DVI-D to VGA adapters - how do they work? - Page 1
Forum section: electronics / general

author: Yansi
author_rank: Super Contributor
timestamp: January 26, 2015, 11:36:57 pm
content: HiA curious question, how do these DVI-D to VGA adapers work?I mean these:http://www.ebay.co.uk/bhp/dvi-d-to-vga-adapterAs 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

author: Whales
author_rank: Super Contributor
timestamp: January 26, 2015, 11:42:32 pm
content: Perhaps there are devices somewhere that (non-standardly) accept D over a VGA D-sub connector, to cut down on connector costs.

author: Yansi
author_rank: Super Contributor
timestamp: January 27, 2015, 12:13:00 am
content: That is possible explanation, but weird. (saves cost on connector, needs adaptor)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=item58bc5fb29bLittle later I have discovered, it is for some dumbass routers, which acceptsUSB via ETH connectors.// It reminds me Dave's rant video about proprietary connectors. That wass great!

author: Unknown
author_rank: Unknown
timestamp: January 27, 2015, 12:30:06 am
content: 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_InterfaceThere 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.

author: Yansi
author_rank: Super Contributor
timestamp: January 27, 2015, 02:22:50 am
content: 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..

author: AlessandroAU
author_rank: Regular Contributor
timestamp: January 27, 2015, 04:27:01 am
content: Quote from: Yansi on January 27, 2015, 02:22:50 amThis is an dumb adapter for almost all modern video cards which still support VGA over the DVI interface.

author: David_AVD
author_rank: Super Contributor
timestamp: January 27, 2015, 09:10:40 am
content: 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.

author: Rasz
author_rank: Super Contributor
timestamp: January 27, 2015, 09:32:00 am
content: 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.

author: wraper
author_rank: Unknown
timestamp: January 27, 2015, 09:36:09 am
content: 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.

author: miguelvp
author_rank: Super Contributor
timestamp: January 27, 2015, 09:47:19 am
content: 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_InterfaceI'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.

author: andtfoot
author_rank: Unknown
timestamp: January 27, 2015, 09:53:48 am
content: Quote from: miguelvp on January 27, 2015, 09:47:19 amThat 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.

author: bktemp
author_rank: Super Contributor
timestamp: January 27, 2015, 10:01:57 am
content: 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.

author: miguelvp
author_rank: Super Contributor
timestamp: January 27, 2015, 10:08:00 am
content: Quote from: andtfoot on January 27, 2015, 09:53:48 amTrue, they are selling them mislabeled, they are just DVI-I to VGA adapters, I should have paid more attention to the postAt 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.

author: rollatorwieltje
author_rank: Unknown
timestamp: January 27, 2015, 10:21:48 am
content: Quote from: miguelvp on January 27, 2015, 10:08:00 amThey 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.

author: Unknown
author_rank: Unknown
timestamp: January 27, 2015, 01:17:02 pm
content: Quote from: miguelvp on January 27, 2015, 09:47:19 amAbsolutely not! DVI-D is digital only. As per the specification.Quote from: AlessandroAU on January 27, 2015, 04:27:01 amNo, 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"

author: wraper
author_rank: Unknown
timestamp: January 27, 2015, 01:21:15 pm
content: 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.

author: Yansi
author_rank: Super Contributor
timestamp: January 27, 2015, 01:23:41 pm
content: No lines of video signal? What's in there then?Or what's their purpose?

author: wraper
author_rank: Unknown
timestamp: January 27, 2015, 01:42:10 pm
content: Quote from: Yansi on January 27, 2015, 01:23:41 pmHere you go. Only VSYNC is used among analog pins.

author: amyk
author_rank: Super Contributor
timestamp: January 27, 2015, 06:42:59 pm
content: 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.htmlhttp://www.aliexpress.com/item/RT809F-hd-USB-LCD-programmer-24-25-93-RTD2120-A-key-to-automatic-identification-speaking-reading/1405765093.html

author: Yansi
author_rank: Super Contributor
timestamp: January 27, 2015, 07:12:17 pm
content: Just wow. Didn't know that it is possibleI should give you 10 points for that answer, thanksBy the way, for what purpose it is needed to modify firmware in the monitor that way?

author: SeanB
author_rank: Super Contributor
timestamp: January 27, 2015, 07:50:34 pm
content: 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.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 28, 2015, 02:35:36 am
content: 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.

author: wraper
author_rank: Unknown
timestamp: January 28, 2015, 04:13:05 am
content: 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=item418f4ab789Look at the feedback:http://www.amazon.com/Male-DVI-D-Female-VGA-Adapter/dp/B007EA48MAhttp://www.amazon.com/KingWin-DVI-D-Female-Adapter-ADP-04A/dp/B009MPB2M4I 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.

author: amyk
author_rank: Super Contributor
timestamp: January 28, 2015, 10:01:12 am
content: 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...

author: Stonent
author_rank: Super Contributor
timestamp: January 28, 2015, 03:30:12 pm
content: Quote from: andtfoot on January 27, 2015, 09:53:48 amEveryone 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.

author: Unknown
author_rank: Unknown
timestamp: January 28, 2015, 06:09:02 pm
content: Quote from: Stonent on January 28, 2015, 03:30:12 pmDigital-to-digital might be one thing which could be performed by an IC under a blob, but digital to analog (or vice versa) is an entirely different type of processing. You essentially need a codec. It's not just a matter of adjusting timing or other elements of the signal, you need to recreate the signal frame by frame. A cable just won't do it, unless of course you're using DVI-I or some other proprietary wiring/signal method which already outputs the desired signal for you.

author: bktemp
author_rank: Super Contributor
timestamp: January 28, 2015, 06:27:36 pm
content: Quote from: Halon on January 28, 2015, 06:09:02 pmDVI to VGA is actually very easy: You simply need to convert the digital data into analogue signals without any timing or format conversions. You can do this using off-the-shelf parts. The other way is more difficult because you must recreate the pixel clock. This can be really tricky.Displayport to HDMI is not as simple as it may sound. True converters are quite rare, because most displayports can output HDMI/DVI signals but only at wrong signal levels. The only necessary conversions are therefore the electrical levels. This can be done by a single chip, but it relies on the video card capable of supporting HDMI mode.

author: wraper
author_rank: Unknown
timestamp: January 28, 2015, 07:41:25 pm
content: Quote from: amyk on January 28, 2015, 10:01:12 amWhat ISP? there is 24CXX eeprom.

author: Unknown
author_rank: Unknown
timestamp: January 28, 2015, 08:54:06 pm
content: Quote from: bktemp on January 28, 2015, 06:27:36 pmI'm not saying it's hard, but it still requires substantial processing. These days it can be done inside a small box. Back in the day (as Dave can attest) this required significant engineering and sometimes expensive gear.

author: helius
author_rank: Super Contributor
timestamp: January 28, 2015, 09:10:18 pm
content: Forgive me if I'm misremembering, but wasn't Dave's teardown of a VGA to LVDS (or TMDS) device?That is going to be more complex than the other direction because it needs to do clock recovery (clock is not explicitly present in VGA signal) and a lot of deskewing.

author: bktemp
author_rank: Super Contributor
timestamp: January 28, 2015, 09:28:52 pm
content: Exactly. VGA to anything digital is complex, because there is no pixel clock. Finding the exact number of pixels per line and the phase alignment requires a lot of digital processing (at least the phase detection and the image position detection this is embedded in hardware in almost all VGA input ics). The really hard part is guessing the resolution. I have build several VGA monitors using those highly integrated ics and detecting the resolution correctly is not as easy as it sounds: There are standards, but many video cards simply ignore them. Every resolution you want to support has to be present in a huge table. If you use a resolution that is not in the table, the monitor either refuses to work, or chooses the nearest entry, but the image will not fit the screen or may even be way outside of adjustment range. And there are some resolution with the same horizontal and vertical frequencies, but different pixel clocks. It is almost impossible to detect them correctly.Digital to VGA is easy, because absolutely no processing is necessary, except the conversion into 3 parallel digital data streams that can be fed into a DAC.

author: David Spicer
author_rank: Contributor
timestamp: January 28, 2015, 10:15:25 pm
content: I may have nissed it, but i didn' t see anyone mention that there are two superficially similar Dvi connectors. One has 4 more signals than the other, arranged as a cross at one end of the connector. These are rgb and hsync . Vsync is somewhere in he other big array of pins, because four is simply not enough. The other type has a flat wide pin only. I can't remember the precise difference between them, but there is a wikipedia article that explains it.As several people have pointed out, the second type needs some heavy lifting to convert, but the firs is simple, just wires. Pinouts are on the wiki article.There are chips which do this from silicon image among others, but they are big evil and unapproachable no fab ip only companies, whose first question is: How many million pieces a year will you be needing?

author: David_AVD
author_rank: Super Contributor
timestamp: January 28, 2015, 10:54:05 pm
content: Quote from: David Spicer on January 28, 2015, 10:15:25 pmYep, mentioned several times. The subject and the first post even specifies "DVI-D".

author: David Spicer
author_rank: Contributor
timestamp: January 29, 2015, 06:33:35 am
content: I stand corrected. Sorry fellas.

author: Rasz
author_rank: Super Contributor
timestamp: January 29, 2015, 11:29:15 am
content: Quote from: Stonent on January 28, 2015, 03:30:12 pmno1 there are products without electronics, chinese one dollar specials is the prime example2 There are Active and Passive "DP to HDMI" convertershttps://en.wikipedia.org/wiki/DisplayPort#Dual-modepassive has NO CHIPS INSIDEQuote from: Halon on January 28, 2015, 06:09:02 pmQuote from: Halon on January 28, 2015, 08:54:06 pmnope, HDMI, DVI, LVDS digital signals regain VGA timings, all you need is deserializer and DAC.Displayport. like MIPI DSI, on the other hand sends video packetized. Lets you do magic like adaptive framerate, update only parts of the image, or suspend data transmission while regaining frozen picture on the screen (display has its own framebuffer).Btw another type of magic chinese cable that fits into this threat is VGA/DVI/HDMI/DP/microDP to chinch video cableexample:http://www.ebay.com/itm/2013-New-VGA-to-Video-TV-Out-S-Video-AV-and-3-RCA-Female-Converter-Cable-Adapter-/171160421648Will not work with >90% of products on the market, but ebay is flooded with this crap.