Why So Much FT8 Splatter?

Discussion in 'Working Different Modes' started by MM0IMC, Aug 28, 2019.

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  1. MM0IMC

    MM0IMC Ham Member QRZ Page

    I guess this is rather a rhetorical question, but why is there so much splatter by some FT8 stations here in Europe?

    Seems that quite a few operators don't know how to monitor or control their rig's ALC levels!

    10m FT8.jpg

    In this case, it was a German station.
     
    KK5JY likes this.
  2. SM0AOM

    SM0AOM Ham Member QRZ Page

    FT8, by definition, cannot cause "splatter", because it is a constant-amplitude emission.

    What you are seeing here is instead in-band harmonic distorsion which causes spurious emissions to occur
    at twice, three-times and four-times the FT8 carrier centre frequency.
    ALC also has very little to do with this form of distorsion.

    Unfortunately, not so much can be done in modern SSB equipment to mitigate this problem,

    It is a design flaw in many transceivers that there is a lot of in-band distorsion caused by speech processing and general non-linearities in the audio chain.
    One possible remedy is to reduce the mic gain below the point where any speech processing sets in.

    Otherwise, another remedy is to either reduce power or to run professional equipment which is designed to have low in-band distorsion products.

    73/
    Karl-Arne
    SM0AOM
     
    Last edited: Aug 28, 2019
    W4RAV likes this.
  3. MM0IMC

    MM0IMC Ham Member QRZ Page

    You're of course, correct. It's distortion. I would call it IMD, personally. My rig uses a low level audio input at the back of the rig. I seldom use compression whilst using SSB, but that's nothing to do with the data mode on my rig.
     
  4. SM0AOM

    SM0AOM Ham Member QRZ Page

    Constant envelope emissions cannot cause IMD.
    This is in-band harmonic distorsion.

    73/
    Karl-Arne
    SM0AOM
     
  5. ND6M

    ND6M Ham Member QRZ Page

    Perhaps, some of the "offending" stations should just follow the setup info for the mode.
     
    MM0IMC likes this.
  6. SM0AOM

    SM0AOM Ham Member QRZ Page

    Unless there is a reliable way to detect harmonic generation in the audio passband, it is very difficult to achieve this.

    Unfortunately, most amateur transmitters have a "mic gain" control instead of the VOGAD used in professional gear.

    A "mic gain" control together with audio speech processing may create serious distorsion, which is made worse by not understanding what audio processing does.

    This form of distorsion is almost impossible to detect by oscilloscope observation of the RF envelope, and has to be observed with an RF spectrum analyser loosely coupled to the transmitter output.

    Former colleague SM5HP has studied the in-band distorsion problem in great detail, and some, even quite high-priced, amateur transceivers are very bad in these aspects.

    73/
    Karl-Arne
    SM0AOM
     
  7. KT5WB

    KT5WB Ham Member QRZ Page

    This is one of my main complaints with FT8. There is no good way to contact the operator and advise them of their signal quality issue.
     
  8. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Use the 'free text" function.
     
  9. G0GSR

    G0GSR Ham Member QRZ Page

    But now the "mic gain" is commonly achieved in software AFTER the ADC so if this is overloaded, no adjustment to "mic gain" can correct it.
    Progress? No, cost saving.

    Frank
     
  10. SM0AOM

    SM0AOM Ham Member QRZ Page

    You are right, this is "backwards progress".

    If there would be a separate "line input" for digital modes eqiupped with a decent VOGAD, either implemented in hardware or software, this would not happen.

    I think that I have become spoiled using professional-grade SSB gear for a long time.

    73/
    Karl-Arne
    SM0AOM
     

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