Shutter Control Signal

This option is intended to produce a square wave output whose timings are derived from the scanning of video frames. The signal has a frequency equal to the frame rate (ie. half the field rate) with independent control over the start and end points of the pulse provided the pulse starts in the first (even) field and finishes in the second (odd) field. The original purpose of this option was to generate timings for a liquid crystal shutter or a pulsed laser to be used in conjunction with a video camera. The aim was to expose the even and odd fields at close to the same instant in time, thus reducing the time lag between the two fields in the frame. The timing singal is available on the DTR line of COM1 (normally connected to Audio channel 2 input of the VTR). This signal may be used for a variety of other timing purposes such as triggering a strobe lamp.

The timing signal is generated in the following manner:

           The incoming video signal is acquired continuously to buffer 0. The
           input look up table is set so that intensity 255 is mapped to 254.

           A single dot (intensity 255) is written to the screen at some point
           on some even line. This point is the ON position. The position is
           specified by the user. When the incoming digitized signal
           overwrites this point, the DTR line is driven high. This occurs
           during the first field of a given frame.

           A single dot (intensity 255) is written to the screen at some point
           on some odd line. This point is the OFF position. The position is
           specified by the user. When the incoming digitized signal
           overwrites this point, the DTR line is driven low. This occurs
           during the second field of a given frame.

           The previous steps are repeated.

A related facility is supplied by [T Timing signal] which is able to have the basic period and duty cycle extending over many video fields.

Line at which shutter to be opened (even) ?

This entry specifies the line at which the DTR signal is to be driven high. After specification of the shutter close line (next entry), the up and down arrow cursor keys may be used to adjust the position of this point. The point will be visible on the digitized image as a dot of intensity 255. The shutter is opened during the scanning of the even field. If an odd numbered line is entered, it will be rounded down.

Line at which shutter to be closed (odd) ?

This entry specifies the line at which the DTR signal is to be driven low. After specification the left and right arrow cursor keys may be used to adjust the position of this point. The point will be visible on the digitized image as a dot of intensity 255. The shutter is closed during the scanning of the odd field. If an even numbered line is entered, it will be rounded up.

<up> <down> to change open time <- -> to change close time O C to set Open or Close time V to change viewing position using Cursor submenu Q to quit

After the initial shutter open and close points have been specified, continuous acquisition starts and the timing signal is generated. <Q> returns control to the parent menu, terminating the shutter signal. The up and down arrow cursor keys may be used to adjust the point at which the shutter is opened. Up arrow makes the shutter open earlier, and down arrow delays it until later. Similarly left arrow makes the shutter close earlier and right arrow allows it to stay open longer. Note that during cursor key operations there may be a small glitch in the timing signal.

Alternatively <O> or <C> may be used to specify directly the shutter Open and Close points. During these operations the timing signal will be stopped. The timing signal is also stopped after pressing <V> which invokes the standard DigImage cursor submenu. This submenu may be used to adjust the viewing position and or magnification of the incoming video signal. Note that you must use <P> to exit the cursor submenu if you wish the current pan and zoom to be maintained on exitting the submenu. Once the submenu is left the timing signal will be started again.

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DigImage User Documentation

Stuart Dalziel, last page update: 19 February 1996