Author Topic: DVI-D to VGA adapters - how do they work?  (Read 15715 times)

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Offline Yansi

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DVI-D to VGA adapters - how do they work?
« on: January 26, 2015, 11:36:57 pm »
Hi
A curious question, how do these DVI-D to VGA adapers work?
I mean these: http://www.ebay.co.uk/bhp/dvi-d-to-vga-adapter

As we already know, DVI-D has only the TMDS digital signal pairs. Is there any smart circuitry built in the plastic blob, or is it some sort of chinese wangle?

Has someone already investigated, whats inside these?

Thanx,
Y
 

Offline Whales

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Re: DVI-D to VGA adapters - how do they work?
« Reply #1 on: January 26, 2015, 11:42:32 pm »
Perhaps there are devices somewhere that (non-standardly) accept D over a VGA D-sub connector, to cut down on connector costs. 

Offline Yansi

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Re: DVI-D to VGA adapters - how do they work?
« Reply #2 on: January 27, 2015, 12:13:00 am »
That is possible explanation, but weird. (saves cost on connector, needs adaptor  ;D )

Some time ago, I came across this strange thing: http://www.ebay.com/itm/Black-USB-A-Female-to-Ethernet-RJ45-Male-Adapter-Connector-Router-Adapter-/381117510299?pt=LH_DefaultDomain_2&hash=item58bc5fb29b

Little later I have discovered, it is for some dumbass routers, which acceptsUSB via ETH connectors.   :palm:


// It reminds me Dave's rant video about proprietary connectors. That wass great!  >:D
 

n45048

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Re: DVI-D to VGA adapters - how do they work?
« Reply #3 on: January 27, 2015, 12:30:06 am »
Yes, well, it depends.

You can get passive DVI to VGA adapters, but they will only work if your video card has a DVI-I socket. DVI-I carries additional analog video signals (as well as digital) that can be connected straight through to a VGA plug with no signal conversion at all. There is a good explanation of the different connectors on Wikipedia: http://en.wikipedia.org/wiki/Digital_Visual_Interface

There are active converters you can buy which will actually process and convert the digital signal back out to analog video however their performance varies. I have seen cheaper Chinese products perform poorly and that's notable in the image quality coming out of the unit, especially at higher resolutions and colour depths, they may even struggle to process images at the full frame rate and you can end up with 'jerky' motion.
« Last Edit: January 27, 2015, 12:34:11 am by Halon »
 

Offline Yansi

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Re: DVI-D to VGA adapters - how do they work?
« Reply #4 on: January 27, 2015, 02:22:50 am »
Thanx for the explanation, but I know the DVI interface to this point. Question was directly specified for the chinese cheapass $1-3 adapters DVI-D (obviously from the photos analog C-pins aren't fitted) to VGA, which looks kinda suspicious to be a heap of normally unusable blob.

Or maybe there really is some absolute low-cost mass produced chip? Maybe buy one for examination, whats inside (if anything).  :-//

Whale's explanations seems valid, but have you ever seen a device with a dual interface 15p DSUB connector, which could be fed with analog and digital in the same 15p connector? Woul'd be interesting to know, which manufacturer likes to bullshit people with this stuff..  >:D
 

Offline AlessandroAU

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Re: DVI-D to VGA adapters - how do they work?
« Reply #5 on: January 27, 2015, 04:27:01 am »
Thanx for the explanation, but I know the DVI interface to this point. Question was directly specified for the chinese cheapass $1-3 adapters DVI-D (obviously from the photos analog C-pins aren't fitted) to VGA, which looks kinda suspicious to be a heap of normally unusable blob.

Or maybe there really is some absolute low-cost mass produced chip? Maybe buy one for examination, whats inside (if anything).  :-//

Whale's explanations seems valid, but have you ever seen a device with a dual interface 15p DSUB connector, which could be fed with analog and digital in the same 15p connector? Woul'd be interesting to know, which manufacturer likes to bullshit people with this stuff..  >:D

This is an dumb adapter for almost all modern video cards which still support VGA over the DVI interface.
 

Offline David_AVD

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Re: DVI-D to VGA adapters - how do they work?
« Reply #6 on: January 27, 2015, 09:10:40 am »
It does seem dubious at best.  As you say, there's no analog pins on the DVI-D plug like a more conventional adapter has.
 

Online Rasz

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Re: DVI-D to VGA adapters - how do they work?
« Reply #7 on: January 27, 2015, 09:32:00 am »
there probably exist GPUs that can switch between digital and analog signals on digital only pins.

Just like ATI (dont know about Nvidia) DVI digital pins can be switched into LVDS mode using low level firmware haxoring (so gpu in laptops can reuse pins).


There are low cost magic all in one chinese dvi to vga chips, but they are in the $10 range, not $.5 with free shipping.
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Online wraper

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Re: DVI-D to VGA adapters - how do they work?
« Reply #8 on: January 27, 2015, 09:36:09 am »
They do not work. There are actually no wires for video signal at all inside this thing. IIRC there are EDID lines, ground and 2 more wires for hot plug detect I think. 5 Wires in total.
 

Offline miguelvp

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Re: DVI-D to VGA adapters - how do they work?
« Reply #9 on: January 27, 2015, 09:47:19 am »
Not sure what the problem is with those, I have many and they work fine. DVI-D has Analog RGB, HSync and ground on the on the cross section of the connector and VSync on pin 8 (upper pin closer to the cross connector.

http://en.wikipedia.org/wiki/Digital_Visual_Interface

I've used many over the years and actually my display I'm using right now is via the monitor VGA connector with a DVI-D to VGA adapter and running at 1920x1080. The monitor has a DVI-D input too but I reserve that for when I need to repurpose my monitor to kits that have HDMI outputs, and I have an HDMI to DVI-D adapter that only uses the differential pair signals.

Just think of DVI-D as a hybrid that supports both TMDS and analog signals. One thing you won't find is HDMI to VGA without some supporting hardware (or VGA to HDMI for that matter).

I've have hacked (literally) some of those apart when I'm in deed of female VGA connector:


BTW if you take one of those apart it's an ordeal because they are totally potted with some translucent plastic stuff, so it's probably easier to purchase the vga connector if you can wait for it to arrive.

Edit, it's a straight wire connection from the analog DVI-D pins + analog ground + HSync + VSync to the corresponding VGA pins, nothing inside other than straight wires.
« Last Edit: January 27, 2015, 09:54:41 am by miguelvp »
 

Offline andtfoot

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Re: DVI-D to VGA adapters - how do they work?
« Reply #10 on: January 27, 2015, 09:53:48 am »
Not sure what the problem is with those, I have many and they work fine. DVI-D has Analog RGB, HSync and ground on the on the cross section of the connector and VSync on pin 8 (upper pin closer to the cross connector.
That would be DVI-I (I = integrated).
DVI-D (D = digital) has digital only (no pins around the spade connector looking part).
DVI-A (A = analogue) is analogue only.

I've seen HDMI to VGA cables, but I believe they are intended for devices that use proprietary schemes to push analogue video out the HDMI port when configured to do so. I'm guessing this DVI-D to VGA might be a similar thing.
 

Offline bktemp

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Re: DVI-D to VGA adapters - how do they work?
« Reply #11 on: January 27, 2015, 10:01:57 am »
The more expensive ones with a larger case around the connector probably have a genuine DVI receiver and DAC to convert the signals.
The cheaper ones probably work only with special video cards with support for VGA signals on DVI pins. There are some video cards with 2x VGA outputs on a single DVI connector.
 

Offline miguelvp

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Re: DVI-D to VGA adapters - how do they work?
« Reply #12 on: January 27, 2015, 10:08:00 am »
Not sure what the problem is with those, I have many and they work fine. DVI-D has Analog RGB, HSync and ground on the on the cross section of the connector and VSync on pin 8 (upper pin closer to the cross connector.
That would be DVI-I (I = integrated).
DVI-D (D = digital) has digital only (no pins around the spade connector looking part).
DVI-A (A = analogue) is analogue only.

I've seen HDMI to VGA cables, but I believe they are intended for devices that use proprietary schemes to push analogue video out the HDMI port when configured to do so. I'm guessing this DVI-D to VGA might be a similar thing.

True, they are selling them mislabeled, they are just DVI-I to VGA adapters, I should have paid more attention to the post :)
At that price there is no way they are digital only.

Edit: the thing is that its use is to hook the on the back of your computer to avoid the cost of a DVI cable, and that is probably a DVI-I connector anyways. I've never seen a video card that only has DVI-D output.
 
Edit again: I must be tired, they obviously show the pictures without the C pins. Maybe I should get one to see what is inside.
« Last Edit: January 27, 2015, 10:22:07 am by miguelvp »
 

Offline rollatorwieltje

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Re: DVI-D to VGA adapters - how do they work?
« Reply #13 on: January 27, 2015, 10:21:48 am »
True, they are selling them mislabeled, they are just DVI-I to VGA adapters, I should have paid more attention to the post :)
At that price there is no way they are digital only.
They are not DVI-I to VGA adapters, the actual analog signal pins are missing. These things are just for devices that don't really adhere to the standards. For example many laptops can output analog video over MiniDP, some desktop graphics cards allow dual analog video on a DVI connector... For those cards you get weird converters like this.

Just expect these converters to not work unless they are specified for your graphics card.
 

n45048

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Re: DVI-D to VGA adapters - how do they work?
« Reply #14 on: January 27, 2015, 01:17:02 pm »
Not sure what the problem is with those, I have many and they work fine. DVI-D has Analog RGB, HSync and ground on the on the cross section of the connector and VSync on pin 8 (upper pin closer to the cross connector.

Absolutely not! DVI-D is digital only. As per the specification.

This is an dumb adapter for almost all modern video cards which still support VGA over the DVI interface.

No, don't fall into that trap. Your adapter needs to support DVI-I as per my original comments. It's not as common as you think. Most modern cards are DVI-D (Single or Dual Link)

... and Dual Link is different to "Dual Head"
« Last Edit: January 27, 2015, 01:22:45 pm by Halon »
 

Online wraper

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Re: DVI-D to VGA adapters - how do they work?
« Reply #15 on: January 27, 2015, 01:21:15 pm »
Trust me, they are useless. Actually I have bunch of them as source of cheap DVI-D connectors. There are no lines for video signal in them, they just cannot work.
« Last Edit: January 27, 2015, 01:45:50 pm by wraper »
 

Offline Yansi

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Re: DVI-D to VGA adapters - how do they work?
« Reply #16 on: January 27, 2015, 01:23:41 pm »
No lines of video signal? What's in there then?  :-DD

Or what's their purpose?
« Last Edit: January 27, 2015, 01:27:21 pm by Yansi »
 

Online wraper

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Re: DVI-D to VGA adapters - how do they work?
« Reply #17 on: January 27, 2015, 01:42:10 pm »
No lines of video signal? What's in there then?  :-DD

Or what's their purpose?
Here you go. Only VSYNC is used among analog pins.
 

Offline amyk

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Re: DVI-D to VGA adapters - how do they work?
« Reply #18 on: January 27, 2015, 06:42:59 pm »
I know what those are for - programming monitor firmware. The ISP interface uses only those pins and some monitors are DVI-only, while ISP adapters usually have the VGA connector.

http://monitor.net.ru/forum/topic37230-32.html

http://www.aliexpress.com/item/RT809F-hd-USB-LCD-programmer-24-25-93-RTD2120-A-key-to-automatic-identification-speaking-reading/1405765093.html
 

Offline Yansi

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Re: DVI-D to VGA adapters - how do they work?
« Reply #19 on: January 27, 2015, 07:12:17 pm »
Just wow. Didn't know that it is possible  :o

I should give you 10 points for that answer, thanks  :-+

By the way, for what purpose it is needed to modify firmware in the monitor that way?
 

Offline SeanB

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Re: DVI-D to VGA adapters - how do they work?
« Reply #20 on: January 27, 2015, 07:50:34 pm »
Not really firmware, just modify what the monitor tells the attached computer, so that you can change things like the display name, the list of supported resolutions ( though you better be sure the monitor will support it as well) along with removing supported modes you do not want it to display. You can also on some change the levels of default brightness, colour calibration constants and such.

Inside most monitors there is an eeprom attached to the connector lines only for this data, some might also have ther data lines connected to the internal micro as well.
 

Offline NiHaoMike

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Re: DVI-D to VGA adapters - how do they work?
« Reply #21 on: January 28, 2015, 02:35:36 am »
I once opened up a cheap HDMI (basically DVI-D) to VGA adapter and it more or less just has some custom ASIC for the DAC.

It's possible to reprogram the EEPROM in some monitors by using software to take control of the I2C bus.
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Online wraper

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Re: DVI-D to VGA adapters - how do they work?
« Reply #22 on: January 28, 2015, 04:13:05 am »
I don't think they they are intended for programming EDID, never seen them advertised as programming adapter. Why do they add Vsync wire then? They are the same as crappy DVI-I adapters but lacking wires from 4 analog pins which do not exist on DVI-D connector, strange product. Too many of them are made for such a tiny EDID programming market. If that would be the case, I would expect VGA male - DVI female, so you can attach DVI cable to it. Like in this useless crap  :) [url=http://www.ebay.com/itm/White-DVI-D-Female-Analog-24-1-to-15-pin-VGA-Male-Connector-Adapter-dual-link-/281576912777?pt=LH_DefaultDomain_0&hash=item418f4ab789]http://www.ebay.com/itm/White-DVI-D-Female-Analog-24-1-to-15-pin-VGA-Male-Connector-Adapter-dual-link-/281576912777?pt=LH_DefaultDomain_0&hash=item418f4ab789[/url]

Look at the feedback: http://www.amazon.com/Male-DVI-D-Female-VGA-Adapter/dp/B007EA48MA   
http://www.amazon.com/KingWin-DVI-D-Female-Adapter-ADP-04A/dp/B009MPB2M4

I have a better idea why they are made. Market have a demand for cheap DVI-D - VGA adapters and China does fulfill it. Doesn't matter if they do not work, the only important thing is that people buy them :palm:.



 

Offline amyk

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Re: DVI-D to VGA adapters - how do they work?
« Reply #23 on: January 28, 2015, 10:01:12 am »
Vsync could be part of the ISP protocol (reset pin). The DVI is male since it is to be plugged into a female on monitor PCB. No doubt many people are buying these completely unaware that they don't do what they want...
 

Offline Stonent

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Re: DVI-D to VGA adapters - how do they work?
« Reply #24 on: January 28, 2015, 03:30:12 pm »
Not sure what the problem is with those, I have many and they work fine. DVI-D has Analog RGB, HSync and ground on the on the cross section of the connector and VSync on pin 8 (upper pin closer to the cross connector.
That would be DVI-I (I = integrated).
DVI-D (D = digital) has digital only (no pins around the spade connector looking part).
DVI-A (A = analogue) is analogue only.

I've seen HDMI to VGA cables, but I believe they are intended for devices that use proprietary schemes to push analogue video out the HDMI port when configured to do so. I'm guessing this DVI-D to VGA might be a similar thing.

Everyone one of those I've seen, even the cheapest looking ones that appear to have no electronics, actually do.

There's always a chip in one end of the cable that converts.

I used to think that displayport to HDMI cables were straight through as well until I found out they use incompatible signalling technologies and sure enough, you crack one end open and there's a small chip inside.
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