Distortion on Receive

Discussion in 'Amplitude Modulation' started by K0OKS, Aug 10, 2021.

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  1. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Has anyone else been experiencing addition distortion on receive lately? I noticed it on 75m on lower S/N AM signals. I thought it might be my old Drake 2B, so I switched my receive to the 7300 and got the same distortion.

    Then I was trying to talk with N2DTS on 40m today on my Valiant and he said I was horribly distorted and uncopyable, while I was at the same time listening to myself crystal clear on the Dallas KiwiSDR (I am in St. Louis).

    There was a lot of lightning static each time. Anyone else experiencing a lot of distortion on receive on less-than-strong stations?
     
  2. K5UJ

    K5UJ Ham Member QRZ Page

    I've worked hams who had distorted audio, but off the top of my head I can only think of three factors: 1. Distortion stemming from selective fading, which is a propagation problem. You get a lot of one or both sidebands, but the carrier is diminished. It's especially noticeable with envelope detectors. Synchronous detectors don't have this problem. 2. Operators using crazy high positive modulation peaks which will show up as distorted audio in an envelope detector. Again, a sync. detector will clean that up, but highly asymmetric modulation exacerbates selective fading distortion. 3. Hams with vintage ham rigs that have ham rig audio. Single ended audio stages, and el-cheapo ham audio iron. Each stage adds some distortion and it snowballs and if they have mic problems and weak signals, the end result are signals impossible to copy. Clean clear undistorted audio can be readable, even with a weak signal and QSB. That's all I can think of. In my opinion, a fellow with a 100 w. rig that has very low distortion, such as 1% THD or less and can handle processed audio, will probably get through better than another ham with a vintage ham 300 watt rig and no processing. Of course this assumes everything else such as antennas are equal. By the way, to my amazement, I occasionally encounter operators who have voices that sound distorted. They just have crappy voices. It really does help to have a clear resonant speaking voice. But that's a separate topic.

    I actually like using a receiver with an envelope detector because it allows me to have an uncolored listen to a ham's audio. If there's something wrong, an envelope detector will reveal it. Product and sync. detectors can mask problems.
     
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  3. K0OKS

    K0OKS Premium Subscriber QRZ Page

    I am using a Valiant with a lot of mods I did myself. It sounds crystal clear (terrific in fact) on web SDRs and on my local receivers. Anyone with a decent receive on me always compliments the audio. I have a full audio chain with mild compression and an Re-27nd mic. It’s not my transmitter…

    It must be the band, as I have heard terrible distortion in many signals lately, only on AM including on N2DTS when he and I were trying to communicate. On his signal it was only occasional, but he also runs more power, and may have a better antenna system than my ladder line fed doublet at 60’.

    Note I have tried multiple receivers and antennas. They receive distortion is something band related.

    I was mainly wondering if anyone else had noticed this lately.
     
    Last edited: Aug 10, 2021
    SWL37632 likes this.
  4. WA3VJB

    WA3VJB Ham Member QRZ Page

    Joe, next time you're settled in and stations on the band sound distorted, try taking a listen on one of the Kiwi SDR nodes in your propagation area. They have the ability to select synchronous detection which could mitigate signal path distortion, and reveal whether the distortion is at your receiver, or the bad luck of encountering station(s) transmitting a distorted signal.

    Here's one in Missouri, pre-tuned to 3875, and pre-selected to synchronous AM reception. Give the link a few seconds to connect. For some reason it is slower than other nodes I hit.

    http://kb0fxkiwisdr.hopto.org:8073/?f=3875samz09
     
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  5. K0OKS

    K0OKS Premium Subscriber QRZ Page

    I have been using the WebSDRs quite a bit. I forgot that Kiwi does have the synchronous detector. (It has so many cool features.) I will give that a shot next time.;)
     
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  6. SWL37632

    SWL37632 QRZ Member

    When I was an avid BCB DXer, I could routinely experience this phenomenon.

    Rigs used were mostly tube types such as NC-100A, NC-101X, modified ARB, 'Souped up' AA5's, as well as transistor auto radios.

    Synchronous detection, introduced in the Sony ICF 2010, was very sought after but was quite expensive when it came out.

    Later, I purchased a Sherwood Synch Detector and it was/is quite amazing ...still use it with my modified 455 KHz IF RX's.
     
    Last edited: Aug 11, 2021
  7. N2DTS

    N2DTS Ham Member QRZ Page

    Joe was horribly distorted over a long time and many transmissions.
    I was working on something in the shack and had the RX on and heard him calling CQ.
    It sounded like a modulated osc with a bad power supply AND a lot of RF getting into the audio.
    It was SO bad I could only make out the CQ part.
    I fired up the TX and told him he was distorted (why no one was responding to his CQ).
    Quite a while later he was back with the same thing, but one short (and one only) transmission he was clear and I said "hey, you got it fixed" but he went back to being all distorted.
    He made some long transmissions but was always badly distorted.
    Something new for me, I have heard selective fading and overload, and excessive modulation, and phase shift of the vfo but never heard anything like this in the last 50 years.
    Joe's signal was not weak.
    I had the icom 705 on, then tried the homebrew and the airspy with sync detection, I also tried FM narrow.
    The sdr display showed nothing but bass, no high end and more audio on the upper sideband.
    Other stations were fine at various distances.
    I could not try the Zero 5 antenna as its disconnected for a new roof on the house and only had the dipole.

    I have no idea what could cause such bad distortion and how it could last so long.
    I was sure it was an old ww2 military transmitter with a bad power supply, carbon mic and a LOT of rf getting into the audio.
     
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  8. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Thanks for the reply, Brett. The weird thing was that after the first time you reported I was distorted I started listening to myself on the Dallas kiwi sdr, and it sounded quite good there.

    No clue what would do this.
     
  9. W9BRD

    W9BRD Ham Member QRZ Page

    More like product detectors and sync detectors can end-run problems apparent in envelope detection.

    A product or sync detector acts as an amplitude-linear frequency shifter to bring sideband audio back to baseband. It uses the energy of a local carrier to do this work. (In doing so it may leverage either or both sidebands, according to the particulars of the receiving setup.) All it needs to render audio is sideband energy -- okay, and for a sync detector, enough carrier for lock -- and you're good.

    High-quality envelope detection is more fragile because it uses the energy of the carrier to do the work of heterodyning the sideband(s) back to baseband. This makes envelope detection dependent on the correct amplitude and phase of the carrier relative to the sideband(s), and (for DSB reception) the phase of the sidebands relative to each other. Propagation can screw that up handily, and does. So can reflections, as we experience in some parts of our town on particular NYC-metro AM stations as a result of reflections from HV-power-distribution lines; position your radio at just the right spot where direct and reflected signals coincide in exactly the wrong way, and you can experience fixed carrier-trough and/or intersideband misphasing distortion that can be alleviated only with a directional receiving antenna; changing your location; or (preferably SSB, to avoid intersideband problems) product/sync detection.
     
    Last edited: Aug 11, 2021
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  10. SWL37632

    SWL37632 QRZ Member

    You just refreshed my memory...

    There is one particular intersection with a powerline junction in my town (So. Cal) where one particular local AM BCB (about 30 miles away but 50KW) consistently shows this signal distortion. The modulation gets distorted and the voices sound muffled....since I've noticed it over several years when driving through the intersection and seeing the multiple powerlines, I assumed that it was some kind of reflection based distortion but thought it was a unique anomaly.
     

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