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Can a basic envelope detector demodulate a DSB-SC signal?

  1. Aug 5, 2017 #1
    ie: http://m.eet.com/media/1072296/RFFig84.jpg

    I am 99% sure the answer is: no, it can't.

    I've read that DSB-SC demodulation requires the carrier frequency to be put back into the signal at the receiving end prior to demodulation (or as part of the demod process), and that if its too out of phase with the original carrier waveform you lose the message. I've seen the math for that, and the math makes sense...what I don't understand is why a basic envelope detector won't work for DSB-SC.

    Is it because of the phase reversal? Wouldn't the phase reversal have predictable effects though on the envelope? Wouldn't it double the frequency if a simple envelope detector was used, and couldn't one also just half the frequency again to correct for this?
     
  2. jcsd
  3. Aug 5, 2017 #2

    NascentOxygen

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    Sketch what the DSB waveform under sinusoidal modulation would look like (as v(t) vs time) then chop off the negative excursions and smooth it, just as is done to illustrate how this recovers the modulation from an AM signal. After applying this process to DSB, what do you end up with as the "recovered signal"?
     
  4. Aug 5, 2017 #3

    sophiecentaur

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    DSBSC looks like a 'string of beads' on an oscilloscope, rather than the diagram in the OP. Just Google images DSB SC waveform for hundreds of pictures. A diode will cut off the bottoms of the beads. This will produce a horrific sounding audio with what is effectively all second harmonic and no fundamental. 'Understandable' speech, perhaps and you would definitely know what the programme material is but not a good 'demodulator'.
     
  5. Aug 9, 2017 #4

    tech99

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    Nothing more to add, other than as a matter of interest, if the carrier is re-inserted at slightly the wrong frequency, the audio still sounds quite intelligible. In effect, we then hear two voices speaking, one a little too high and the other a little too low. So the result is not too bad. I have found that the de-tuning range is less than for SSB, maybe 100 Hz, and in practice the ear tends to hear the higher pitched voice. All the text books say the carrier must be inserted with the correct frequency and phase.
     
  6. Aug 9, 2017 #5

    Averagesupernova

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    Quite correct. Yes, the carrier must be inserted with the exact frequency and phase to be identical in quality to AM. But that is not to say that voice (for instance) is unintelligible with the carrier off a hundred hertz or two.
     
  7. Aug 9, 2017 #6

    sophiecentaur

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    We used to call it "Donald Duck" voices.
    SSB used a locally generated oscillator for the demodulation. Quite good enough for spoken communication - especially after a bit of acclimatisation.
     
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