This article describes the converter for computer VGA signals and NTSC standard to PAL color TV signal, the circuit's main technical parameters are as follows.
Power supply: +5 V regulated power supply, current less than 300mA.
Video input: RGB + HSYNC + VSYNC signals from the VGA card, refresh rate compatible with the NTSC standard.
Video output: composite video and S-Video (Y / C).
Supported video standards: PAL B, G, H and NTSCM.
Circuit requires VGA card can be sent with the PAL or NTSC standard video timing compatible RGB format video signals.
Work
Circuit such as the photo shows.
The circuit based on analog integrated circuit AD722, AD722 is the RGB signal can be converted to NTSC / PAL encoder signals, resulting in PAL or NTSC standards and consistent, the corresponding luminance (baseband amplitude) and color signal (amplitude and tone phase), brightness and color signals can be mixed to produce composite video output, all outputs can directly drive standard 75O video cable port, no additional amplifier, the device only a +5 V power supply, and no external delay lines or filters .
The converter only few external electronic components, subject to an external clock crystal oscillator, the circuit can be provided for the PAL 4.43MHz, to provide for the NTSC color carrier frequency of 3.58MHz, also provides for power supply decoupling and the appropriate video Line port (input and output) required components.
The most complex part of the circuit is the synchronization signal processing. Because of VGA card is sent from the sync signal polarity is arbitrary, when the signal into the conversion circuit U1 (four 2-input XOR gate for polarity conversion) when the U1 and around the VGA circuit is always in sync to ensure proper polarity, the circuit is always in working condition; However, some VGA cards for some graphics mode, the width is very difficult to produce accurate HSYNC signal, while the width of HSYNC must be the right color video signal receiving consistent. In the circuit, around the U2 (555 timer) and monostable multivibrator signal generator suitable for the LSYNC pulse width, pulse width of the circuit adjusted by the R4.
The use of circuit
Using the circuit must be inserted from the VGA card drivers to switch to TV, VGA card and its function is to produce the right format and refresh rate of the graphics signal. The circuit card and from the driver to switch to SCART VGA card the same driver circuit. Then, select the video output to match the output of the standard criteria. Then set the switch SW1 and Schedule SW2, is used to match the video output standard.
R4 can be adjusted using the width of the HSYNC signal, NTSC signal requires 4.59ms, while the PAL requires 4.6ms. If the HSYNC signal pulse width of the wrong color television signal at the receiver will be a problem, or simply can not keep pace with the video signal. Best to use debug HSYNC signal oscilloscope, vector display or frequency counter (with periodic measurements) and so on. Without these machines, you can debug R4, so that the color TV signals can work well, usually on the PAL and NTSC standards (the timing between the two is very close to) do the same setup.
Finally, fine-tune the color carrier frequency, C6 carrier frequency for fine-tuning NTSC, PAL and C7 for fine-tuning the carrier frequency, carrier frequency adjustment is intended to form the best color, use a vector display or video analyzer, otherwise, only get close to the TV set.
Also a need to prepare the following materials.
75O to BNC plug (can be installed PCB).
4-pin mini-plug (can be installed on PCB).
Metal box.
Production
Production, carefully thread with the AD722 integrated circuits connecting each pin, and then fasten it to the PCB (not an easy task, but doing it very interesting.) Production of PCB, you can order the appropriate resolution, the circuit output to laser printer (6.83 × 3.87 inches), to create their own circuit boards.
Improvements
Has been eliminated as AD722, AD724 AD722 is the closest you can use to replace the device, fortunately, AD724 and AD722 are very consistent and have the same pin functions, the only change is needed, RGB input section foot of the need to increase a 0.1µF ceramic or plastic capacitors, the capacitor connected to the 75O input port between resistance and AD724RGB.