Visual Target Tracking Sysyem

  
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The Visual Target Tracking System (VTTS) is a system for visually tracking targets using a gimbal mounted camera and digital control circuitry. A small black and white camera is mounted on a stand that can be moved with two directions of freedom. When an object is placed in the camera`s field of view, the camera moves such that the object becomes centered in the camera`s field of view. The camera`s progress is monitored by a Television monitor which will display both a center square and a crosshair overlay on the center of the object. Figure 1 shows how the VTTS is connected to the real world. The VTTS was broken into four distinct modules, the digitizer, target detection, the output unit, and the camera control. Figure 2 shows a schematic diagram of how the system was modularized. The digitizer takes an NTSC video signal from a standard video camera and presents a 64x64 pixel grid of black and white pixels for output and image detection. The digitizer outputs a 7 bit X address, a 6 bit Y address line, a data line and communication signals data_available and wait. Figure 3 shows a system diagram of the digitizer. The astute reader will realize that to address a 64x64 grid only 6 X address bits are required. The original design called for 192x128 (8 bits X, 7 bits Y) resolution, but as details about implementation became clearer the resolution was dropped to 64x64, yet the digitizer`s design is capable of capturing the full 192x128 pixels. Unfortunately, much of the digitizer`s complexity is due to the fact that it is capable of capturing in 192x128 mode, even when a lower resolution was finally used in the project. Internally, the digitizer still creates a 128x64 pixel grid, and the low X address bit is ignored by the other modules. The digitizer generates two communication signals in addition to the pixel address and data. The wait signal is high while the digitizer is buffering the next NTSC frame from the camera. When the wait signal is high, the other kits can perform the computation necessary for image detection and crosshair overlay.
Visual Target Tracking Sysyem - schematic

When the wait signal goes low, new data and address information is sent. To signal that the data and address lines contain valid information, the data_available signal is brought high. The digitizer keeps the address and data lines stable while data_available is high. Figure 4 shows a timing diagram for inter-kit communication. The standard for broadcasting TV signals in the United States is called NTSC (National Television Systems Committee). NTSC was first proposed in 1940 and was standardized in 1953. Televisions sweep an electron beam horizontally across a phosphor coated screen, starting from the upper left and working downward. The phosphors emit light when struck by the electrons, and by changing the strength of the electron beam, the phosphors that are excited change and create a moving image. There are 525 horizontal scan lines in each NTSC frame (a frame is one complete screen). Each NTSC frame is divided into 2 fields of 262. 5 horizontal lines each. The first field contains the even horizontal lines, and the next field contains the odd lines. Even and odd fields are interlaced between each other. Figure 5 shows the relationship between the even and odd horizontal lines in each field, and the format of the digitizer`s output. 30 frames (or 60 fields) are sent each second, and the first 22 horizontal lines in each frame are blank (called vertical blanking) to allow time for the electron beam to return to the upper...



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