[RTTY] AFSK RTTY on USB question

[Kok Chen]

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