LVDS Single-to-dual converter Background
Motivation:
Before college I bought a laptop I thought'd be my main system for years to come... dropped quite a bit of cash into going top-of-the-line, including upgrading the display to 1600x1200 for all my coding and design needs. Well, I made some mistakes, and that laptop was stolen a year or so after college. Combine that with my being flat-broke, and I now have (and have grown to love, as I've modified nearly every part) a graciously-donated PowerBook G4 as my main system... It came with 1024x768, which, frankly, isn't even enough to browse a website, as far as I'm concerned. I *had* to do something.
The search for a better display:
I did quite a bit of searching... Came across this guy who instilled a bit of hope: http://www.ifixit.com/Guide/PowerBook+G4+Aluminum+12-Inch+1-1.5+GHz+LCD+panel+upgrade/3117
But looking on ebay, even used this HV121P01-100 display was *way* out of my budget.
The only other display I was able to find at this size and resolution (SXGA+, 1400x1050, the max I've seen in a 12.1in display) was the LTD121KM1K/LTD121KM7K, often found in a Toshiba M200-series pen-tablet. This guy was running for less than $30 (with the wacom digitizer!).
This display is *in no way* compatible with my system, but seeing as how I have nothing but time these days, I took on the project.
(In)Compatibility/Differences:
Numero Uno: This display is dual-pixel, whereas my laptop is single-pixel (see an explanation below). This is a big problem, not at all tackleable by the faint-hearted. (until now, maybe?)
Numero Dos: The mounting of this particular display is *significantly* different than the original display. The original was otherwise pretty much the standard mounting used in 12.1in displays. The LTD, on the other hand, has mounting I've never seen before. Also, its backlight CCFL was too long to even fit in my display's lid. These problems, I figured, I'd come back to when the time came.
Numero Tres: This particular display does not have an EDID chip! But that's irrelevent, since if it did it would tell my single-pixel computer that it's dual-pixel... and they wouldn't work together. (see below)
Misc: The connector's obviously different, to make up for extra signals, ...
Single- vs. Dual-Pixel displays:
Most digital video-signal technologies these days (e.g. HDMI/DVI, "Flat-Panel Display Link" aka FPD-Link, also often poorly-referred-to as "LVDS", Open-LDI, etc.)--come in two variants: "Single-Pixel" and "Dual-Pixel." (Sometimes called "single-channel" and "dual-channel"). It's pretty straightforward, ,a dual-pixel display uses twice as many wires to send two (adjacent) pixels at a time, rather than one. The end-result is that the transmitter and receiver get to run at half the necessary pixel-rate, which means frame-rates can be higher, signals can be transmitted further, wires and circuitry can be of lower-quality, etc. etc. etc.
Standardization:
Thankfully, these things are pretty darned standardized, these days. From what I've pieced together... Most laptop displays now use LVDS (FPD-Link). Most use the exact same configuration, as far as the actual data sent down the lines (18bits per pixel; 6bits per color, the order these bits are transmitted, the same sync signals, etc.). The only differences, usually, are pinout. Single-channel displays usually have one of two pinouts on a standardized 20pin connector, dual-channel displays usually have one of two pinouts on a standardized 30pin connector. If your new display has the other pinout from your old display, it's just a matter of swapping the order of the pins. Further, for displays which have a different configuration (e.g. 24bits per pixel) it usually just involves an additional LVDS signal, which carries the additional least-significant bits. In other words, if you want to connect an 18bpp display to a 24bpp cable, you just don't connect the extra LVDS signal. Amazingly well-thought-out by the standardizers, considering how nasty and incompatible displays were in the old days. (Note that the dual-pixel connector can often be used to transmit single-pixel data, as well, depending on the display's needs).
Compatibility Caveat (EDID):
Unfortunately, some computer manufacturers decide to cut corners... Especially some of the smaller companies or cheaper models. E.G. Dell often sells laptops which can be configured for multiple different displays; "you want to spend $200 more, we'll be happy to pop in the better display." This is handy for us hackers, because it means that these displays have an EDID-chip, which tells the computer what resolution to run at and whether it's single or dual-pixel. But when companies cut corners (by, e.g. not installing an EDID chip) your options for upgrading and/or reusing the display in another system are significantly limited, if not impossible.
The LTD display I chose *does not* have an EDID chip... But it doesn't really matter, in my case, because even if it did, the data it would send to my computer would be irrelevant since the computer itself doesn't have the ability to send dual-pixel data.
Converter Feasibility:
So all this leaves me with a few challenges:
I have to somehow convert single-pixel data to dual-pixel.
(Has this stuff become so standardized that such a conversion is possible with very few components? YES! Texas Instruments has exactly the chips I need).
I have to create a custom EDID chip
(Does the computer even pay attention to the EDID? -- Yes, since the other dude was able to run his display. Though, oddly, some of the displays used in this laptop model do *not* have EDID chips, so I guess they must have a default...)
Can the single-pixel channel in my computer handle the extremely high transmission-rates necessary?
(Barely, we'll come back to that. But those transmission-rates can't be *that* different from "the other dude's", so it should work).
Am I capable of designing a circuit-board that can handle these data-rates?
(Heh, that's a good question).
The end-result: Yes, it's possible.
Check out LVDS Single-to-dual converter Hardware
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