DVI-D to VGA adapters - how do they work?

[n45048]

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.

Digital-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.

[bktemp]

Digital-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.

DVI 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.

[wraper]

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...

What ISP? there is 24CXX eeprom.

[n45048]

DVI 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.

I'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.

[helius]

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.

[bktemp]

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.

[David Spicer]

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?

[David_AVD]

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.

Yep, mentioned several times.  The subject and the first post even specifies "DVI-D".   

[David Spicer]

I stand corrected. Sorry fellas. 

[Rasz]

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.

no
1 there are  products without electronics, chinese one dollar specials is the prime example
2 There are Active and Passive "DP to HDMI" converters

https://en.wikipedia.org/wiki/DisplayPort#Dual-mode

passive has NO CHIPS INSIDE

Digital-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.

I'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.

nope, 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 cable

example: http://www.ebay.com/itm/2013-New-VGA-to-Video-TV-Out-S-Video-AV-and-3-RCA-Female-Converter-Cable-Adapter-/171160421648



Will not work with >90% of products on the market, but ebay is flooded with this crap.