Black Box IEEE 1284 Betriebsanweisung PDF herunterladen (Seite 64)

Page 64 of 98 The Black Box Toolkit

In the screen shot below the cursors have been positioned exactly on the leading and

trailing edge of the bitmap display (opto-detector centre screen on line 3).

On line 3 we can see each refresh of the bitmap display (35 refreshes). We can see the

exact position of the cursors by looking at the upper or lower status bar. The green cursor

A is positioned at 1245ms on line 3 and the red B cursor at 1598. This gives an elapsed

time between the cursors of 353ms. This is the M measure.

As the opto-detector was positioned mid screen we should remember that the actual time

is for mid-screen to mid-screen. If we used a frame-by-frame full-screen measurement the

duration would have been 360ms as we would need to account for the theoretical 5ms

before and after the leading edge of the opto-detector being triggered. This is because the

screen must be redrawn as a whole. These periods are indicated by the troughs between

“on” periods in the graph.

In the screen shot above we have dragged the green cursor -5ms from the leading edge of

the first image being detected. This represents the top of screen position on the first

refresh of the image being displayed. The red cursor has been dragged +5ms from the

leading edge of the last sample. This represents the bottom of screen position of the last

refresh trace of the stimulus image display. As can be seen the M measure matches the

theoretical 360ms which is based on 36 refreshes of 10ms each when a monitor is driven

at 100Hz. When using a CRT, corrections such as these need to be made as if an opto-

detector is positioned mid screen, there will always be a delay before the top of screen

redraw reaches the sensor and again from the sensor position to the bottom of the screen.

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Black Box IEEE 1284 Betriebsanweisung Seite 64