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Re: Laptop with reverse VGA
From: Neil Harris <neil () tonal clara co uk>
Date: Tue, 21 Feb 2012 17:19:10 +0000

On 21/02/12 14:48, Jay Ashworth wrote:
----- Original Message -----
From: "Jake Khuon"<khuon () neebu net>
I think the form-factour is already there. I have a Motorola Atrix
smartphone. It's available with a laptop-dock unit. This is
essentially a USB hub and display. The display is connected by
outputting from the phone's HDMI port. The rest of the input/output
device (keyboard and trackpad) are seen as USB connected devices and
interfaced via the phone's USB port (Atrix supports USB host mode).

Essentially, this laptop dock is what people are talking about except
for a generic host instead of for a phone. We would want to expose the
HDMI input generically and probably with an additional VGA input. Of
course there are also VGA-HDMI converters. Anyone wanna ring up
Motorola to see if they're interesting in adapting the Atrix
laptop-dock technology?
As someone who's done video for 20 years, I can tell you, Jake:

It ain't that easy.

The interface on the Atrix is purpose-built, and it's almost certainly
just a DVI/HDMI digital interface to a panel that expects that.

What's necessary for a standalone KVM of the sort we're talking about
is what the video people call a "genlock" circuit -- most machines that
need this at all have analog VGA out, and you have to have a chip that
can lock up to it, and extract the video from that analog signal cleanly.

This is, to quote the Jargon file, decidedly non-trivial to do well.

That's the reason why a single port unit, not on sale, is generally around
$400.  If it was DVI/HDMI *only*, it could be substantially cheaper, but
I've never seen one that was.

- jra

High prices are more likely to do with the small market for such devices, than to do with the cost of the underlying technology.

It isn't so much genlock, as accurate pixel clock recovery, that's the hard thing.

It is indeed hard to do well, but fortunately the chipmakers have done all that for you. It's a common enough need (think flat panel monitors) that there are inexpensive single-chip solutions for it that not only do the A/D conversion, but handle the pixel clock recovery for you as well: see, for example, the Analog Devices AD9884A or ADV7441A.

Data sheets at http://www.analog.com/en/audiovideo-products/analoghdmidvi-interfaces/ad9884a/products/product.html and http://www.analog.com/en/analog-to-digital-converters/video-decoders/adv7441a/products/product.html respectively.

-- Neil

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