Scan Converter General Idea
General idea for a line buffered , single channel digital scan converter circuit. (PAL/NTSC to SVGA/VGA)
Circuit comments :
1- The main clock frequency must be at least 2 times more than the highest frequency of input video signal.(4 times or more in practice) or an anti aliasing filter is essential for input to eliminate the higher frequencies of the picture.
2- Clock generator , must have the ability to produce enough sharp edge pulses with stable frequency and phase.
3- In case of using a dual port RAM (like those in graphic cards) , there is no need to digital switches because there are separate pins for RAM input and output , anyway it should be a static type , a FIFO buffer is a good idea.
4- Noise , interference between analog and digital lines and between low value address and data lines and parasitic capacitance in routes are major problems in PCB design.
5-In case of implementing a microcontroller for timing purpose , the need to basic digital circuits is minimum but a high speed microcontroller is essential.
6- It is supposed that the amplitude of input signal is always fix.
7-All of logic circuits , A/D , D/A and RAM chips must be high speed ones , it's even possible to design a D/A circuit using some resistors.
8-Out put buffer is important because it matches the output impedance of D/A and 75 ohms input impedance of monitor.
9- Timing and synchronization is very critical and different delays in signal routing can be an important factor , a separate sync generator circuit is mandatory for producing appropriate synthetic H and 2H sync signals.
10- This circuit only modifies the horizontal frequency of the video signal and vertical scan rate remains the same.
11- Setting a mechanism for GenLock , can improve the quality of output picture.
12-Some of the connections or details are not presented here , for example it's obvious that the A/D converter needs the clock signal to run.