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VGA to PAL and NTSC converter

2016-01-17 08:16  
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Note: There is one mistakes on the circuit diagram which should be corrected.The AD722 pin which is marked as to be pin 17 is really pin 15.On some version of circuit diagram R4 is marked to have value of 10 kohm. R4 should really be 1 kohm as listed in component list and small circuit diagram picture. C5 should be 15 nF (not 1 nF as listed in some picture versions).

Circuit operation

This circuit is based on AD722 IC from Analog Devices. The AD722 is a low cost RGB to NTSC/PAL Encoder that converts red, green and blue color component signals into their corresponding luminance (baseband amplitude) and chrominance (subcarrier amplitude and phase) signals in accordance with either NTSC or PAL standards. These two outputs are also combined to provide composite video output. All the outputs can drive standard 75 ohm terminated video cables directly without extra amplifiers. The chip is a very compact solution, because operates from a single 5 V supply and it need no external delay lines or filters.

Because the AD722 IC does almost everything necessary, only very little extra electronics is needed. The chip needs external clock crystals to operate, so the circuit provides the color subcarrier frequency crystals: 4.43MHz for PAL and 3.58MHz for NTSC. The circuit here also provides necessary components for power decoupling and proper termiantion of video lines (inputs and outputs).

The most complicated part of the circuit is the sync signal processing. Because the sync signals from VGA card can be at any polarity, U1 and circuit around it will guarantee that the sync signal are always correct polarity when they enter AD722 conversion IC (U1 makes polarity inversion if needed). The circuit as this would already work, but with some VGA card on some grpahics modes it is hard to generate HSYNC signal which has exactly right width. The HSYNC width must be quite accurately correct for proper color video signal receoption, so I thought it was a good idea to add a circuit which would generate a sync signal which always has right pulsewidth. In this circuit monostable multibirato built around U2 (555 timer) generates the right pulse width for HSYNC signal. The pulse width of this circuit is adjusted using R4.

Circuit use

When using this circuit a right VGA to TV driver must be loaded. This driver does the job of making the VGA card to generae the picture signals at right format and refresh rate. The drivers which cna be used with this circuit are the same as with my VGA to SCART circuit. You have to select a driver which matches the video standard you want to output.

Then you have to select the output standard on the circuit to match the vide output standard. Use the following table to set the switched SW1 and SW2 to match the video output standard you want to use:

Standard     SW1        SW2

PAL          Closed     Up

NTSC         Open       Down
You must adjust also the width of HSYNC signal using R4. NTSC signal asks for 4.59 microsecond HSYNC signal and PAL asks for 4.60 microsecond HSYNC signal. If the HSYNC signal pulse width is not right, quite many TVs have problems in receiving the color signal correctly or even sync to the video signal at all. Best option to adjust the HSYNC signal is to use some measuring equipment like oscilloscope, video vectorscope or frequency counter (which has period measurement capabilities). If you don't have the necessary measusing equipments, you have just to adjust R4 so that you get the colors workign well with your TV. Basically uou have to adjust this setting once, because when you get the setting right, the same setting usually works for PAL and NTSC standard (they timing is so near each other in this).

The last setting is to to fine-tune the color subcarrier frequency. C6 adjusts NTSC subcarrier frequency and C7 adjusts PAL subcarrier frequency. You have to adjust those so that you get the best color reproduction. You have access to vectorscope or similar vidoe signal analyzer, you can use it to fine-tune the color subcarrier frequency to be exactly right. Otherwise you have just to approximate what is the best setting for your TV.

Component list

site part   value
==== ====== ==========
C1          22uF
C10         100nF
C11         10uF
C12         220uF
C13         220uF
C14         220uF
C15         100uF
C16         100nF
C17         100nF
C18         100uF
C2          22uF
C3          1 nF
C3'         220nF
C4          100nF
C5          15nF
C6          10-30pF
C7          10-30pF
C8          10uF
C9          100nF
D1          1N4001
P1          15 pin VGA connector
R1          2K2
R2          2K2
R3          10K
R4          1K
R5          75R
R6          75R
R7          75R
U1          74LS86
U2          ne555
U3          AD722JR-16
U4          7805 regulator IC
Y1          3.58MHz crystal
Y2          4.43MHz crystal
Other items needed:75 ohm BNC connector (PCB mountable)4 pin mini-din connector (PCB mountable)Enclosure made of metal

Building the circuit

I have built my first prototypes to a veroboard. This method worked very well with other componets on the circuit than the AD722 IC, which was only available in SOIC enclosure. I built my first prototype by carefully solvering small wires to each pin in the AD722 IC and then soledered those to veroboard (not an easy task and fun to do).

The later prototypes I built to the following circuit board (click the picture for larger view, 300x300 dpi scanned picture):

Circuit board

You can print out the picture at correct resolution to a laser printer (6.83x3.87 inches) to a slide and propaly use it as a mask for making a circuit board of your own. If you have a postscript printer or a suitable viewer/printing program you can download zipped Postscript version of the circuit board which automatically prints to right size.

I don't have a nicely drawn out component placement diagram. I found an old paper version of one component placement diagram which might be helful:

Component placement picture

It is not very clear and does not show orientation of all components, so you have to figure out that part of circuit construction yourself. That's all I can provide at the moments (I don't have my old protoype circuit boards anymore).

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