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Re: [30MDG] PSK ?

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  • Andy obrien
    And while on the subject of QPSK, here is an old article about PSK31 QPSK that I wrote around 2000 for the Logger32 help files, it includes quotes from Peter
    Message 1 of 6 , May 2, 2011
      And while on the subject of QPSK, here is an old article about PSK31
      QPSK that I wrote around 2000 for the Logger32 help files, it
      includes quotes from Peter Martinez that he granted permission for me
      to use .

      Andy K3UK



      QPSK (Quaternary Phase Shift Keying) Operation
      Andrew J. O'Brien, KB2EOQ

      QPSK is referred to as an error-correcting PSK mode. Strictly
      speaking, it is not error-correcting in the traditional sense, but
      we'll
      leave that for another discussion. You will find it very useful in
      copying very weak signals. You will also discover that it is used
      less than BPSK.
      The usual convention among QPSK'ers is to use BPSK to establish a QSO
      and then switch to QPSK You will rarely find someone
      calling CQ in QPSK mode. Some radio amateurs will switch to QPSK if a
      BPSK QSO with weak signals is producing poor copy.
      PSK veterans will point out, however, that while QPSK outperforms BPSK
      when weak signals are the issue, QPSK will perform no
      better than BPSK if noise is the signal limiting condition.
      In the ancient, early days of PSK31, in the last century, QPSK was
      used as the mode of choice for some PSK nets, but that does
      appear to be the case in the 21st century.
      .
      Tuning a QPSK Signal
      snip....

      QPSK can perform better than BPSK when band conditions are poor due to
      polar flutter. At such times you may be able to get
      copy from a QPSK signal even though the "cross" in the tuning window
      is poorly formed.
      Note: QPSK requires that BOTH stations in the QSO be using the same
      sideband! While this is not important for BPSK QSOs, it
      vital when using QPSK.

      QPSK-Reversed
      BPSK is demodulated the same way whether you are on upper or lower
      sideband. This is not true for QPSK, which operates
      like RTTY. You must shift the signal in a way that the decoder expects
      or it will not decode. In the case of QPSK, this leads to
      problems with standards, since the mode is so new.
      In actual practice, most hams appear to be operating BPSK using AFSK
      and upper sideband. This means that, when they switch
      QPSK (without reversing), they must be decoded by the other station
      using upper sideband and QPSK (without reversing).
      However, the standard of RTTY operation is lower sideband, and this
      means that most hams, operating as just mentioned, are
      operating QPSK-reverse, based on the RTTY standard.
      What this really means is that, when you and another station decide to
      switch to QPSK, if you do not know whether that station is
      using upper or lower sideband, there is a chance that you will not
      decode that station. If you click again on the mode pane, it will
      switch from QPSK to QPSK-reverse, and you should begin to copy.
      One trick is to set all three Rx windows on the signal to be copied,
      set one aux window at QPSK and the other at QPSK-reverse.
      When you see which Aux window starts to print readable copy, switch
      your main Rx window to that mode and you can then

      So What Is QPSK? How QPSK Got Its Name
      Peter, G3PLX, says in an article entitled "PSK-31, A new
      radio-teletype mode with a traditional philosophy," says that he
      called it
      "quadrature polarity reversal keying" (which of course would have come
      out as QPRK), but that everyone else calls it quaternary
      phase-shift keying (QPSK). It is an error-correcting mode that relies
      on four phase-shifts rather than two, to create the basic
      scheme.

      Peter Martinez' (G3PLX) Description of QPSK
      The QPSK mode used in PSK31 takes the binary data-stream, at the point
      where it would otherwise go direct to the BPSK
      modulator, and feeds that through a 5-bit shift-register. A logic
      operation then forms the parity function of the 1st, 2nd, and 5th
      stages, and another forms the parity function of the 1st, 3rd, and
      5th, giving two bit-streams at the same 31.25 bps rate as the
      original data. These two bits form a binary number, the four values of
      which are mapped to the four possible phase-shifts in the
      QPSK modulation. Thus a single data-bit from the source results in a
      5-bit-long predictable sequence of 90-degree and 180
      phase-shifts, interleaved with those of following and preceding bits.
      In the receiver, a Viterbi decoder is used to keep track of all 32
      possible combinations of guesses at the transmitted datastream,
      a running total of how well the received pattern of phase-shifts
      matches each guess. The clever thing about the Viterbi decoder is
      that it can be sure that if it throws out the worst 16 guesses before
      it receives the next signal (which doubles the number of
      to 32 again), it can never get it wrong, and after about 20 more
      signals, it can be pretty certain that its guess 20 signals ago was
      right. The snag is that it can only output its best guess after a
      delay of 20 signals, or 640mS in the case of PSK31. Longer Viterbi
      decoders are possible, but the amount of computation doubles at each
      extra signal delay, and for PSK31, which is used for live
      QSOs, the delay would be too long.
      More Facts to Consider About QPSK
      The relationship between BPSK and QPSK is that both have the same
      bandwidth, but that QPSK uses that bandwidth for two
      signals, shifted-from each other by 90 degrees. The extra signal is
      used to transmit redundant information for error-correction.
      reduces the signal-to-noise ratio for QPSK by 3 dB, in comparison to
      BPSK. The expectation is that the error-correction will more
      than make up for this difference. The results are not in, and one goal
      of current PSK31 activity is to evaluate and compare BPSK
      QPSK, to determine the situations in which each method is superior.
      According to Peter, BPSK should be superior under conditions of
      ordinary noise (white noise or random noise) but that under
      conditions of fading or real-life interference (as contrasted with
      random white noise), QPSK may be superior. More evaluation is
      needed.
      For a good introduction see NB6Z's excellent discussion of QPSK at
      http://www.teleport.com/~nb6z/psk31.htm.
      *Peter Martinez has granted Logger32 permission to use his above description


      On Mon, May 2, 2011 at 10:30 AM, Bob N3PPH <rsheskin@...> wrote:
       

      You will find by far more BPSK31 signals than QPSK31. A page you may want to check out just to see and hear many of the different modes http://www.w1hkj.com/FldigiHelp-3.20/Modes/index.htm.

      73,
      Bob N3PPH

      On 5/2/2011 10:23 AM, Bernie wrote:
       

      As you will see, I am quite new to digital. One question I have is what is the difference between QPSK and BPSK. I use HRD and DM780 and when I open it up, how do I know which one to choose. Please excuse the "dumb" question but I have been confused by the choice.

      73 de Bernie - W8NBC
      SKCC#7783, 30MDG#3936


    • Bernie Hildebrand
      Andy Thank you for the very understandable description. Your help is very much appreciated. 73 de Bernie - W8NBC SKCC#7783, DMC#3936 30MDG#4835
      Message 2 of 6 , May 2, 2011
        Andy
        Thank you for the very understandable description. Your help is very much appreciated.

        73 de Bernie - W8NBC
        SKCC#7783, DMC#3936
        30MDG#4835
      • Siegfried Jackstien
        Bpsk shifts phase between 0 and 180 and back Qpsk knows also 90 and 270 degrees in the phase switching ... that is why it is sideband sensitive Dg9bfc Sigi Ps
        Message 3 of 6 , May 2, 2011
          Bpsk shifts phase between 0 and 180 and back
          Qpsk knows also 90 and 270 degrees in the phase switching ... that is why it
          is sideband sensitive
          Dg9bfc
          Sigi
          Ps most what you will find is bpsk31 (almost standard) and some bpsk63
          I never found a qpsk station on air ... I think some use it after asking
          qsp-partner to switch ... but do not know really

          > -----Ursprüngliche Nachricht-----
          > Von: 30MDG@yahoogroups.com [mailto:30MDG@yahoogroups.com] Im Auftrag von
          > Bernie
          > Gesendet: Montag, 2. Mai 2011 14:24
          > An: 30MDG@yahoogroups.com
          > Betreff: [30MDG] PSK ?
          >
          >
          >
          > As you will see, I am quite new to digital. One question I have is what is
          > the difference between QPSK and BPSK. I use HRD and DM780 and when I open
          > it up, how do I know which one to choose. Please excuse the "dumb"
          > question but I have been confused by the choice.
          >
          > 73 de Bernie - W8NBC
          > SKCC#7783, 30MDG#3936
          >
          >
          >
          >
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