Frequenctly asked questions about Tomi Engdahl's VGA to TV converter
Why doesn't my TV work any more???
Because it doesn't like 30 kHz. Zong! Your TV might not like the normal 31 kHz VGA horizonal refresh rate which you get if you don't have proper drivers loaded before connecting your circuit to TV.
My TV does not have SCART connector. Can I use the SCART connector on my VCR ?
The answer is no. The circuit designed by me uses RGB signal wires on SCART connectors. TV can utilize those signals, but VCRs don't understand them. So if you try to connect this converter through your VCR you will not get any picture. If you build those more complicated circuits with composite video output then you can connect them to your VCR.
What this circuit can do ?
This VGA to TV converter can be used for viewing VGA picture on standard TV which has SCART/EURO AV connector in them (practically all modern european TVs). The circuit uses SCART/EURO connector RGB input which guarantees a good picture quality (more bandwith than in standard broadcast TV).
The circuit needs drivers to make the VGA card to generate picture signals suitable for standard TV. There are DOS drivers for practically all VGA card for standard VGA modes (some problems with S3 based VGA cards). Some DOS drivers also support some SVGA modes in some VGA chipsets. Windows 95 support is provided for Matrox Millenium and Mystique. Linux Xfree86 users can use the example configuration file to make suitable settings for their systems.
How an I use the circuit with Windows 3.1 or Windows 95 ?
Basically my circuit is inteded to be used with DOS and Linux, because those systems provide enough flexibility for own configurations. Cenerally windows used high resolution modes and special drivers supplies by card manufacturer which don't usually leave much customazion possibilities to end used. To get Windows screen to TV you need a graphics card which provies enough flexibility for user customized monitor type. The only suitable cards I have found are Matrox Millenium and Matrox Mystique.
Is it possible to use the circuit without drivers ?
My circuit needs support from the VGA card and PC hardware side to generate the vidoe signal at right refresh rate. This can be made using any of the following methods:
- TSR driver program (for DOS and Windows)
- Configuring existing display drivers (Win95 Matrox Drivers, Linux Xfree86)
- Modified VGA BIOS (I have not tested this but should work if the BIOS is modified to do the same things as my DOS drivers)
My TV has only RF input. Can I use this VGA to TV circuit with it ?
No. This Circuit in only designed to be used with TVs which have EURO/SCART connector. If you have a PAL TV you can build the Paulo Coelho's VGA to TV circuit and connect it to an external RF modulator. More information is available at Using Tomi Engdahl's VGA to TV design with TV which does not have SCART connector page.
What is the difference of this circuit and some simpler designs ?
The simpler VGA to SCART adapters have a simpler circuit to make make the sync signals but those simpler circuit need the sync signals at certain speific polarity (right polarity depends on the circuit). The sync polarities can be controlled by the driver so if you select a driver specially designed for that circuit those simpler versions will also work.
I designed my circuit to be universal so that it would work with any VGA to TV driver designed any converter which uses the same conversion method (78LS86 IC is part of a special circuit which converts any polarity sync signals to correct composite sync signal). With a little more complicated electronics my circuit is much more compatible with already available drivers.
My VCR has scart input. Can I record the signal to VCR ?
No you can't. VCRs don't have RGB pin functions implemented in their SCART connector, so my circuit doesen't give any picture. If you have a PAL VCR you can try to build the Paulo Coelho's VGA to TV circuit which has PAL composite video output.
Does this circuit have flicker fixer circuit ?
No. My circuit does not do any filtering to the circuit. If you want to avoid flicker you have to avoid graphics which cause noticable flickering (especially single pixel wide horizontal lines).
The picture on TV flickers very much. Is it anything I can do for it ?
Because TV picture in interlaced and the frame rate is low, TV picture always flickers. The picture content have very much effect on the flicker. For example single pixel horizonal white line flickers very much. You can reduce the flicker by turning down the intensity in the TV and setting the screen graphics so that it does not flicker. In low-resolution grpahics modes my adapter does not use interlacing there are no interlace flicker in those modes.
Some commercial VGA to TV converters have flicker removing filters which can help the situation more or less. This type of flicker filter is quite complicated electronics and is is not easy to make such circuit.
I have 100 Hz TV. Why the picture still flickers ?
100 Hz television doubt to have non-flickering picture, but most of those TVs have solved only part of the problem. Many 100 Hz TVs have still interlace flickering because of the way they draw the picture to screen. 100 Hz technology can remove the large are flicker, but usually still leaves the line flicker. New 100 Hz TVs have For more information about 100 Hz TV technology read the introduction to 100 Hz TV technology article from Philips Web site.
Some DOS programs fill the complete TV screen while others only fill about 60% of the screen height but full width. Do you have any explanation for this?
There is one very good reason for that: VGA display uses three different vertical resolutions:
- 350 line mode is used for EGA high resolution emulation
- 400 line mode is used normally for text mode and 200 line graphics mode using line doubling
- 480 line mode is used for 640x480 graphics
Standard VGA monitor get the information what vertical resolution is in use from the signal going to monitor and selects the suitable screen height setting from the values set to monitor. For more information on this check my web document How multisync monitors detect different screen resolutions which covers standard VGA mode detection and SuperVGA modes.
TV has only one fixed vertical resolution it can use. Normal NTSC TV can show about 483 visible vertical lines. If you have PAL TV the it has about 560 visible lines on the screen. When the graphics mode different resolution than TV some extra black lines must be added to the beginning and end of the VGA graphics screen to make the piture signal to match the TV standard.
I can get 640x480 mode in Windows 3.x to show in TV screen. Unfortunately I can't get the cursor work right in 256 color mode but when I set Windows to 16 colours the cursor works o.k.
Windows VGA graphics mode 640x480 with 16 colors does not use any special features beyond standard VGA, so the VGA to TV drivers should work in this mode without much problems. But in SuperVGA modes special hardware features are used (usually hardware cursor) and those can't usully work well with the tweaking the VGA to TV driver does. The most common problems is that the Widnows driver can't handle the cursor right in interlaced mode which is needed to get the full resolution to TV.
How can I view TV program on my VGA screen ?
This circuit is only designed for showing VGA picture on TV screen. Doing the other way is much more complicated. The circuit would have to do the following thing:
- Take composite PAL or NTSC signal, extract sync, decode colour.
- Digitise each frame as it comes in.
- Read out the picture several times faster that it comes in and you have to read each line out twice
Tomi Engdahl <firstname.lastname@example.org>