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.
San Juan Island, Wa.
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.
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.
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
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.
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
All this is beside the point when it comes to selective fading distortion however.
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.
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.
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.
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.
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.
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
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.