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Re: [MT63] Re: Crest Factor measurements + 80 meter MT63

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  • Tomi Manninen
    ... Well, at least in this way you can be pretty sure you are not over- modulating. You might be a bit on the safe side, but that is only good... There are
    Message 1 of 5 , Jan 10, 2004
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      On Fri, 2004-01-09 at 21:08, wb8wka wrote:

      > That seems to be my emphical observation as well. I adjust my three
      > watt warbler on either PSK31 or MFSK31, by peaking the power, and
      > backing it off slightly. With this method, I don't have any problems
      > working anyone, and reports of signal quality are good.
      >
      > Now, if I go to MT63, I noticed the power (average) is at least 10db
      > lower then what I am seeing on MFSK16 for the exact same settings.
      > This would tend to confirm the high crest factor of MT63.
      >
      > I'm assuming this is the correct way to set up MT63, lacking an ALC
      > meter.

      Well, at least in this way you can be pretty sure you are not over-
      modulating. You might be a bit on the safe side, but that is only
      good...

      There are several complications in this issue. One of them is that
      the modem code needs to be set somewhat experimentally. If you
      assume that the sound card uses signed 16 bit samples, you can
      write sample values between -32768 and +32767. With modes like
      MFSK16 (where the envelope is constant) and PSK31 (where the envelope
      is not constant but well defined) it's easy to write a modulator
      that makes the best use of the available sample value range without
      any danger of overflows.

      The stochastic nature of the MT63 envelope makes this much harder.
      One can calculate the theoretical maximum value of the envelope
      but this value is bound to be *very* rare. In fact it's likely to
      never occur. So in practise the output range needs to be experimented.

      What this means is that it is likely that the output level from a
      MT63 modem is a bit lower than the optimum. Which in turn means that
      your method results in being on the safe side.

      Another issue is that most power meters read somewhat random values
      with MT63. Most meters are by nature peak detectors (detecting the
      peak RF value and then "calculating" the RMS value). They show
      correctly only with a pure carrier. With the complex envelope of
      MT63 they show "something" that depends on the time constants
      involved (filtering + meter time constant). I guess typically they
      read something between the real peak and average values.

      I think adjusting levels is the most common problem with MT63
      (and Q15X25 which is similar in this respect).

      --
      Tomi Manninen / OH2BNS / KP20ME04
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