BNC 1420 - Video Microscope - Manual (Page 19)

17
3 VIDEO AND ILLUMINATION TIMING
The master clock for the Model 1420 is provided by the video signal
timing. The CCD camera outputs video in standard RS170 format (NTSC
in the color version), which is an analog, interlaced format compatible
with standard analog video monitors or video recorders.
Video
LED drive
Even Odd Even Odd
33 ms full frame
Fields
Frame
LED on
(variable)
Video
LED drive
Even Odd Even Odd
33 ms full frame
Fields
Frame
LED on
(variable)
Timing sequence of the illumination in relation to the video signal
The RS170 interlaced video signal is composed of two fields, called even
and odd fields, each containing every second line of the image. The
interlaced format was defined in the early days of television to avoid
flickering TV images. The even field contains lines 0, 2, 4,
524 and the
odd field lines 1, 2, 525. The field frequency is 60 Hz, with one even
and one odd field adding up to a full video frame each 33.3 ms,
corresponding to 30 Hz frame frequency.
To ensure that all lines of the video signal are equally illuminated, the
LED s flash twice during an image, once in every field. The LED pulse
starts in the frame blanking period, and its width can be varied from 0 to
near 100% of a field period, 16.6 ms.
Due to the interlaced readout of the camera sensor, images of fast moving
objects, which move a noticeable distance during the 16.6 ms between two
consecutive fields, may appear jagged at the horizontal edges. See section
below for a discussion of the implications and tools to control the
interlacing effect.