On Mar 19, 2010, at 11:00 AM, Phil Sussman wrote:
> Note: You mention "PSK", if you're using PSK-31, as I understand it, there
> is no reverse polarity problem (NRTZ) ...
That statement is correct for BPSK31 (and BPSK63), but not for QPSK31.
"PSK31" is actually DPSK (*differential* phase shift keying) together with
sinusoidal envelope shaping to control the bandwidth. Instead of absolute
phases, the demodulator looks for a *change* in phase from one symbol to
another.
Since a 180 degree phase change is identical to a -180 degree phase change,
BPSK31 won't care if you use LSB or USB.
QPSK31 however, is based on four different phase changes 0, 90, 180 and -180
degrees. The 90 degree and -90 degree phase shifts will be reversed when you
switch between USB and LSB.
For "modern" digital modes (PSK31, MFSK16, DominoEX, etc), most software modems
default to generating tones to match a USB transmitter/receiver (i.e., higher
audio tone corresponds to higher RF carrier). Since it is so easy for the
software to reverse the generated audio spectrum, they also (or should, if they
don't) allow you to choose to use an LSB transmitter/receiver.
73
Chen, W7AY
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