Tektronix SPG8000 Technical Reference page 94

SDI button menu (Option SDI only)
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Horizontal Step. Sets the number of samples to be scrolled per frame set in the
Period menu item. Use the left (◄) or right (►) arrow button to set the value
from –252 to +252 in 4-sample steps.
Vertical Step. Sets the number of lines to be scrolled per frame set in the Period
menu item. Use the left (◄) or right (►) arrow button to set the value from
–252 to +252 in 1-line steps.
Period. Sets the time interval between occurrences of the scrolling. Use the left
(◄) or right (►) arrow button to set the value from 1 to 16 in 1-frame steps.
Random. Sets whether or not the picture scrolls randomly. Use the left (◄) or
right (►) arrow button to select between Disable and Enable. Press the ENTER
button to confirm the selection.
SPG8000 Technical Reference