What is the most common frequency shift for RTTY emissions in the amateur HF bands?
Why Amateur HF RTTY uses frequency shift keying, where the transmitter alternates between two tones, mark and space. The separation between those two tones is the shift, and the amateur standard on HF is 170 Hz paired with 45.45 baud Baudot code. Your mark and space filters in the decoder must match that shift or the signal will not print.
Watch out 850 Hz was the older wide shift used in commercial and military teleprinter circuits and is still seen in some legacy gear, but it is not the amateur HF norm; 85 Hz and 425 Hz are narrow and commercial shifts not standard for ham RTTY.
Ham RTTY = 170 Hz shift at 45 baud. Think "170/45"; 850 Hz is the old wideband commercial shift.