The reason that LSB historically has been used for RTTY was that the equipment in the early days had difficulty dealing with FSK. On the demodulator side you had to choose frequencies that would not produce harmonics to fool the demodulator. Back then the frequency shift was 850Hz, and it was decided that the optimal demodulator frequencies were 2125 (mark) and 2975 (space). Mark is the idling frequency, so many demodulators had the ability to look at the absence of mark as space. To make the transmitter transmit FSK you added a capacitor to your VFO, and switch this capacitor in and out for mark and space. The standard was to have the mark be the high RF frequency, and the space be the low RF frequency. If the demodulator was looking for a 2125 mark, and the transmitter was transmitting the high RF frequency, LSB had to be used in the receiver.
Today, we don't have these limitations. We do need a standard, and the standard is still to transmit the mark as the high frequency, and the space as the low frequency. The shift for FSK is now 170 Hz. Terminal units that used filter techniques, had filters set up for 2125 and 2295 Hz for Mark and Space respectively. With these standards it was still a requirement to receive on LSB. Today most sound card programs don't care what the mark and space frequencies are for demodulation, as long as the shift is correct. This allows us to shift to the more intuitive USB mode. Mark is transmitted as the high RF frequency,and Space is the low RF frequency. On the demodulator the high audio frequency and space is the low audio frequency. This is much easier to understand. 73, Mark N5RFX >I ask this before but tell me again why al the sound card >modes are on USB when all the *pre* sound card modes >(RTTY, PACKET, AMTOR & PACTOR and others) are >all LSB Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org Other areas of interest: The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion) Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/digitalradio/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/digitalradio/join (Yahoo! ID required) <*> To change settings via email: mailto:[EMAIL PROTECTED] mailto:[EMAIL PROTECTED] <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
