Convert SSB recording on tape to AM?

Discussion in 'Microphones, Speakers & Audio Processing' started by KA2PTE, Aug 12, 2019.

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  1. WA7PRC

    WA7PRC Ham Member QRZ Page

    Audacity can shift the frequency up/down without affecting the speed/tempo. From the Effects menu, select Change Pitch...
     
    N2EY likes this.
  2. AG6QR

    AG6QR Subscriber QRZ Page

    That won't work, but I encourage you to try the experiment instead of taking my word or anyone else's for it. It's a cheap experiment to perform.

    Whatever you transmit with an AM transmitter, when you look at it in the frequency domain, it will always have a center carrier frequency, and two symmetric sidebands above and below the carrier. The original SSB transmission was not symmetric at all. I hope that's enough to explain why the AM transmission will not contain the same information as the original SSB had.

    For another thought experiment, consider what would happen if the audio fed into the original SSB transmitter consisted of a single pure 800Hz tone. The SSB transmitter would transmit a single frequency, 800Hz above the suppressed carrier. The AM receiver would hear this as a dead carrier, slightly mistuned, but the result would be silence in the speaker. If the tone had been 1000Hz or 600Hz, the AM receiver would have still heard a silent carrier. This may help explain why the audio recording from the AM receiver didn't capture enough info to recover the entire original audio.
     
  3. KT1F

    KT1F Ham Member QRZ Page

  4. KA2PTE

    KA2PTE Ham Member QRZ Page

    Thanks @KT1F , I see someone there was programming something and did some kind of decent conversion.

    I wonder if the fact that we have a solid 400hz tone in most the recording, that provides a stable reference to go with.
     
  5. KT1F

    KT1F Ham Member QRZ Page

    I don't think that helps. As others have said, the problem is that a lot of the information is missing.

    Could it be possible to build some really smart system that can learn enough to fill in the gaps and make something intelligible like what K7TRF described? Maybe but it's not a matter of just inserting a "BFO" signal and resolving it. The fact that a human can understand it to a limited degree depending on various conditions suggests that some machine interpretation might be possible. However, I think some of that human ability comes from recognizing common words etc. e.g., it's easy to tell if someone is calling CQ.

    What do you mean by a 400 Hz tone? If you're hearing a 400 Hz tone on the recording of SSB received in AM mode then ... I think that means it's either being introduced into the recording after the demodulator or the SSB signal has two tones 400 Hz apart. If the original audio had a 400 Hz tone then wouldn't hear it. It would product a steady "carrier".
     
    Last edited: Aug 14, 2019
    AG6QR likes this.
  6. KA2PTE

    KA2PTE Ham Member QRZ Page

    You can hear it in the first few seconds of the sample recording mp3....and a little in the 2nd half.

    Yes, its embedded in the original transmission broadcast in SSB. I have worked in avionics so I recognize the noise.
    Its a constant frequency background noise, I'm guessing its originally around 400hz because
    the recording was done on an aircraft. Aircraft 3 phase 115VAC power is always running at 400hz and people who work on
    airplanes with communications say that tone is always there and hard to get rid of, especially on older designs. This recording is from 1970.

    So if you cut these portions containing that resonant 400hz tone off and came up with some programming that merely converted it to a 400hz sine wave, and applied that formula
    to the entire clip, I think the rest should fall in place pretty decent.

    After all, voice modulation is a sine wave - a complex sine wave with varying time and amplitude, but a sine wave nonetheless.





     
  7. WA7PRC

    WA7PRC Ham Member QRZ Page

    In addition to the frequency shift I previously mentions, Audacity also has a notch filter. Select the part that you want to effect. From the "Effects" menu, select "Notch Filter...". Then, enter the frequency and Q. :)
     
  8. SM0AOM

    SM0AOM Ham Member QRZ Page

    The only way to get a 400 Hz tone component after demodulating an SSB signal in an envelope detector, is to have two components spaced 400 Hz apart, such as a 400 Hz "cockpit whine" and its second harmonic. The difference frequency or "second order product" may then be heard.

    As others have stated, once the SSB signal has passed through an envelope detector, both the frequency and amplitude characteristics of the baseband have been lost due to the mixing properties of the envelope detector.

    Using signal theory language, the signal has been convolved with itself. No reasonable amount of processing can restore the original baseband after this have happened.

    73/
    Karl-Arne
    SM0AOM
     
    Last edited: Aug 15, 2019
    AG6QR likes this.
  9. AG6QR

    AG6QR Subscriber QRZ Page

    Well, there is one other way. If the recording is made on board an aircraft with 400Hz power, the 400Hz tone may be introduced into the audio chain somewhere after demodulation, for example if power supply ripple gets into a receiver's audio amplifier, or if an audio cable between a receiver and a recorder is inadequately shielded. If this happened, the tone would have nothing to do with the SSB signal originally sent via RF.

    I don't know if that's what happened, but the description does say that the recording was made on board an aircraft.

    Regardless, you are completely correct about the futility of attempting to reconstruct the original audio.
     
  10. SM0AOM

    SM0AOM Ham Member QRZ Page

    This is indeed the case, and if a 400 Hz "cockpit whine" is injected into the audio chain it does not need to be present in the RF signal.

    It then comes to the question how this recording was made. Was there an AM receiver onboard the aircraft that was tuned to an SSB "off the air" transmission, or was there an SSB transmitter onboard a large aircraft or on a nearby A/C that was tuned in on a another AM receiver?

    This could explain the strong 400 Hz tone in the beginning of the clip.

    At this time (1970), the airline industry still was in the transition period between AM and SSB HF for air traffic control, and the systems existed side-by-side.
    Both 7 kHz (AM) and 3 kHz (SSB) channels could be encountered depending on age of the aircraft and where in the world the operations were conducted.

    73/
    Karl-Arne
    SM0AOM
     

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