My BeagleBoard is still catching dust, somewhere in a dark corner. I never used it, which is kinda sad, isn’t it.
Well, it only has a digital video output, so I cannot connect a normal vga (crt) monitor to it. (Of which I have a few in the basement)
So, I’ve obtained a used flat screen with DVI-D input, back in the days I still worked at Unilogic, and they were replacing their monitors.
Unfortunately, it’s had 5:4 aspect ratio, and it’s native resolution is 1280×1024, which is not supported by the BeagleBoard. Well, it is supported, but with a refresh-rate of 50Hz, which is not supported by the monitor.
Well… I am using this monitor at my desktop nowadays… But meh…. those modern flat monitors with non-standard (non 4:3) aspect ratios, and things like a ‘native resolution’.
A normal CRT monitor would be much simpler… they usually have the same aspect ratio, and the image doesn’t get distorted when running at a lower then maximal resolution.
But…. there is no VGA output on the BeagleBoard… so… I was googling again…. and I found a Hawkboard.
Another ARM board, and it has a VGA-out. Looking at it’s schema, I saw it used a THS8135 to provide in VGA output. So, I started googling if there was a converter for the BeagleBoard using this chip…. but in stead I came accros this
http://www.linusakesson.net/hardware/beagleboard/vga.php
Very simple circuit… this should do the trick. Why should it be complicated?
So, why are HDMI(DVI-D) to VGA converters to expensive? This circcuit that I’ve found is just a few resistors and two transistors.
So, this circuit should costs about 4 euros!!! Why are those converters a few hunderd euros? (Okay, thet are connected to the DVI-D port, and this circuit goes to the LCD interface….)
So, I guess I should get some resistors, transistors and a soldering iron, much cheaper then a hdmi to vga converter or a new monitor…
Also, this seems to be a DVI-D to raw converter….
http://www.harbaum.org/till/dvi2par/index.shtml
so, why are the dvi-d to vga converters so expensive???? It can be made for dirt-cheap!!!
The circuit contains three basic N-2N digital analog converters (separate converters for Red, Green and Blue).
You can probably remember ProHES at Fontys where we've used the same circuit for digital-analog audio conversion. The sound quality was acceptable, but not high quality.
The reason why those converters are so expensive is because they contain some high quality low-noise digital to analog converters.
You can build this circuit and I'm sure you'll see the colors, but you'll never get the high quality colors the beagleboard outputs. There is simply too much tolerance difference between the resistors to get high quality digital to analog conversion.
High quality conversion is only possible when all resistors have exact the same tolerance and exact the same mixture of compounds. This is only possible when all converters are integrated on a single chip with one single production process.
Of course there is some tolerance in cheap resistors, but do you expect a normal CRT monitor to be sensitive enough to actually see this?
I was thinking about something like this before, but I never expected this would produce a VGA compatible signal. But apparently is does.
btw. I might expand my version to do full 24 bit colour.
Well… the circuit may be a little more then those € 4, since I forgot to count the connectors. But still, it won't get close to the € 180 a pre-built converter costs.
S The converter IC for getting DVI-D data to parallel data is how much? (http://focus.ti.com/docs/prod/folders/print/tfp401a.html) is the equivalent, so lets look it up (http://fr.farnell.com/texas-instruments/tfp401apzp/ic-receiver-495mbps-smd/dp/1470430?Ntt=TFP401A) in single unit quantities it's 9.72 Euro. This is one part, need a connector how about hdmi in VGA out so (http://fr.farnell.com/itw-mcmurdo/hde15poltd/sub-d-hd-male-15p/dp/1075294?whydiditmatch=rel_3&matchedProduct;=HDE15POLTD&Ntt;=HDE15POLTD) is 5.46 Euro.
HDMI (http://fr.farnell.com/tyco-electronics/1746679-1/hdmi-receptacle-19pos-pcb/dp/1869507?Ntt=1746679-1) 4.75 euro.
The PCB is going to cost a bit (estimate is 250 Euro for 5 4 layer boards). So it's not as expensive as it first appears.
I haven't converted the digital output to analog yet nor have I passed the EDID information into the VGA connector there are a few other things that need to be done.
I know you are attempting to keep costs down and use what you have, however DVI-D is completely different than VGA so there is a considerable amount of 'things to do' in between. Use this as a starting reference (http://en.wikipedia.org/wiki/DVI-D ). the TFP401A is the successor to the TFP101A which is about to be absoleted. I believe the primary difference is speed.
I am not quite following.
Apparently, you want to take the HDMI output that comes out of the TFP410 and convert it back to parallell using a TFP401?
Why on earth would I want to do that, when I can directly take the parallell signal?
So, I won't need a HDMI connector either, a simple flat cable would be sufficient.
I do need the VGA connector, and the Digital to Analogue conversion circuit, as described on that link.
Indeed, using that I still have do DDC/EDID information, but do I really need it if I just want a picture on my screen?
Well… DDC/EDID… it's just a I²C bus. The only "problem" there is that the BeagleBoard and the monitor use different voltage levels. If you look at the site I linked you see a sparkfun level shifter is suggested http://www.sparkfun.com/products/8745
Looking at farnell, I guess it comes down to this: http://nl.farnell.com/texas-instruments/sn74cb3t3245pw/translator-logic-level-converter/dp/1436823
If implementing this DDC/EDID interface would mean the timings are configured automatically, I might consider adding it. But if I will still have to enter the timing manually, I don't see the point of implementing the DCC/EDID interface.
Oh… maybe I get it now, as the link I posted in my first comment suggests something like this, please note that it is about a revision B BeagleBoard. THe revision B BeagleBoard doesn't offer the RAW LCD interface, which the revision C BeagleBoard does.
The main post was alltough, about converting the LCD RAW to VGA.
THe link in the first comment was just meant as an extention. But even when looking at that it just shows a 2 layer BCP?
A normal CRT monitor is always much simpler, they usually have the same aspect ratio, and the image doesn’t get distorted when running at lower then maximal resolution.
The one you’re linking is quite expensive I must say. Looking at prices like that is the reason why I would consider building the discussed converter. There are much cheaper ones available on sites like dx.com. I’ve mentioned one of these in my more recent posts. Even though that was an HDMI to VGA converter. But that’s the connector the BeagleBoard has.
However, there is another problem with that solution in combination with the BeagleBoard. It violated the HDMI specification by drawing too much current. There is a polyfuse on the BeagleBoard causing the video output to stop. It is safe to replace this polyfuse with a heavier model. I don’t have the numbers here at the moment at the current which is safe for the BeagleBoard to draw. Anyhow, I got the replacement polyfuses at home. I haven’t replaced it yet, as I am not sure if it is any good because of the USB problems I am experiencing with my BeagleBoard.
Why do you find that to be very expensive?
The converter i’ve linked costs $139, yours $159, the beagleboard costs $125. If the converter costs more then the device I am trying to connect, I don’t think it’s worth it. Besides, comparing the price of a converter to the price of a new monitor makes me wonder if it’s worth using such a converter with an old monitor in stead of buying a new monitor.
Nowadays I’ve found converters that cost much less, and are in fact cheap enough to make sense to be used in combination with a Raspberry Pi.