Detecting TVs with PC's graphics card

Question

I'm interesting of how can graphics cards detecting what type of TV has connected with S-Video? TVs sent some indefiniter signal back to the computer? And how it's working (protocols, signals, freq, etc)? Thanks.

Answer 1

If the connector that enters your TV is a standard 4-pin S-Video, then the only signals are luminance and chrominance.

The graphics card detects the presence of a TV based on the load impedance on the lines which is 75 ohms. If you were to connect only luma, you would have black&white picture.

No matter what resolution you set on computer, the output signal is still a standard NTSC or PAL video.

I did some experiments with a homemade CVBS adapter and an old CRT TV.

Without the 75 ohms load on chroma the TV detection is not working as it should and sometimes it would output 1080p signal - or at least that what it looked in the monitor settings.

Usually, there's no need for a 75 ohms on luma because it enters straight into TV and the CVBS input has this impedance already. But if you can't get it working I suggest building something like above and adding an extra 75 ohms resistor on the luma wire (from the hot wire to ground).

If neither this work, you could play with the following:

• see if your graphics card has I2C lines in the S-Video connector (here)
• program an I2C EEPROM with a custom EDID
• connect the memory to the I2C lines and supply it with 5V

Answer 2

The answer is on Wikipedia should you care to ask google:

Non-standard 7-pin mini-DIN connectors (termed "7P") are used in some computer equipment (PCs and Macs). A 7-pin socket accepts, and is pin compatible, with a standard 4-pin S-Video plug. The three extra sockets may be used to supply composite (CVBS), an RGB or YPbPr video signal, or an I²C interface. The pinout usage varies among manufacturers. In some implementations, the remaining pin must be grounded to enable the composite output or disable the S-Video output.

-- Wikipedia

So some systems can use a (not part of the official S-Video spec) I²C interface in the same way that VGA interfaces use I²C to identify the monitor connected.

For other setups, where it just detects the presence of a screen, it is as simple as measuring the impedance (75Ω) at the remote end of the connection. A simple enough thing to arrange.