HP 54710A User's Reference Manual page 244

Measurements
Time-interval measurements
Rather than a simple straight line interpolation between sample points, the
HP 54720 uses an approximate (sin x)/x digital filter for reconstruction. This
yields more accurate time-interval measurements than a simple straight-line
interpolation.
Filling in between samples in this way is called reconstruction. But is it
valid? Does reconstruction add "new" information that was not contained in
the original samples? The Shannon sampling theorem assures you that
reconstruction is valid, if you do not violate the Nyquist criterion.
The time base accuracy and resolution specified for the HP 54720 digitizing
oscilloscope, in real-time sampling mode, is much better than the 250 ps
sampling interval. Reconstruction is what makes this accuracy and resolution
possible and real.
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