VGA


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by EddieG » Mon Aug 29, 2011 12:58 am
Hi

I notice the device doesn't provide VGA as standard ... which seems odd.

Does your HDMI output adhere to the DVI standard (and also provide the analog VGA signal)? i.e. is it a case of using a $5 pin converter cable, rather than a $50 digital-to-analog converter box?
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by kaidenshi » Mon Aug 29, 2011 3:43 am
I'm not positive, but I believe if they are using an industry-standard HDMI output scheme then it should be pin-compatible with DVI. HDMI is basically DVI with audio and optional copy protection. I don't think VGA is in the spec, though I may be wrong.
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by EddieG » Mon Aug 29, 2011 5:13 am
Thanks for the info. So, HDMI is pin compatible with DVI-D, but doesn't have the the DVI-A (analog) wiring? Sorry for the ignorance ... haven't had a TV for years, so the HDMI connector is all new to me.
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by abishur » Mon Aug 29, 2011 5:21 am
The answer is that you will be able to purchase a very simple HDMI to DVI-D convertor if you want to use DVI. But no, you'll need a special converter if you want to go to VGA. That said, there is also a composite (RCA) video out as well. The idea going on here is that everyone has a TV that can accept either composite or HDMI video. This product isn't meant to be a main computer replacement, so why worry about a clunky VGA port? Even in third world countries there is an abundance of old TVs with composite video, so the r-pi could be sent to such a location and work quiet amazingly.
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by EddieG » Mon Aug 29, 2011 5:56 am
Yep, but I wasn't really interested in DVI, more the fact that the DVI standard is backward compatible with VGA (via DVI-A). I guess the (in turn) backward compatible HDMI -> DVI doesn't inherit this.

There is also an abundant supply of old monitors (in the third world), with a much better quality output than a composite TV.
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by Attackid » Mon Aug 29, 2011 7:23 am
sorry for being noob at all these components but I just want to connect my vga monitor to the device. Do I just use a hdmi to vga dongle converter I have like 20 at my house.
I realize sound can't travel through that so, I hope someone makes a setup video cause I'll not computer tech guy at all.
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by RobinJ » Mon Aug 29, 2011 8:11 am
Quote from Attackid on August 29, 2011, 08:23
sorry for being noob at all these components but I just want to connect my vga monitor to the device. Do I just use a hdmi to vga dongle converter I have like 20 at my house.
I realize sound can't travel through that so, I hope someone makes a setup video cause I'll not computer tech guy at all.

May I remind you that the Raspberry Pi is meant to learn how to program (and do it properly)?
http://www.raspberrypi.org/?pa.....#038;t=182
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by EddieG » Mon Aug 29, 2011 8:37 am
@RobinJ

Thanks for that link. I had searched for VGA, but had come up with nothing (someone else had the same negative search result in the the other thread).

Regarding the programming aspect: I'm finding it very difficult to find out anything about the Broadcom hardware (pinouts or block diagrams) ... something that's going to be useful if people want to do low-level coding. I assume that the Broadcom SoC has direct HDMI/composite out, but no VGA. If that's the answer to my original question? It's not obvious, as other SoCs do have VGA out (so no extra circuitry required).
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by Michael » Mon Aug 29, 2011 8:59 am
EddieG: That's correct: the SoC has hdmi and composite outputs but no built-in S-Video or VGA capability.

The nearest thing we have to a block diagram at the moment is this from abishur:
https://lh4.googleusercontent.com/-kS2971V7gJ0/TkrfCY6svZI/AAAAAAAAAig/d3KH-BBuzik/Alpha%252520Board.jpg
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by Chris » Mon Aug 29, 2011 9:02 am
VGA was not standard most likely since the concept was to hook it to a TV, not many TV's have CGA ports but most have HDMI and ALL have the standard tv connectors.

You could always use a HDMI to VGA cable if need be.
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by toxibunny » Mon Aug 29, 2011 10:57 am
I thought we'd decided that the plain cables won't work, and that you'd need to buy a more expensive converter box like http://www.hdmitovgaadapter.net/...
note: I may or may not know what I'm talking about...
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by abishur » Mon Aug 29, 2011 1:22 pm
Really guys, I can understand missing something said in a different thread since the search functions of this forum can by a little off, but you at least need to read the thread you're posting in before posting, especially when it's only a couple of posts long ;)

As I said in the fourth post of this thread the r-pi does HDMI and composite (RCA) video out. The HDMI will be able to do a simple gender bender cable to DVI-D. As a result, if you want VGA you will have to get a special digital to analog video conversion box. You can argue the logic of this all day long, but it's already been decided so it might be like spitting in the wind :) If you just want to know their logic for no VGA, this is simply my understanding of how it was decided

Their goal with the r-pi is a lot like the BBC micro, it's a box which can turn a TV into computer. It's not really designed to attach to traditional monitors. Almost everyone has a TV lying around that you can with minimal effort attach a cable to for a little bit and mess around on the computer. Low income families tend to not have old spare monitors just lying around. Ergo HDMI and composite video were chosen over DVI or VGA.

Now for just a guess in the dark on my part but if you think about the size of each of these ports (VGA = 1.5x0.75 CM, HDMI = 1.39x0.445 CM, RCA = 0.9 CM) you can get twice the video output options for almost the same footprint on the board (to be fair on VGA I'm only talking about the size of the VGA plug, not the entire port). Since a small form factor is part of the goal here, sacrificing VGA (a plug for a device they're really not concerned about) for HDMI and RCA outputs (which hook up to a screen almost everyone has access to) makes a whole lot of sense.
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by EddieG » Mon Aug 29, 2011 11:57 pm
You could always use a HDMI to VGA cable if need be.


See my previous point about the HDMI/DVI-D/DVI-A. Simple cable if the HDMI feed contains the DVI-A signal...but as it probably doesn't then there isn't really an easy solution. Expensive digital-to-analogue converter (HDMI->VGA), or expensive scan doubler (composite->VGA). The latter is normally pretty rubbish with PAL (rather than NTSC) as you don't have nice frequency multiples.
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by EddieG » Tue Aug 30, 2011 12:22 am
@abishur
Michael answered my original questions.

To iterate, the HDMI/composite video outputs are defined by the pinouts available from the Broadcom System-on-a-Chip (SOC). As the SoC doesn't provide a VGA line, additional circuitry would be required (adding expense to the unit).

My initial assumption was that a VGA line was there, but not being used ... and could perhaps be utilised using a couple of cents worth of components via a breakout board (for example. see the 'reversable(sic) video card' for the Parallax Propeller microcontroller). This is not the case.
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by EddieG » Tue Aug 30, 2011 12:29 am
Quote from abishur on August 29, 2011, 14:22
Their goal with the r-pi is a lot like the BBC micro, it's a box which can turn a TV into computer.


That's a pretty odd choice of analogy :D The rich kids with the BBC micros had monitors ... it was the rest of us with ZX Spectrums, C64s etc who used TVs.
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by abishur » Tue Aug 30, 2011 12:36 am
The analogy is actually one I stole from Eben in his latest video ;)
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by jeremy » Tue Aug 30, 2011 12:46 am
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 info@ today:

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!


Jeremy

EDIT:
***- wikipedia says HDCP is "mandatory" for HDMI, but the source for that isn't cited. http://en.wikipedia.org/wiki/L.....interfaces
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by EddieG » Tue Aug 30, 2011 1:15 am
Quote from jeremy on August 30, 2011, 01:46
As for a $50 converter, I haven't found that.


The ~$50 converters are for composite->VGA

I just did a search and found a model from startech.com for $46. It has rubbish reviews though.
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by jeremy » Tue Aug 30, 2011 1:49 am
Ahh. I didn't even consider going that direction.

As for converting composite to VGA, the converters are probably reviewed badly because of the lack of resolution on a composite signal? 480~ visible for NTSC or under 300 for PAL-- I assume this is a PAL type of composite signal... I guess composite could also mean SECAM, somewhere in the 600 lines area I think? But all of those are interlaced and will make normal-sized text unreadable. I don't know about the different versions of composite video, but I can't see how they would be compatible with each other. As for the abundance of composite-ready tv sets in the world- are those PAL composite? It seems like in third-world countries they would be SECAM which makes the point moot if the R-pi is using PAL. But then again, the HDMI seems completely counter-intuitive to the argument for not including VGA-- "we want it to be available on cheap tv's" but there are exactly zero cheap sets with hdmi input!! :)

Either way, it seems crazy to pay $50 (or $179!!) for an adapter to hook a $50 display to a $35 board. Is it so very costly to stick a chip that can put out even 1024x768 in rgb?

If all else fails, for my application I'll just start migrating vga-only monitors out and getting newer dvi monitors. I guess I may have a few used ones for sale in the near future. :) (wonder what the depreciation on those is going to be after a year of use-- 95%!?)

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by abishur » Tue Aug 30, 2011 2:38 am
I got my convertor of Amazon.com for 25 bucks, it works amazing.

@Jeremy, well composite video certainly doesn't look pretty, but you can do text work on it just fine at the right resolutions, which fits in perfectly with the r-pi's goals. As for HDMI, it actually makes a lot of sense. For one thing the chip already supports it so you're not paying more for the feature, the connector isn't significantly more expensive than a DVI connector, but it is a good deal more compact (which means the PCB cost is lower). Plus, for a 5 bucks you can buy a plain Jane cable that will let you hook up to DVI devices (yes the opposite is true as well, but again, HDMI = smaller footprint), and it's a nice advertising feature. It draws in hobbyist to purchase their device so they can send more off to children who need it.

It's all very well thought out by the r-pi team. The video outputs definitely reaches the maximum percentage of people for the lowest overall cost. I mean seriously, I can easily name you 10 families in America, in rich, middle class, and lower income families who don't even have a monitor VGA or otherwise anymore because we've gone the way of the laptop. HDMI/Composite for video outputs was definitely the best way to go ;) (Of course that's still rough for hobbyists like you or I who have spare monitors with VGA lying around. I'm just saying I understand and support the decision.)
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by jeremy » Tue Aug 30, 2011 2:56 am
I didn't mean to say it wasn't well thought out. If there is a $25 hdmi->vga converter at amazon, that would certainly fill my needs. Do you have a link?

I thought your deleted post was more interesting (damn the emailer, right!?) -- we only get cheap (like I said, $50~) lcd monitors and of the last ~100 monitors I've bought in the last 24 months, none have had dvi. They are SAMSUNG model E1720NR if you're interested, but I just bought CDW's last 24 and they are discontinued! Probably replaced with a model that has only DVI!! As for family and friends, yes-- laptops (of the $300 variety) are common, but you can't hook hdmi or composite into those, so they aren't really valid for this conversation! :)

I understand completely the decision, I'm just saying that if there was a "C" version with a vga connector for $50 available now I'd order 25 today! And ask for overnight shipping! ;)

Jeremy
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by EddieG » Tue Aug 30, 2011 2:59 am
Of course there is the $10,000 licensing fee for HDMI (to consider).
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by jdarling » Tue Aug 30, 2011 3:15 am
I'm another one of those automation freaks named Jeremy (not the same Jeremy that started all of this) with lots of 15 and 17" flat screens lying around who would love to hook them up to Pi's. I'm wondering if something along the lines of using a minimalist (read sub $5) ARM, FPGA, or ASIC for conversion from DVI to VGA would work that could be sold as an addon. I'm no expert with HDMI or DVI (actually I know little to nothing about them), but I'm willing to start looking into it as I have HDMI and DVI monitors/TV's around as well for testing, unfortunately I don't have any devices with these outputs on them LOL. This might be a case where Atmel, TI, Parallax, etc and a cheap (relatively speaking) SchmartBoard would be useful.

Will have to dig through these piles of "Free Samples" and "Demo Kits" they keep sending me to see if anything is useable.

Any suggestions on where to start with DVI->VGA in software would be? No, I'm not lazy, but I hate digging through piles of Google results when someone has a good link handy.

- Jeremy (2?)
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by jeremy » Tue Aug 30, 2011 3:16 am
Is there any validity to an HDMI "requiring" HDCP? If it requires content protection, then I don't see how converting it to DVI would even work-- there's no way to pass HDCP onto the DVI is there? I'm not saying there isn't a cable that does that (I don't know) there's just something that doesn't add up there. Maybe TMDS can have HDCP for all I know.

Really? $10,000!!?? Now I've got to go see what consortium of manufacturers owns that patent and is milking oem's for licensing fees!

Jeremy
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by jeremy » Tue Aug 30, 2011 3:22 am
EDIT: this is DVI-D to VGA. I don't know how you get DVI-D out of HDMI.

Well, I'm good with an iron and resistors and all, but this just looks like a nightmare. Here's the link!!

(I mean, seriously, it's just "two transistors and a bunch of resistors." how hard could that be?!)

http://www.linusakesson.net/ha.....rd/vga.php

(don't say I didn't warn you, but apparently it's "easy" to build this to convert 24-bit color!)

Jeremy (2) :)

PS: oh wait- are you two or am I two?!
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