Starter course- VGA is an analog signal requiring 5 wires and a ground. The three colors are sent (R, G and B) over each of three pins and then there is a horizontal sync wire and a vertical sync wire. DVI implies a digital signal (it turns out to be TMDS), but it happens to usually carry an analog component for backward-compatibility. That's what I knew before yesterday.
What I learned yesterday: Generally, DVI on a PC ("A", "single", or "dual") will also output 5 analog signals, RGB H and V that are easily re-wired to a 15-pin VGA plug-- it's a passive adapter and I've got a bin full of them. A digital monitor with a DVI connector ignores these signals entirely. DVI-D is DVI *without* the 4 analog pins, and nothing connected to the v-sync that otherwise would be on pin 8 of a dvi connector. http://upload.wikimedia.org/wi.....pinout.png
(note the "C" pins and pin 8. On a DVI-D plug there are no C1-4, but some form of ground spade is always there.)
HDMI is an encapsulated form of DVI-D with (optionally) audio and (optionally***) the use of some "security" protocols (think DRM, HDCP) and as such has no analog signaling whatsoever.
"Composite" (in this case, in the style of RCA) is everything on one wire (plus ground - think Nintendo or C64) and will get you nowhere with a vga monitor. While TV's are "everywhere" in third-world countries, VGA monitors are everywhere in the entire world!!
As for a $50 converter, I haven't found that. The only DVI-D to VGA converters that I found were one for $179 US and another site with instructions on how to build one out of resistors and transistors, and then some additional tweaking and hacking. There is a "DVI-D to 15-pin D-SUB" with a ton of comments about how it "doesn't work". The description pretty clearly states that it isn't going to give you a vga signal off a dvi-d output. That kind of magic just doesn't happen. You've actually got to have something that can figure out the digital bits and turn them into analog scan lines. Three of them. With sync.
So, I just found the forums. This was my email to [email protected]
I build automation systems for factories and can only think of a million places to use these. I recently switched to global scale technologies' dreamplugs for all input-output that doesn't require a monitor, but all of my display systems run on cheap cheap cheap 17" VGA LCD monitors. Right now they are driven by circa-2004 IBM ThinkCentres. (overkill!)
The only thing I'd want is an output I can plug into VGA. I would be using beagleboards already, but the DVI-D doesn't include the 5 channels needed to pull an analog signal. The DVI-D to VGA adapters are $180 US! The monitors were only $53!
HDMI and composite are great, but there are so many older analog VGA monitors out there. Most of the USB-VGA adapters seem to be "secondary" and require some kind of other vga adapter before they will work, and the ones that don't seem either expensive or completely lacking support. How many pi's do I need to buy to get someone to stick RGBHV output on them?
Either way, I can't wait until you are ready for production! Ground breaking!
***- wikipedia says HDCP is "mandatory" for HDMI, but the source for that isn't cited. http://en.wikipedia.org/wiki/L.....interfaces