This technique started on idea that VGA screen at 640x480 resolution is non-interlaced picture 60 Hz refresh rate and horizonal scan frequency of 2xNTSC horizonal scan frequency. The converter needed to convert this to interlaced TV signals has to do the following things:
- Store first odd number scanline shown in VGA screen
- Send out the stored can line at half speed and encode it to NTSC video signal
- Store next odd scanline to buffer
The converter handles 640x480 graphics nicely without any drivers and it is possible to have your VGA monitor showing the same picture at the same time. To ble able to show other VGA resolutions nicely a driver is need to set all VGA modes to 60 Hz refresh rate (the driver makes VGA card to add blank scanlines to the beginning and end of the picture to make it 480 line picture).
When conversion is needed for PAL TV, a driver is needed which sets all VGA modes to 50 Hz operation. The horizonal scan rate for PAL output is still made by halving the VGA horizonal scan rate, which gives a little bit too high horizonal scan rate for output PAL signal. This problem usually messes up the colors in many products (for proper PAL signal decoding the horizonal scan rate must be very accurately right in conjuction with the color subcarrier frequency).
The electronics itself become quite complicated. A typical circuit which connnect to VGA monitor output needs fast A/D converters for digitizing incoming video signal, small memory biffer for storing those signals and digital PAL/NTSC encoder. Because the basic conversion need quite much electronics, the manufacturers have typically added a flicker reducing circuits to their products. Flicker reduction circuit stores fre scanlines information and interpolate between those scanlines to get rid of some of the flickering. There are some special ICs in the marked which can do all this.
Application fields where this technique is used: Most inexpensive small external VGA to TV converters, VGA chipsets with built-in TV output
