Successful LVDS LCD installations?

Giuliano · 01-04-2005, 08:52 PM

I searched through ~140 threads in this group to find this answer, but I didn't see it:

Has anybody here done a successful installation of a LVDS LCD panel?

I see lots of threads (including my own) about motherboards with LVDS onboard, and various LCD's that may or may not be LVDS, but no actual installations.

I've been planning on going the LVDS route (with a Commell LV-671 board), and I believe I have a reasonably good chance of success due to my understanding of electronics, among other subjects.

If nobody else has done an LVDS installation, I'll write up a good FAQ about it, once I start my installation.. which may still take another month or two.

Anybody?

Jackboot · 01-04-2005, 11:22 PM

how about more info about LVDS than just the acronym for us dummies here?

Yuriy · 01-05-2005, 04:52 AM

LVDS as in laptop LCDs? There have been a lot of laptop installs (including some of my own) but I doubt anyone actually connected something like that to a custom PC, the controller prices are simply horrid.

Giuliano · 01-05-2005, 06:51 AM

Quote: Originally Posted by Jackboot

how about more info about LVDS than just the acronym for us dummies here?

LVDS stands for Low Voltage Differential Signalling, and is one of the two commonly used LCD communication types, with the other being called TTL/CMOS.

LVDS and TTL/CMOS are most often used in laptops, where a direct digital connection to the LCD is required.

TTL/CMOS is the more common of the two signalling technologies, but it has some limitations, such as short cable length, and high number of pins/wires, often numbering in the 30-40 pin count.

As a result, LCD's with TTL/CMOS technologies cannot be very far from their controller, which is why you see LCD's like the Lilliput/Xenarc models with VGA controller boards.

LCD panels are not like analog glass-tube monitors - LCD's have a native physical resolution, and perform best only at that resolution, such as 640x480 or 800x600, or even the difficult to obtain 800x480 of widescreen LCD's.

VGA controller boards get around this limit by scaling down (or up) your PC's analog VGA signal to whatever physical resolution the LCD is capable of, which results in a blurry picture.

In addition, the process of conversion from digital to analog (VGA video port), and then back from analog (VGA LCD controller) back to digital (LCD itself) can lead to signal degredation and a fuzzy picture, because analog transmissions can easily be affected by EMI (electromagnetic interference).

TTL/CMOS wiring uses 1 wire for each bit of color resolution that the LCD is capable of - so if you have an 18-bit TTL/CMOS LCD capable of 262k colors, you have (at least) 18 wires to transmit signals on, not including power and ground lines, etc.. that many number of wires, and the method of signalling, can cause crosstalk (interference) problems in longer cables.

Because of this, TTL/CMOS data cables can not be longer than 6 or 12 inches, due to the interference over long lines.

LVDS solves this by transmitting the three color signals (Red, Green, Blue) on 6 wires, or 3 differential pairs. Because the signal is differential, or the difference between the signal on one wire and another, data can be transmitted farther than TTL/CMOS can, up to 12 meters with proper (shielded?) cabling.

The reason for LVDS's newfound popularity is simple - there are now Mini-ITX boards on the market that have LVDS transmitters on-board or as an option, and can easily drive the proper LCD's with the right cabling and knowledge.

Previously, in order to use a TTL/CMOS or LVDS LCD panel, you either had to have a VGA controller card attached to the LCD, a PCI LCD controller card attached to the PC, or you had to purchase a special "embedded" type of motherboard, usually in non-PC form factors such as PC/104 or boards the shape and size of a 5 1/4" drive.

Quote: Originally Posted by Yuriy

LVDS as in laptop LCDs? There have been a lot of laptop installs (including some of my own) but I doubt anyone actually connected something like that to a custom PC, the controller prices are simply horrid.

See, that's the point - you can now buy MiniITX motherboards with LVDS controllers built in (such as the Commell LV-670/671 P4 series), or as an option board (some Via Epia models).

The motherboards take care of all the LCD specific timing issues, and backlighting control, making it easier to use LCDs with these boards.

However, it won't work with just any LVDS LCD panel - the LCD panel has to support a specific resolution, color depth, and channeling (single/dual) such as 800x600 18-bit Single Channel, or 1280x1024 24-bit Dual Channel, and the video timing parameters for those LCD types must be supported by the motherboard's system bios for them to work.

This is why non-standard LCD sizes, such as the ever popular 7" widescreen 800x480 LCD's are not currently easily supported - it's because the motherboard manufacturers have not tested them and included the timing parameter set in their bios.

And that's even assuming you can locate and purchase a 7" widescreen LVDS panel - I've tried, and I can locate them easily enough, but it's next to impossible to buy one from a retailer or distributor, unless you want to buy tens of thousands at once.

The easy availability and standard resolution support of 4:3 aspect ratio LVDS LCDs make the possibility of an all-digital display solution that much more appealing, which is why I'm working on fitting a 8.4" 4:3 high-resolution screen in my WRX instead of a 7" widescreen Liliput or Xenarc.

petesonline · 01-05-2005, 08:04 AM

There was a previous post with tons of sites in the east( korea, china) they seem to love lvds and have successfully used it. Lvds is the future, but I think we have a way to go before it reaches us. The site I mentioned also sold kits and the prices seemed reasonable, but ordering overseas and language barrier can seriously hamper things.
Pete

Jackboot · 01-05-2005, 11:14 AM

well written Giuliano. Makes sense to me now...

...unfortunately, I think there will be few here that have the technical knowledge to help you much. Best of luck though!

My question: is the difference in display quality between VGA and LVDS drastic enough to warrant such effort? Of course, I guess the same could be said concerning our efforts to install an entire PC in our cars rather than go buy an AVIC-N1 or similar...

DeltaFX · 01-05-2005, 11:27 AM

Lvds is the key if you need to create a laptop/tablet-like pc. For a car-puter, using lvds is ... well an exercise of style.

Giuliano · 01-05-2005, 11:38 AM

Quote: Originally Posted by Jackboot

well written Giuliano. Makes sense to me now...

...unfortunately, I think there will be few here that have the technical knowledge to help you much. Best of luck though!

My question: is the difference in display quality between VGA and LVDS drastic enough to warrant such effort? Of course, I guess the same could be said concerning our efforts to install an entire PC in our cars rather than go buy an AVIC-N1 or similar...

I think I've got what it takes in technical skills to pull it off, which is why I'm going forward with my plans for an 8.4" LVDS LCD.

The upshot is that by hand-picking specific models of LVDS lcd's, you can pick and choose what resolutions and brightness you will accept, as opposed to having to pick from what the retail LCD market offers.

Right now, I am planning on getting an 8.4" LVDS LCD that has a native resolution of 800x600 - most small size LCDs on the market today cannot do this, even if the back of the case says 800x600 or 1024x768.

Those stated resolutions are what the VGA controller is able to accept, it simply scales the resolution up/down to fit the physical panel resolution.

Right now, I also have the option of buying an 8.4" LCD that does 1024x768 - except it's too big to fit in my WRX's dash right now, without having to do some additional major modifications. I'm sure I will get it later, in "Rev 2" of my system.

So the upshot of going the LVDS route is that you can pick from small-size LCDs that have higher native resolutions, resulting in a sharper and clearer picture.

The fact that it's digital as opposed to analog is also a benefit, because there is less A/D conversion going on, and less signal loss in the process.

Giuliano · 01-05-2005, 11:39 AM

Quote: Originally Posted by DeltaFX

Lvds is the key if you need to create a laptop/tablet-like pc. For a car-puter, using lvds is ... well an exercise of style.

Good point..

Can you do a CarPC without a bare LCD, LVDS or not?

Sure you can..

But I've always believed in doing things with style.

Yuriy · 01-06-2005, 06:06 AM

Hehe, well, I got lucky and actually extended my laptop cable to about 2 meters (6 feet).
Works great with just a little interference (barely noticable).
That's some good info though, the LVDS sure sound like an option. You'd need pinouts and connectors tho, as I would guess..

Giuliano · 01-06-2005, 09:22 AM

Quote: Originally Posted by Yuriy

Hehe, well, I got lucky and actually extended my laptop cable to about 2 meters (6 feet).
Works great with just a little interference (barely noticable).
That's some good info though, the LVDS sure sound like an option. You'd need pinouts and connectors tho, as I would guess..

Got the pinouts, and they're fairly simple.

Power, Signal A+, Signal A-, (and B+/-, and C+/-), grounds, and Signal Clock +/-, that's pretty much it.. maybe 16-17 connected pins out of 20, on the LCD side.

The connectors I got, too, except they're almost the right kind.

I posted a picture here - they're tiny:
http://www.mp3car.com/vbulletin/show...4&postcount=22

Giuliano · 01-06-2005, 12:15 PM

Here's an example of the LVDS pinouts required for the motherboard (Commell LV-671) and LCD (EVD084S01-DT from eVisiondisplays.com, aka AUO B084SN01 w/enhancements) that I have chosen for my WRX CarPC project.

The document shows three things:

The LCD Type number table, used to set the LCD Type in the BIOS.
The LVDS connector pinouts on the motherboard.
The LVDS pinouts on the LCD panel.

The LCD I chose is an 18-bit panel at 800x600, and is a single-channel model, so the appropriate LCD Type setting in the BIOS would be number 2, which is highlighted in Bold.

If the LCD was different, I would have to pick a different type, such as a 1024x768 24-bit Single Channel panel - the appropriate LCD Type would be 8.

What's the difference between an 18-bit panel and a 24-bit panel?

An 18-bit panel can display about 262K colors, and a 24-bit panel can display 16 Million. Most smaller LVDS LCD panels will only be 18-bit, as most 24-bit panels are larger and have a higher resolution.

For the 18-bit 800x600 single-channel LCD I chose, the pins I would need to connect to on the motherboard are highlighted in bold - there are a total of 18 pins, to match the 18 of 20 pins on the LCD panel connector.

The LVDS data is transmitted on the ATX0-2 +/- pairs, and each pair carries 6-bit's worth of data, so for an 18-bit single-channel panel, I would need to use the first three ATX pairs. For a 24-bit single-channel, I would need to use the first 4 ATX pairs.

You may notice similar pairs named BTX0-3 - those are for the 2nd channel, if you needed a dual-channel connection. For an 18-bit dual-channel, you would use ATX0-2 (3 pairs), and BTX0-2 (3 pairs). For 24-bit dual-channel, you would use ATX0-3 (4 pairs) and BTX0-3 (4 pairs).

The transmitted data is kept in sync by the ATXCK (clock) pairs - for a dual channel LCD, you would use both clocks, and possibly the PANELCLK and PANELDATA pins.

One remaining thing I have to verify is the polarity of the +/- pairs.

That is, whether the - on one end goes to the + on the other, or whether it's a same-to-same connection, like - to - and + to +.

I think it's same-to-same, but I have to verify that.

Giuliano · 01-06-2005, 12:18 PM

Quote: Originally Posted by Yuriy

Hehe, well, I got lucky and actually extended my laptop cable to about 2 meters (6 feet).
Works great with just a little interference (barely noticable).
That's some good info though, the LVDS sure sound like an option. You'd need pinouts and connectors tho, as I would guess..

Actually, this had me thinking...

Do you know what the model number of the LCD panel is?

And what kind of cabling/wire did you use?

There's a possibility that your LCD panel is LVDS.. and if you extended it to 6 feet with little interference, that's a good sign.

Yuriy · 01-06-2005, 06:47 PM

Quote: Originally Posted by Giuliano

Actually, this had me thinking...

Do you know what the model number of the LCD panel is?

And what kind of cabling/wire did you use?

There's a possibility that your LCD panel is LVDS.. and if you extended it to 6 feet with little interference, that's a good sign.

The LCD is Toshiba LTM12C263, it came with the Compaq LTE 5300 laptop (all 5xxx series have the same connectors except for 5250). As me and Oracle later found out, this screeen is actually a widely used one and is used on a lot of Compaq laptops (such as the Armada) and possible some (if not all) Toshiba models.
I do not have the pinout, but it's approximately 24 pins in a row (similar to the ones you have) on the LCD side and 2 rows of approx. 12 pins on the motherboard side. But! The same LCD used on the Armada laptop only has about 6-8 wires (excluding the inverter) that connect it to the motherboard!

Oh yeah, the one in the Armada is a slightly different LCD with the LTM12C283 and it has a resolution of 1024x768 instead of 800x600 that my LCDs have. The connectors that go into the LCD itself are exactly the same.
http://www.mp3car.com/vbulletin/atta...achmentid=3462

One more thing I noticed is that when you connect an LCD with a lower resolution (I have some with 640x480) to a laptop that outputs 800x600 you will still get the image but it will be moved 160 pixels to the right and 120 down, just like the black "frame" you always seem to notice around higher resolution screens running at lower res then native. The image was chopped off but otherwise was pretty clear. I bet it could be fixed with the right software.

Anyway, you got me thinking too, seems like we are one step closer to connecting a laptop LCD to a PC

I am thinking about sending you one of my LCDs and see if you can connect it, and then if you succeed we would all benefit from it

Yuriy · 01-06-2005, 06:57 PM

Oh, I used the printer cable to extend it. The most recent extension I made was a bit shorter (about 3 feet) with no interference. As I found out later the interference I had previously was caused by some wires occasionally touching each other. One common problem that all my extensions experience is the inability to display resolutions lower than native. But once Windows kicks in it all displays fine. The current extension I'm using displays all resolutions because each wire is independently wrapped and carefully connected.