VGA/SCART/Audio Automatic Switch

This circuit is designed to go between the video card and computer monitor. Everything will work as normal until TV freq video detected - then the audio and video are switched to the TV. Primarily for watching (divx,xvid,etc) video but it's also good for games, emulation and annoying anyone watching TV..

scartsw_circuit.png (12680 bytes)
The µC monitors the horizontal sync line. In VGA operation the video goes through to the vga out via RY1-3 and IC3A/B are switched on, allowing the H&V sync signals to pass. When H. Sync drops below ~17khz the video is switched to the SCART by RY1-3. Pins 8 and 16 are raised high to force the TV into RGB mode. H&V sync signals are XORed together then inverted by IC3D, IC3C is closed to let the sync though, audio is switched by IC4 (biased to 2.5v by R5 and R6).

PCB Layout - at 600dpi
PCB Component Overlay
Autotrax PCB file
Hex for the PIC µC and the asm source

The PCB is 102*57mm and should (just) fit into one of those 130x68x42 plastic boxes. The circuit uses pretty standard parts that should be available at any decent electronics shop. The relays have a "mini fujitsu" footprint - the closest thing I could find to a standard. Most shops have 'em, if not in 12v then in 5v (or 6v).

If using video resolutions higher than 1024x768 there may be some ghosting on the VGA monitor. Maybe this can be reduced by using thin coax (like the type inside a monitor cable) to connect the VGA out socket to the PCB.

board_little.jpg (15236 bytes)   finish_little.jpg (11721 bytes)   connectors_little.jpg (15711 bytes)

I've changed the board layout slightly since this one was made. The audio coupling caps are bent over IC4, there's not much room for them otherwise.

SOTWARE

The best software to run Windows at TV frequencies is EnTech's PowerStrip. There's a couple of guides to using power strip for this purpose here and here. It's written for ATi Radeon cards but the info applies to almost all modern video cards.
The ideal setup is two PowerStrip shortcuts, one to switch into into a TV mode and one to switch out of it.

For DOS or Linux drivers have a look at Tomi Engdahl's VGA to TV converter page.

3/7/04

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