Re: [MT63] Re: Crest Factor measurements + 80 meter MT63
- On Fri, 2004-01-09 at 21:08, wb8wka wrote:
> That seems to be my emphical observation as well. I adjust my threeWell, at least in this way you can be pretty sure you are not over-
> 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
modulating. You might be a bit on the safe side, but that is only
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