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There are a lot of devices on the market right now (especially mobile devices) with a Micro-HDMI or Mini-HDMI port and no VGA or D-Sub output. Most manufacturers of said devices sell a cable that looks something like this:

hdmi-vga-adapter

I have yet to find a cable like this that claims to work on a wide array of devices. In general, these cables claim to work with one specific device only.

The way these cables work, I think, is that analog VGA signals are sent from the HDMI port on the device. This should work for devices that have special hardware on the motherboard/GPU capable of driving this.

Is it the case that these cables have to be custom designed for each device? Or, is it rather that any device which possesses this special "signaling of analog VGA over the HDMI port" can be made to work with a cable that is physically compatible (i.e. the HDMI end plugs into the device and the VGA end accepts a VGA monitor cable)?

Note that I am not looking for a product recommendation, just a conceptual clarification on what exactly these devices are doing. Also, a few remarks:

  • The cables like the one depicted here are not digital to analog converters. I know about these: they are expensive, and they are the ONLY solution if your device only outputs a digital signal and is incapable of driving analog VGA over the HDMI port.
  • The cables like the one depicted here are not straight crossover cables from VGA to HDMI, either. The crossover cables are designed to send a digital HDMI signal over the VGA port's wires; that is, the wire protocol is HDMI (digital) but the physical pinout is the same as VGA, even though nothing analog is happening. Once again, this is not the behavior that, I believe, the devices which I'm talking about in this question are doing. The cabling and devices that this question is about transmit the analog VGA data over the HDMI port (the HDMI port is in the device outputting the data, and the VGA side is the monitor/projector).
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4 Answers 4

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The way these cables work, I think, is that analog VGA signals are sent from the HDMI port on the device. This should work for devices that have special hardware on the motherboard/GPU capable of driving this.

The cabling and devices that this question is about transmit the analog VGA data over the HDMI port

Your guess that "analog VGA signals are sent from the HDMI port" is not likely true.

  • VGA signals require a characteristic impedance of 75 Ohms. Aside from a good/proper connector (five BNC is preferred over a HD15), that implies coaxial cable for the RGB video signals. The cable in the photo looks undersized.

  • In the current highly competitive marketplace, reduction of manufacturing cost and extending battery life are crucial. Inclusion of VGA circuitry so that there's "VGA data over the HDMI port" would not be used nor appreciated by the vast majority of mobile device users, and is therefore nonsensical.

  • There are no spare (unused) pins on these HDMI connectors to support inclusion of VGA signals. (The DVI connector does have enough pins for this, e.g. DVI-A, DVI-I.)

  • That's a very bulky, oversized shell for that HD15 connector in the photo of the "cable". That shell could easily house some converter electronics.

  • The visual similarity between the "cable" in your photo and the converter in Dave M's link is probably not coincidence.

enter image description here

What you insist is just an adapter "cable" is more likely a HDMI-to-VGA converter with active electronics, even though you claim that they "are not digital to analog converters" with no supporting evidence.

If there is a dependency between a host device and a VGA converter, then that might be due to power requirements and video resolution(s).

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Indeed, I suspect you are right, that there is actual conversion being done in the housing built up around the VGA female connector. This is the most plausible explanation. Thanks. –  allquixotic Nov 6 '12 at 4:46

The only potential problem I can think of that might prevent a cable like that from working on different devices is HDCP.
Obviously, if a cable like that is sold for a device without HDCP, I'd expect it not to work on a device that does have HDCP.

I have had some experience with MHL cables and, while each manufacturer claims that their specific cable will only work with their devices, it just always worked in other devices, as long as they are MHL enabled.
This should not be any different.

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1  
Are you sure you didn't mean HDCP instead of DHCP? –  AndrejaKo Nov 5 '12 at 22:32
    
you are very much right, corrected –  Mark Spencer Nov 5 '12 at 22:36

Several vendors seem to offer "generic" converters. This one from Startech is one example. Not suggesting a specific product but saying there seems to be some adapters that will work for a number of devices. Startech offers a presales support chat and an 800 number so they may have a definitive answer.

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The cables don't have to be custom-manufactured for each device; however, only analog signals will ever be output from a VGA port, and only digitial signals will ever be output from an HDMI port. So, it's really not an HDMI adapter at all; rather, it's a simple DE-9 to HDMI adapter.

As there is no standard for the conversion between these two connectors, the pin configurations will vary between implementations. Thus, no cable can be guaranteed to work with any one device. In addition, plugging one of these into a digital-only display might fry something, as the signal is still analog. (And vice versa, if you're using it in the other direction.)

You can't convert between analog and digital signals with an electromechanical adapter such as this one. That's why you don't see DVI to component video, etc. adapters, except for highly specialized circumstances.

By the way, when analog video is transmitted through a DE-9, IIRC there are actually different implementations of the "VGA" video standard, which are not compatible with one another.

See, this is why we don't often use analog video anymore.

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I agree with your first statement: VGA is analog only, HDMI digital only. However, I have such an adapter and use it every time I am in class, as the cables to the projectors are still VGA and the laptop only outputs HDMI. It works with every VGA cable I have encountered. So there must be some real digital to analog conversion going on. –  vinnief Jul 9 at 23:49

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