Generation of a Video Signal

A video signal is a recoverable train of electrical impulses generated by scanning a two-dimensional image produced by the optical train of a microscope. The image is sequentially scanned in narrow strips and combined to produce the final signal. This interactive tutorial explores the relationship between the microscope image, scan lines, and the video signal.

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When the tutorial initializes, the field diaphragm iris is closed to reveal only about 30 percent of viewfield area and the horizontal video scan starts at line 10, just above the open portion of the diaphragm. The Field Diaphragm slider can be utilized to increase or decrease the opening size of the aperture. Visitors can also use the mouse cursor to click anywhere in the viewfield area and then drag the scan line up and down. Once the scan has been halted, the user must click on the Start button to reinitiate video scanning. The specimen is a differential interference contrast (DIC) image of a bovine pulmonary arterial cell grown in tissue culture.

Beneath the microscope port is a graph illustrating the video signal voltage scan as a function of time at constant speed. Image brightness along the scan line is converted to amplitude, in proportional terms of voltage, to produce a faithful, time-dependent, electrical representation of a single narrow strip of the optical image. In this manner, dimensions in space along the horizontal axis are converted to time delays in the electrical signal, and the intensity of each small area of the image is converted to the amplitude of the electrical signal, measured in volts.

The horizontal scan lines are shown diagrammatically to the right of the microscope port in the Image Scan Lines plot. After each sequential scan, the scanning spot is made to fly back to the beginning of the next scan line, as illustrated by the dashed line in the plot. The Vertical Deflection Drive plot traces the vertical progress of the horizontal scan lines, and also "flies back" when the last horizontal scan has been completed.

To portray a faithful rendering of a two-dimensional image, the video scan lines must cover the entire picture area, and the electrical signal must contain the amplitude distribution of all points on all of the scan lines. In most video systems, the output from the scan lines is formatted serially, or in sequence, as a single linear signal rather than as many parallel signals.

Conversion of an optical image to a video signal in the vidicon tube was originally accomplished by scanning the stored image with an electron beam. Image storage occurs on the photosensitive surface of a video camera either by alteration of the conductive properties of the target of a vidicon tube camera or by the accumulation of charge in a storage well as in a CCD camera. Read out of the stored image in a vidicon tube camera occurs when a scanning beam of electrons encounters a region of the target that has been altered by the absorption of photons. The magnitude of the electrical current flowing through this region is then proportional to the number of photons that had impacted the target during the integration period. The Java tutorial illustrates the relationship between the optical image and the electronic signal resulting from scanning a spot across the image from left to right. Formation of an entire video image requires sequential scans from the top to the bottom of the optical image. Each scan line then represents a different region in the optical image so that as complete an electronic reproduction as possible can be produced.

Although solid-state video cameras do not employ a scanning electron beam for reading out the stored charge, their electronic output is indistinguishable from that of tube-type devices. The format of the electronically scanned image is dictated by the appropriate video standards rather than by the method of charge storage and read out.

The speed of the scanning spot, the number of scan lines and the frequency of the video image formation were chosen so that the average viewer would see a flicker-free image that was devoid of scanning lines. The number of scan lines required for comfortable viewing depends on the observer's distance from the screen, the size of the screen and other environmental variables. The frequency of image refreshment must be faster than the critical flicker-fusion frequency of 50-60 images/second.

Contributing Authors

Kenneth R. Spring - Scientific Consultant, Lusby, Maryland, 20657.

John C. Long and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.