How Can I Detect SCART Aspect Ratios Using a Serial Port?

[MikeA]

I'm in the process of making a homebrew PVR and have a cable STB connected via a SCART->composite cable to a WinTV-PVR 350. This therefore loses the ability to know if the TV picture being sent is in widescreen or 4:3. I was looking into making some form of circuit/device that allowed the computer with the TV card into detect what type of TV picture was being sent via the cable. I know that the 8th pin in a SCART cable is used to send this info (~6v for widescreen, ~12v for 4:3).

This is where I become stuck . I was hoping someone here could recommend a circuit that would be able to connect to a serial port (RS232) and allow me to find out if the SCART cable was sending a widescreen or 4:3 picture.

(PS. I hope this in the correct sub forum )

 

[anorlunda]

ping @Tom.G

 

[Tom.G]

For a 9-pin serial RS232 connector on a computer you have:
Pin . . . . Signal
=============
4. . . . . . .DTR (Data Terminal Ready) (Output)
5. . . . . . .Signal Gnd
6. . . . . . .DSR (Data Set ready) (Input)
7. . . . . . .RTS (Request To Send) (Output)

On Terminal equipment (the computer) DSR is an input and can be read from the serial board Status Port. That's the easy part.

The voltage specs for an RS232 receiver is:
Binary 0 (Space or On): between +3V and +13V
Binary 1 (Mark or Off): between -3V and -13V

With your signal swing always being above the 3V threshold and varying only 6V, both signal gain and offset is needed.

The simplest approach I could come up with was an IC comparator with hysteresis. I finally found one that could handle the RS232 voltage levels and cost less than US $1, the TLV1805 by Texas Instruments. the data sheet is at:
HTTP://WWW.TI.COM/LIT/DS/SYMLINK/TLV1805.PDF

The chip itself is available at the usual online suppliers such as digikey and mouser.

Here is a marked-up circuit from pg19 of the data sheet showing use of the +12V and -12V computer power supplies and a rough guess of resistor values. The resistors are chosen for operation approx. at 9V input with a deadband (hysteresis) around 1 or 2 Volts.

If it is a real problem getting the +-12V computer voltages, you could use the serial port DTR and RTS (pins 4 and 7) to supply the circuit. However the actual voltages available at those pins is unknown and the needed resistor values would likely change.

Have fun! And please keep us updated on the project.

Cheers,
Tom