Minimalist Video Digitizer.

Were you looking for the Simplified Analog Electronics Simulation page? It is still availiable. There is also a section on a tracking Analog- to- Digital converter . (Plus introduction to more converter types.)

Advantages include: Circuit simplicity, no need for on board memory and the associated interfacing, no need for a fast and expensive converter; the image is built column- by- column over several frames.

Disadvantages: The image must be still for over 5 seconds, as only a single column of pixels is sampled during each frame. The main software loop runs at the TV line frequency, so an assembler routine is necessary: Almost all high- level languages are not fast enough to acquire and store in excess of 12, 000 samples per second.

The skeleton video digitizer block diagram is in figure 5. The crystal oscillator may be at the TV color frequency, or its multiples. The simple counter will have a minimum of 8 bits, while the programmable counter block will need to be made of more than one ICs in cascade. The programmable counter data inputs (D's) are supplied by the corresponding outputs (Q's) from the plain counter. The Load' signal to the programmable counter is from the mixed sync splitter block. The carry- out pulse triggers the pulse lengthener. The pulse lengthener provides both the control signal to the sample- and- hold, and the SOC' (start- of- conversion) signal to the analog- to- digital converter.The ADC output bits, end- of- conversion (EOC') and mixed sync outputs are led to the computer input port.

Software Description:

  1. The computer resets the counter and disables interrupts.
  2. A delay loop waits for a positive frame sync signal edge and also skips the first few (and last) lines of the image (which do not contain picture information) by detecting a corresponding number of edges on the EOC' signal.
  3. A pixel value is stored after the occurence of a negative edge on the EOC' signal. (At this point, it may advantageous to simply store the number in linear memory space; it can be moved to the screen space later, when the whole image is complete, using a high- level language).
  4. The last step is repeated until a complete column is built during one frame.
  5. The counter is advanced, and all steps save (1) are repeated until a complete image has been grabbed.

The simplicity of the arrangement can be quite deceiptive: It certainly doesn't imply the results are not worthwhile! The reproduction of a 320x512 monochrome screen of 40x32 characters was rendered essentially fault free.

 

 

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