diode detectors but it's certainly terrible on this version of SDR. The TS-2000 sounds just fine, not hi-fi but quiet readable. 
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k0eoo

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There should be no "search for the SWEAT spot" necessary for AM....  To go from SSB to AM all I do is change the AGC decay from FAST to SLOW or MED....  Something must be wrong with something else in your setup if you have tried the setup Ken or I put forth...  Wish I could visit your shack to hear what you're hearing....
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Jim - N7CXI

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The hiss you're talking about is probably due to the 6000's flatter response. NONE of the older receivers had anything even approximating a flat response.

Have you tried setting an RX EQ curve that rolls off 3-6dB per octave above 3kHz? That might be a start on getting you there.

HTH,
Jim N7CXI
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k0eoo

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I'll throw my log on the fire here Steve....  I have many high quality AM receivers in the shack and none of them equal the consistent AM audio quality I get from my 6500 has as well as the SDR-1000 or Flex-5000 I owned....  So, you might have something amiss with one of your settings....  

Use Fast decay with AGC-T at just above the noise level (30-40 on 75m) and start with flat EQ and the slice audio gain at about 60-70 and the global audio gain set to suit....  YMMV....

By the way, I'm a retired design engineer for Audio Research Corp and enjoy the superior audio quality the Flex radios provide, especially on AM and SSB...  Good luck, Dennis, k0eoo
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Steve N4LQ

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AM from BC stations and hams using 100% modulation and decent signals sound awesome on the 6000 but when someone is using a rice box or low level screen modulation it's really hard to decipher. The older receiver run rings around it. 
I'm trying to find time to set this up so I can record a video. I may also run some test using the signal generator and various levels of modulation to see if I can pin this down. 
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Steve N4LQ

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Here is a video I made to attempt to show the difference between the 6000 and other receivers on AM. 
Note: Same speaker is being used via separate inputs on the Bose. Same antenna.
No amount of juggling the AGC-T or EQ helps. 
https://www.youtube.com/watch?v=2HBg6uc8CNY
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k0eoo

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Hello Steve,   (I notice Bob is looking at this issue but I just wanted to pass on my observations on your video while we are waiting his findings....)

I finally took a look and listen to your YouTube video, nice job by the way....  First thing I noticed is you had the bandwidth on the flex set too narrow.  You were listening to Robert, W0VMC and is signal was 10kHz wide (+/-5kHz) and the flex window showed about 4kHz (+/-2kHz), much too narrow!  On the TS-2000 you had 4kHz set which is 8kHz bandwidth in AM mode.  So we are comparing 4kHz BW on the flex with 8kHz BW on the TS-2000.....

I usually run my AM BW at 8 or 10kHz depending on the width I see on the panadapter, and the EQ as shown in the clip:



In addition I noticed you had the Rx EQ set to peak in the mid-band and be flat in the lows and highs; so you were lacking in amplitude in the presence band which helps in clarity.....  But, because you had the BW set too narrow to hear Roberts mids and highs, EQ becomes a mute point....

Dennis, k0eoo
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Steve N4LQ

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Nice try Dennis but believe me none of these has an effect on phase distortion due to selective fading. 

The TS-2000 was set for a "HI-CUT" of 4000 hz. The total bandwidth is actually 8 khz or 4 khz each side of zero-center frequency. I can open it up to a max of 5 khz HI-CUT and with the Lo-Cut set for zero we have 10 Khz max BW.

Flex's bandwidth indication showing 10 khz is really 10 khz total or 5 khz on each side of center. 

Adjusting the EQ is lots of fun but it cannot compensate for distortion. If you notice, I made several adjustments while filming and I tried to maximize the vocal range between 300 and 3500 hz. 
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k0eoo

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Steve, Where in the video did you show the Flex BW set to 10kHz?  The Flex BW did not cover Roberts whole signal.  I could not see the scale on the Flex screen and from the panadapter you can plainly see the highlighted blue BW area did not cover even half of Roberts transmitted signal, so as far as I saw and heard the comparison was not apples to apples.....  

Steve, I don't want to point fingers, I'm just a customer as you are and everyone's perceptions are valid even if we don't all agree....  Flex is looking at the issue so if something is amiss they'll find it I'm sure....

Regards, Dennis, k0eoo
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Steve N4LQ

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Dennis:
With the Flex bandwidth set for 5.6 Khz the carrier is centered within the passband and there are 2.8 khz bandwidth on each side. Only one sideband is necessary for demodulation of AM. The Kenwood TS-2000's bandwidth is indicated as lo-cut and hi-cut. lo-cut was set to 0 and hi-cut to 4000. That gives a total bandwidth of 4 khz or 2 khz for one sideband. The maximum I can set this for is 5 khz or 2.5 khz for one sideband. That's one reason the Kenwood sound a bit bassy and the reason I reduced the Flex to 5.6 khz with is the most narrow pre-set bandwidth on Ssdr in AM mode. 
That is pretty close to apples vs apples. So in reality, the Flex was set slightly WIDER then the Kenwood. As for Robert's signal width. I can't do much about that. 3 khz one side of zero is plenty = 6 khz total. 
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N7AIG

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Sounds to me like the Kenwood has a high-frequency emphasis that you may not be aware of. Even your own voice as narrative lacks the crispness and clarity of the Kenwood in the film clip.

Most natural sounds have a roll off of around -6 to -12 dB / octave above 1 kHz. Voices have a dead zone between the first and second formants around 1 kHz.

So in my own systems here, running through the EQ of a MOTU 828mk3, I use a low frequency cutoff of 100-200 Hz, a high shelf filter of about 6 dB boost above 1 kHz, and a Q3 notch filter centered at 1 kHz. (instead of boosting the highs, I actually cut the lows with a low shelf filter by -6 dB at 1 kHz - that helps avoid overdriving the output)

73 de Dave, N7AIG

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Steve N4LQ

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The Kenwood has 3 preset EQ positions in the menu, Bass boost, F. Pass and High boost. F. Pass is set to emphasize the vocal range which is where I had it set. High boost is really crisp. You can also set custom levels via the computer.
All this is beside the point when it comes to selective fading distortion however.
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k3Tim

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Hi Steve,

A few questions if I may.  In the video the Flex has the filter bandwidth (passband) reduced compared to the signal showing up on the Panafall.  How many kc. wide was signal and the filter setting?  The original query states the AM on Flex sounds bad, somewhat implying SAM sounds fine,  Is this inference correct - ie. SAM sounded good on the Amateurs AM signal?  I assume you've tried the broadcast AM in MW and HF and results for AM versus SAM were?  
For the other receivers do you know the exact method of AM demodulation?

With the update to SAM, setting the RX EQ, and running the audio thru PC to perform equalisatiion, I was simple blown away by Japan Radio broadcast booming in from the Pacific in the early morning hours.  Their signal is 12Kc wide, and the SAM de-mode makes it sound like studio quality.

_..--
 TiM  
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Steve N4LQ

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Tim: Thanks

Filter width: Varied throughout the video from maybe 4 khz up to 8 khz. This has no effect on "selective fading distortion" but certainly on the fidelity. The TS-2000 uses a separate hi-cut and low-cut passband adjustment to set the bandwidth. I had low cut at 200 khz and varied the hi-cut from 4 khz up to 5khz which is max.

I only compared AM to AM since the TS-2000 does not have SAM however I can tell you that even in SAM mode I still experience bad distortion with the smallest amount of fading. I had first referred to listening to AM guys here within 100 miles on our 75 meter AM net during the late afternoon. SAM helps some but not totally and is somewhat bothersome to use in a net where everyone is using boatanchors and as much as 5 khz off frequency. 

Again I will say that the Flex/ssdr sound very nice on the AM BC band for whatever reason I can't explain. 

I haven't listened to Japan lately so I can't comment. 

Finally....as I mentioned previously, I can use any of my old receivers and they sound better than this however the lowly TS-2000 does sound better on AM than about anything else I've got. 

Steve N4LQ
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Jim - N7CXI

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Steve,
Any chance you could cut the Flex RX EQ at 63 and 125 to the minimum, and see if that has any effect on the distortion you're hearing?

Thanks,
Jim N7CXI
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Steve N4LQ

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Jim
No....Cutting the LF audio doesn't improve the distortion however......

I was able to make a big improvement by simply eliminating one sideband while in AM mode. IOW...Set the bandwidth to 6 khz, center the AM signal in the passband then grab the edge and pull one of the sidebands in toward the carrier thus eliminating it. Most of the selective fading stops and audio fidelity is unchanged. I assume the sidebands are out of phase due to fading, which is typical, however the net effect is more drastic in the 6000 than in most receivers. 

Doing this enabled me to copy a weaker AM ham on 3590 khz just 5 khz away from a huge AM guy on 3885 khz. The TS-2000 was totally swamped by the stronger station, K4KYV who runs a virtual AM broadcast station!

Well this is a workaround and more needs to be done.

Steve N4LQ
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Jim - N7CXI

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I don't know how AM is demodulated in SmartSDR - I'll guess just the magnitude of the I&Q stream if it's like other common DSP demodulators. It will be interesting to see how AM is demodulated in the TS2000. If it's done in hardware, I should be able to find it in the schematic. Thanks for the detailed testing - you've made an interesting set of observations.
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Jim - N7CXI

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I had a look at the TS2000 service manual. The AM demod is done in the DSP, so there is no opportunity to learn how it works.

Since the 6000 series and SmartSDR have the benefit of much lower overall distortion, (including no filter phase distortion) there's no reason our 6XXX radios couldn't have the best demodulators available. The problem may be identifying what's the best and implementing it. It's an interesting problem to have, since it could be fixed as easily after-the-sale as before.

As an active AM experimenter and founder of two DSP engineering companies, I suppose I should look into it. Overcoming entropy (laziness) is usually the problem around here, at least... ;-)

73,
Jim N7CXI
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k3Tim

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Laziness is a good motivator...

I checked the TS2000 manual also  

{   Google:  "TS-2000X_Service_manual.pdf"    }

and agree with you on AM detector / DSP.  

On page 15 it says:

In AM mode, a 455kHz signal passes through the AGC amplifier (Q51) and amplifier (Q48 and Q45) and is detected by D58. The detection signal retrieved for the AGC is rectified, passes through the DC amplifier (Q39) for AGC control and goes to the Q37 gate terminal (G2).

but believe this is referring to the AGC voltage under AM mode.

From "Digital Signal Processing in Communication Systems", by Marvin Frerking (aka. W0EKC) one can demodulate the AM signal by:
   
     demod-AM = sqrt(I^2 + Q^2)

----
The text is recommended for anyone wanting to know the digital processing details.  Be advised a "little" math is involved...

_..--
k3Tim 


PS:  listened to a two hams running AM in AM mode and never heard distortion.  On SW bands don't hear it either except when there is a deep carrier fade, as one would expect.
(Edited)
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Burt Fisher

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I was on AM on 7.290, got less than glowing reports on the 6300, was using the recommended profile for the FHM mike, switched to a Heathkit DX-60, got immediate praise. A 50 year old rig beats the 6300?
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Jim - N7CXI

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"   demod-AM = sqrt(I^2 + Q^2)"

That's the IQ stream magnitude I was referring to. It's the most basic form of AM demodulator. Easy to code.

Burt, SmartSDR 1.4.3 changed the game for the TX EQ. I have mine reworked to a degree, but still have lots of room for improvement. It's at least possible that the image I attached will show up... Your mic and voice will no doubt be different, but I've had compliments with this curve and the PR781.

Jim N7CXI
(Edited)
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James Whiteway

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Bert, I think using the built in profile for the FHM mic may not always work as well as setting things up. I have a Heil PR781 and have used the included profiles with it, and "most" of the time, everyone says it sounds like me. But, I have also found that with some careful tweaking of the settings, I can get a clearer signal that I have been told, sounds more natural. I think the profiles that are included in SSDR, should be viewed as a starting point and not a carved in stone absolute. Sometimes, depending on the person's voice, the default profiles will work fine. Other times, maybe not.
james
WD5GWY
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Ken - NM9P

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It is only a matter of getting the adjustments right. The 6000 is capable of excellent audio even on AM. But you cannot use the processor at anything more than NOR. And it might be best not to use it at all. And getting the EQ right takes work. The default profile may not be right if it wasn't changed with the 1.4 update. The EQ characteristics were changed with 1.4.
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k3Tim

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Having a local receiver helps one in setting the mic/equ up (statement of the obvious).  Even a Softrock SDR should be good enough.  

I tried the AM transmit mode with input to BAL and it sounds fine in local receiver.  Same with FM / SSB.

k3Tim
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Burt Fisher

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My 6300 sounds good to me but not to others. I need a Flex owner to help me on the air, if anyone has time.
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Steve N4LQ

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I'm willing.  
Email me steven4lq@gmail.com

n4lq
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Jim - N7CXI

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Burt,
If you haven't resolved it by the weekend, I'll be home by then and happy to work with you.
email is barberaudio@gmail.com .

Jim N7CXI
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Burt Fisher

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Thanks Jim, I hooked up with N4LQ, it was interesting that the FHM stock mike sounded so much worse than a D-104. In the end even the D-104 was hardly impressive. How does a DX-60 beat the ultra modern 6300?
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Norm - W7CK

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I have never gotten good audio reports from my 6700.  Its amazing that an average microphone can sound good on so many different rigs, but plug it into a Flex and you have to muck around forever just to get it to sound acceptable.  I think maybe the software still needs some fine tuning in this area.  I have to have different profiles for AM, FM and SSB - doesn't make too much sense.  We'll see what the future holds....
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Alex - DH2ID, Elmer

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I get very good audio reports for my 6500 with a YAESU MD-100 microphone.
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Ken - NM9P

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Burt,
I am pretty much occupied all week, but Saturday Afternoon I might have some radio time and perhaps we could do some tests if you still need it.

Ken - NM9P
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Steve N4LQ

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Burt and I worked on this for about an hour yesterday. We about wore out the virtual sliders.  It seems like there is some degree of distortion no matter what we tried. Possibly he has some RFI on the mic cable but we didn't get into that. 
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Ken - NM9P

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Burt, I don't remember from your earlier posts...
Have you had the factory check/correction for the mike connector grounding issue done?  Perhaps, if you have the issue, yours only is showing up on AM?  Just a thought.
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Steve N4LQ

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Tell us more about that Ken. News to me.
Also we are running some issues together. Burt and I tested on SSB only and this thread is about AM receive so we've deviated off track a bit. 
Good advice though. 
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Ken - NM9P

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Check the end of this thread.
https://community.flexradio.com/flexr...
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Burt Fisher

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I could always use a second opinion on my audio.
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Steve N4LQ

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Ok...Sounded bad twice.
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Burt Fisher

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Not helpful
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Steve N4LQ

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Do you want to try 20m tomorrow? 
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KY6LA - Howard, Elmer

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The new Slice Record and Playback function works rather well... it now includes the ability to record Equalized and Processed Audio from your mike and from DAX.   At least with SSB (I have never tested AM) it effectively alleviates the need for a second receiver to set up your audio
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Jim - N7CXI

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No, it doesn't work on AM.

It's just OK, though. We AM experimenters are a rugged bunch - we don't need no
monitor nohow. ;-)

Jim N7CXI
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Tim - W4TME, Customer Experience Manager

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Official Response
I have entered this problem report into our bug tracker for additional investigation. Thank you for the defect report.
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Barry N1EU

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I'm a little late to this thread but that video seems to show the TS-2000 set for a 4KHz audio passband and the Flex set for a 2KHz audio passband (after you narrowed it with the mouse early in the video).  That's the fundamental reason the Kenwood sounds so much better.

Barry N1EU
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Steve N4LQ

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Barry etc. I started with 6 khz bw on the Flex then 5.6 khz and tried various widths by dragging the bw bar. "Sound" or hi-fidelity isn't really the issue here. The issue is susceptibility to phase distortion due to selective fading. Actually, setting the Flex for a wider bandwidth exacerbates the problem while dragging one side of the passband toward zero tends to improve things slightly, having eliminated one sideband  

A question I have is...What role does the received carrier play in the demodulation process? In a typical diode detector the relationship between carrier strength and sideband strength is important. If the carrier fades below the sideband strength, overmodulation occurs and the resulting audio is distorted however what goes on inside a DSP chip is beyond my pay grade. 

 
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Jim - N7CXI

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We don't know what flavor or variation of AM detector is used in SmartSDR, *so what follows is speculation.* 

The "AM" detector *could be* a simple "AM = sqrt(I^2 + Q^2)" magnitude detector, as described elsewhere in this thread - an envelope detector in analog terms. In this case, the relationship to the carrier is only that found in the signal - subject to selective fading and all other forms of phase distortion.

Beyond the textbook magnitude detector we venture into the realm of pure speculation, since details vary widely between implementations. Most of the more sophisticated detectors use some variation of a synthesized carrier, phase-locked to the signal carrier. (SAM) The more sophisticated ones make some attempt at identifying/correcting selective fading by comparing the integrated magnitude of each sideband against both the signal carrier and the synthesized carrier, as well as each other. That computed relationship is used to modify gain envelopes that control the relative levels of the carrier, synthesized carrier and the sidebands before the actual demod. The detector can also usually be switched between modes to force LSB, USB or DSB or "auto" sideband detection modes. "Auto" mode allows the detector to select which sideband to use based on the above calculations, as well as user input for threshold parameters.

My reason for this extended speculation is to point out that AM detectors can be very simple or very complex. For usn's amateurs, the beauty of it as deployed in "distributed" SDR schemes like the Flex radios is that both firmware and software can be updated after the fact based on the development priorities of the vendor. In the case of SmartSDR, I believe it's even possible for a third party to sign up for a slice IQ stream and perform custom demodulation on it. Now whether that can be routed back to the hardware audio I don't know, but if not it seems to me that DAX or even a direct connection to the PC audio device could be used to work around it.

In other words... If we don't like the AM demod, some enterprising young tinkerer could build one we do like, even if Flex doesn't give it the priority we would prefer.

My .02,
Jim N7CXI
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Steve N4LQ

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Perhaps we should implement the 1N34 mod?
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N7AIG

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ROFL!! I love it...

73 de Dave, N7AIG
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Steve N4LQ

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Or perhaps we can find room for a 6AL5 and use the extra diode for hardware AGC.
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N7AIG

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I have been thinking about the question posed earlier, "in a DSP, how is the carrier dealt with?". I concur that the typical implementation of AM detection is normally by way of envelope detection, formed as the RSS of the I and Q amplitudes. In that kind of situation, the Carrier normally appears as a DC offset, and is simply discarded. 

Another viewpoint is that you are forming the square root of the conjugate squared signal, which in the Fourier domain is a convolution among all the sideband spectra on both positive and negative frequency sides, and the Carrier, which is a delta function at DC. 

Either way, the Carrier sort of falls out as unimportant and even extraneous. I have to think back real hard to remember why it was ever considered important...

73 de Dave, N7AIG
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N7AIG

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sorry, my bad...  The conjugate squared signal is an autocorrelation in the Fourier domain (not a convolution), and that carrier is used to sweep across the sidebands to produce a nice strong audio component. So the carrier really does take part in the calculation. You'd want the carrier to be much stronger, (e.g., 4x?) than the sidebands in order for its contribution to the autocorrelation to stand out. If you don't have a carrier, then you have to inject one yourself.
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Jim - N7CXI

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4X is the standard recipe, yes.

Where it gets tricky is that one or the other of the sidebands will often be selectively faded or phase distorted, and the carrier may or may not have adequate SNR against the sidebands to correlate with anything. This goes back to the issue at the top of the thread. If you choose to address that, then various time-domain tricks are normally employed.
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Jim - N7CXI

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You also can't assume the frequency of the carrier is in the middle of the filter. The synthesized carrier has to correlate to the signal carrier when the SNR is good enough to do so precisely, but not update when it isn't. With the SmartSDR "SAM" demod you can be off to the side a bit on an AM signal using the "AM" demod, then click "SAM" and listen to it slide over to the signal carrier frequency and lock. If the signal carrier fades away, the SAM carrier just stays put until the SNR comes back up.
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Steve N4LQ

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Speaking of SAM. I notice hearing the signal slide over to the carrier frequency at times but not visibly. The scope's carrier stays put. I can drag the passband around but I don't hear the carrier return. I don't think PowerSDR worked quiet this way.