The Automatic Scart Switch

 

If you have connected the TV-out of your graphics card to the scart connector of your TV this little electronic circuit might be interesting. It enables TVTool to automatically switch your TV to the appropriate AV channel when TV mode is turned on. When it is turned off again the TV switches back to normal tuner mode. This is possible because there is special pin on the scart connector which is made for this purpose. Especially when the TV has several scart connectors this is a comfortable way, because you don´t have to use your remote control and go through a number of AV channels until the right one is enabled. This is also nice in conjunction with the multiple soundcard support of TVTool. All you have to do is to turn on TV mode, everything else is done automatically.

 

The connection diagram

On the German part of the site there is currently a relatively complicated circuit shown for this purpose. But in most cases it can be done much easier. Thanks to Andreas Freiherr, who has suggested this method.

 

 

Scart switch schematics

 

The wiring is really easy as you can see on the illustration. All you need is a cable with a diode (standard type 12V) inside, which connects pin 4 (DTR) of the serial port and pin 8 of the scart connector. The second cable is the ground line. This is only necessary if there is no other connection between the PC and the TV yet (like audio or sound).

 

How does it work?

When TV mode is enabled TVTool sets pin 4 of the serial port to high level. This results in a voltage of about 5-12 Volts on this pin. This voltage level is connected through the diode to pin 8 of the scart connector. When there is a certain voltage level on this pin the TV selects the corresponding AV channel automatically.

When TV mode is disabled again TVTool sets pin 4 to low level. The voltage level is now about -5 to -12 Volts. Therefore the diode is required, because the TV might be damaged by a negative voltage. Don´t leave the diode out! The diode can be compared to a gate which shuts when there is a negative voltage level. Now the TV will switch back to normal tuner mode.

  • Note: Actually the voltage level defined for the switch pin on the scart connector is 12 V. Many serial ports don´t output this voltage, but only about 5 Volts. In most cases this is enough to trigger the switch function of the TV. However, in some cases the serial port voltage might be too low. Then you have to choose the more complex circuit which can be found here (German only at the moment).

 

How to configure TVTool?

To make the Scart Switch work TVTool has to be configured before. This is done in the registry directly. Please open the registry with the Windows utility regedit.exe. Then go to this key:

  • HKEY_LOCAL_MACHINE \ Software \ TVTool7

Look for the subkey 'Com Port' and set this key to the value 1-4 corresponding to the serial port you want to use. After the next start of TVTool this port is used to turn on and off the Scart Switch. If you select 0 the function is turned off.

 

Com port selection in the registry

 

That´s it. After the next start of TVTool the scart switch will be active. Enjoy the new comfort! :-)