mini-DVI to DVI adapters and analog signals
Posted by bitguru on January 15, 2008
Mini-DVI is an Apple-specific port they use on products (such as their 12″ and 13″ laptops) where a full-size DVI port would take up too much space. [update: The 13″ MacBook Air, announced today, would seem to use micro-DVI, not mini-DVI.]
It seems obvious that it would be possible to build a mini-DVI to DVI-I adapter that carries both digital and analog signals. After all, Apple sells adapters that convert from mini-DVI to both DVI and VGA, so both analog and digital signals must be available in the mini-DVI port.
Apple says of mini-DVI, “The computer detects which adapter is present by reading its EDID (Extended Display Identification Data) via DDC. The EDID for video is in the adapter; the EDID for VGA and DVI are in the display.” But I think we can presume (can we?) that for both DVI and VGA that the DDC/EDID conversation between the mini-DVI port and the monitor is passed straight through the adapter unadulterated. It’s only the adapter providing S-video and composite video ports that plays DDC/EDID tricks.
So the only reason for a miniDVI-to-DVI adapter not to pass through the analog signals is to save a few conductors in the cable. Yet it seems both games in town have in fact done this.
It’s obvious Apple does it, since the female DVI connector on their adapter lacks sockets for the analog pins. [See this image to confirm that it has a DVI-D socket connector that won’t physically accept the analog pins (above and below the blade pin) of common DVI-to-VGA cables such as this one.]
The only other manufacturer of mini-DVI to DVI adapters I can find is Dynex. Dynex’s mini-DVI to DVI cable has a female DVI-I connector, so analog cables will physically fit, but this thread confirms that the analog signals aren’t passed. The DDC/EDID data gets through, but either there aren’t conductors for the analog signals or possibly the RAMDAC that would be generating the analog signals is somehow getting shut down.
It would be nice to be able to carry around a single mini-DVI adapter that could provide both digital and analog signals. Comments?