ATI's magic DVI to HDMI Dongle

[BoostGTR]

Hey guys, i'm just tidying up my first HTPC build and with my current cabinet setup i had been looking at using a DVI to HDMI cable to connect my Sapphire 4670 to my Sony HSS1300 Speakers as I dont have alot of space so needed a compact solution.

Obvioulsy just jumping straight in I bought a 1.3 spec DVI to HDMI cable and swapped out my current DVI Dongle to HDMI cable and thats when I realised i was getting no sound.

I've done a bit of trawling round the t'internet and found out that the supplied dongle is actually special in a sense that it specifically enables the sound to be passed out and without using it a standard DVI to HDMI cable will not work.

I was just wondering if anyone knew exaclt what the dongle did and would it be possible to adapt a standard DVI to HDMI cable or is there some special hardware in the Dongle? I'm hopng its just a cross wiring of one of the pins.

Cheers!

[mslide]

Originally Posted by BoostGTR

I was just wondering if anyone knew exaclt what the dongle did and would it be possible to adapt a standard DVI to HDMI cable or is there some special hardware in the Dongle? I'm hopng its just a cross wiring of one of the pins.

Short answer: You have to use ATI's dongle. It's not just a simple re-wiring of the pins. In fact, the wiring the dongle does is exactly the same as a standard dongle.

Long answer: This is what I've gathered from my curious research on that stupid little thing. It's no magic at all. Actually, it's very simple and quite clever. ATI does not use a proprietary DVI connector on their cards, contrary to what most people say on internet forums and articles. There are no dedicated audio pins on a standard DVI or HDMI connector, nor does ATI employee any on their connector. DVI and HDMI connectors contain 3 sets of data signals that are used to transmit video data. The manner in which the video data is sent is basically the same for DVI and HDMI (thus why it is so easy to convert one to the other). For HDMI, audio is interleaved between the video data, on the same pins. This is how ATI transmits audio over their DVI port and is also the reason why you don't loose the audio when you use their dongle and convert is back to DVI to HDMI to DVI to HDMI, etc.

So, what does the dongle do? It simply tells the ATI card that it is plugged in and to start sending the audio (again, not over dedicated audio pins but over the 3 sets of data signals). Basically, when the dongle is plugged in, ATI pretends the DVI connector is an HDMI one and electrically treats it like that.

How does it detect that the dongle is plugged in? The DVI and HDMI specs include a 2 wire serial bus called I2C ("I squared See") that is widely used in electronics (including motherboards). This bus is used to transfer the EDID information. The ATI dongle includes a small memory chip, either an EPROM or some other non volatile RAM device, on that bus. Any time an adapter is plugged in, the ATI card tries to probe for that eprom and read the data from it. If it's a real ATI dongle, it then turns on the audio.

This is all just my theory but I'm 95% certain I'm right. I would love to crack open one of these to make sure, but I'm not THAT curious. I also once saw a schematic of that dongle somewhere that described this also.

[Smitty2k1]

Originally Posted by mslide

Short answer: You have to use ATI's dongle. It's not just a simple re-wiring of the pins. In fact, the wiring the dongle does is exactly the same as a standard dongle.

Long answer: This is what I've gathered from my curious research on that stupid little thing. It's no magic at all. Actually, it's very simple and quite clever. ATI does not use a proprietary DVI connector on their cards, contrary to what most people say on internet forums and articles. There are no dedicated audio pins on a standard DVI or HDMI connector, nor does ATI employee any on their connector. DVI and HDMI connectors contain 3 sets of data signals that are used to transmit video data. The manner in which the video data is sent is basically the same for DVI and HDMI (thus why it is so easy to convert one to the other). For HDMI, audio is interleaved between the video data, on the same pins. This is how ATI transmits audio over their DVI port and is also the reason why you don't loose the audio when you use their dongle and convert is back to DVI to HDMI to DVI to HDMI, etc.

So, what does the dongle do? It simply tells the ATI card that it is plugged in and to start sending the audio (again, not over dedicated audio pins but over the 3 sets of data signals). Basically, when the dongle is plugged in, ATI pretends the DVI connector is an HDMI one and electrically treats it like that.

How does it detect that the dongle is plugged in? The DVI and HDMI specs include a 2 wire serial bus called I2C ("I squared See") that is widely used in electronics (including motherboards). This bus is used to transfer the EDID information. The ATI dongle includes a small memory chip, either an EPROM or some other non volatile RAM device, on that bus. Any time an adapter is plugged in, the ATI card tries to probe for that eprom and read the data from it. If it's a real ATI dongle, it then turns on the audio.

This is all just my theory but I'm 95% certain I'm right. I would love to crack open one of these to make sure, but I'm not THAT curious. I also once saw a schematic of that dongle somewhere that described this also.

Great work! I feel like this should go somewhere in "Guide to building an HD HTPC" that is at the top of this forum...

[dondu]

Originally Posted by mslide

If it's a real ATI dongle, it then turns on the audio.

And of course, this means that ATI could easily have a setting in their driver to override the dongle, and force HDMI audio always on (or off).

So... Why do they have the custom dongle at all? If it could just be controlled by software?

<sigh />

--Don

[BoostGTR]

mslide your a legend, cheers for the info that has certainly raised my interest in the matter.

It certainly raises the question as to why ATi have designed the card like this. When you think that if someone isnt interested in getting Video and Audio from the card then by using a standard DVI to VGA cable works without a problem as does DVI to HDMI.

Like dondu said why not just add an option in the Catalyst Control Center for the option of sending decoding Audio if ATi were specifically after reducing overheads in the cards performance when users aren't looking for Audio.

And yes I'm surprised this "feature" is not more widely documented in discussions.

All in all a very strange design!
Come on ATi please let us use 3rd party cables, my HTPC looks stupid sticking out 2 inches of my cabinet!!!

[Bastian74]

The ATI DVI-HDMI adapter uses a lot more pins on the DVI interface than you usually see. I would think that it uses these extra pins for hdmi

[dondu]

Originally Posted by Bastian74

The ATI DVI-HDMI adapter uses a lot more pins on the DVI interface than you usually see. I would think that it uses these extra pins for hdmi

No, those extra pins are not used for HDMI.

The DVI connector has several options: One option is to include VGA output, and that adds pins If you can connect a DVI-to-VGA dongle and have it work, then the DVI out has the VGA pins. Also, lots of video cards these days are dual-link DVI, and that adds pins too.

So the "extra" pins you are seeing are the pins to support VGA, and the pins to support dual-link DVI.

--Don

[renethx]

It is interesting that the DVI port of Radeon HD 3200 (AMD 780G chipset IGP) carries audio with any generic DVI-HDMI adapter (confirmed by a couple of people including me).

[bokis]

Originally Posted by renethx

It is interesting that the DVI port of Radeon HD 3200 (AMD 780G chipset IGP) carries audio with any generic DVI-HDMI adapter (confirmed by a couple of people including me).

I´m gona try this to my Onkyo 576, I have a HD3850 dongle that should work

Edit Yes their is audio, but put in the Onkyo in the mix no go it´s still a dvi device

[mslide]

Originally Posted by BoostGTR

mslide your a legend, cheers for the info that has certainly raised my interest in the matter.

Oh, I'm certainly no legend. I'm just an electrical engineer by day who's very curious about things like this.

Originally Posted by BoostGTR

It certainly raises the question as to why ATi have designed the card like this. When you think that if someone isnt interested in getting Video and Audio from the card then by using a standard DVI to VGA cable works without a problem as does DVI to HDMI.

Well, DVI to VGA is never an issue as there are dedicated pins on a DVI connector for the analog video signals. For DVI to HDMI, read on...

Originally Posted by BoostGTR

Like dondu said why not just add an option in the Catalyst Control Center for the option of sending decoding Audio if ATi were specifically after reducing overheads in the cards performance when users aren't looking for Audio.

Honestly, I only read enough of the DVI and HDMI specs to satisfy my curiosity of this dongle. I've often wondered why not just have a soft-switch in the drivers somewhere, as you and dondu are suggesting and let someone use an ordinary DVI to HDMI cable. I'm guessing that they do it this way just to cover their butts and to make sure they are 100% in compliance with the DVI spec. I'd bet almost anything that having the audio turned on all the time, or accidentally, wouldn't hurt a DVI device (i.e. connect the ati card to a monitor via a DVI cable, but have the audio turned on by accident), but they probably don't want to take the chance.

The ATI DVI-HDMI adapter uses a lot more pins on the DVI interface than you usually see. I would think that it uses these extra pins for hdmi

There are no spare pins on a DVI connector. Every single one of them has a function, whether it be for transmitting the analog VGA signals, or dual-link DVI support. Even if they did steal pins just for the audio, it wouldn't be possible to interleave it back into the HDMI data stream using a connector like that. There would have to be a processor inside the dongle that could process the stream and insert the audio in. Why do it that way when the processor on the actual graphics card can do that? They also can't start mucking around with the dvi pinout, and reassigning (say the VGA pins) to something else, because then that could make them non-compliant with some DVI devices. DVI and HDMI are actually very similar when you take away the physical connector differences.

It is interesting that the DVI port of Radeon HD 3200 (AMD 780G chipset IGP) carries audio with any generic DVI-HDMI adapter (confirmed by a couple of people including me).

I'm guessing that maybe they finally realized that it's no big deal to just send out the audio all the time. Things like this dongle sometimes come about out of ignorance. I can see the meeting now:

"Can we just send the audio out all the time so that anyone can use a standard DVI-HDMI connector?"

"Huh? No one here really knows? Okay, well lets just make a dongle that only causes the audio to turn on when it's plugged in then. Now we're 100% compliant with the DVI spec."

So, again, I bet they did it this way just to cover their asses.

Disclaimer: I don't work for ATI, not do I have any sources that can back up my theories on how this works. The way to prove my theory would be to use an ohm-meter to see if the pinouts of the dongle match that of a standard dongle or DVI-HDMI cable. Then, crack it open to see if there is a small eprom connected to pins 6 & 7 of the DVI connector. I would do this myself, but I don't own one of those dongles, nor am I THAT curious to spend money on one just to destroy it in the process.

[dorian11]

I like your theory about the I2C bus being at the root of the special ATI dongle. I'm trying to figure a way around it 'cause I just bought a 50 foot DVI to HDMI cable and it doesn't send the audio as is... and I don't want to go and buy a new 50 foot HDMI to HDMI cable. I was hoping it would be an easy mod -- like figuring out which pins they repurpose for for the I2C bus and hooking them up. I'm hoping there isn't an I2C EEPROM in the dongle... ideally, there is just something (like a pulled up or pulled down pin) that tells the card that the dongle is there an then it turns on the audio.

I had suspected that the I2C bus needed to be there so the downstream device could be told how to decode the audio or to even look for the audio, but peoples experience (renethx) with generic adapters suggest it is just a mater of enabling the interleaved audio data in software...

I'll buzz out my adapter and see if I can figure it out.

[walford]

First you have to have the special ATI DVI-HDMI adapter for your card in order to tell caard to multiplex the audio of the same pins as the video in accordance with the HDMI spec.
Then if use an HDMI(male)-DVI(female) adapter to connect the HDMI out of the special ATI adapter to the DVI end of your long DVI-HDMI cable you will be all set.
The reason that ATI uses a special is to insure that your are not sending out audio out of its DVI connector since that could damage the unit on the other end when just using a DVI-DVI connection.

[mslide]

Originally Posted by dorian11

I like your theory about the I2C bus being at the root of the special ATI dongle. I'm trying to figure a way around it 'cause I just bought a 50 foot DVI to HDMI cable and it doesn't send the audio as is... and I don't want to go and buy a new 50 foot HDMI to HDMI cable. I was hoping it would be an easy mod --...

I'm still hoping that someone will crack open one of those dongles and either prove or disprove my theory. You can try to buzz out the connector, but it won't prove whether or not an EEPROM is inside it. If it is there, it just hangs off of the I2C bus, which would be passed through anyways as that I2C bus is part of the DVI and HDMI standard. That's how the EDID info is sent. I'd be surprised if they simply used a pull-up resistor to tell if it's plugged in.

I had suspected that the I2C bus needed to be there so the downstream device could be told how to decode the audio or to even look for the audio, but peoples experience (renethx) with generic adapters suggest it is just a mater of enabling the interleaved audio data in software...

I agree that it could just be done in software. I don't really know why some would require the dongle where as some other cards, or IGP mobos, can use a generic adapter. Maybe ATI initially wanted to cover their butts, so they came up with the dongle so there was no possibility of damaging a DVI monitor and then later realized it wasn't a big deal?

Disclaimer: I don't own one of these cards, so take what I say with a grain of salt. I really wish an engineer, who worked at ATI, would speak up

edit: I had more written, but Walford's post contains the same solution I was going to suggest (hdmi male to dvi female adapter in between the ati dongle and your cable).

[dorian11]

I thought at first the I2C wasn't part of the DVI interface... But I buzzed it out and it looks like it is -- DDC clock and DDC data are SCL and SDA respectively.

I also found that there is about 500k between 5V and ground on the ATI adapter where as a generic adapter is open, which fits with the idea of there being an active circuit that uses 5V.

I'm able to get EDID info from my Panasonic TV without the dongle... I'm starting to think that maybe there is an EEPROM in there, but as some have suggested, the ATI card just uses it to verify that the dongle is there before sending audio data so as not to screw up a standard monitor (I guess the monitor, not knowing anything about HDMI, would attempt to decode the extra data as image data).

Anyway, I'll probably never find out for sure -- I'll just get the extra adapter as you both mention

thanks

[walford]

I have seen references to an an eprom in the dongle and I beleive it is used to modify the EDID data coming from the destination and going to the graphics card so that the card knows it can also send audio that the card can generate.
An I beleive that this is why not all DVI-HDMI audio capable use the same version of the dongle

[nalawod]

Isn't it also true that you need to use the ATI provided dongle in order to properly detect your HDTV and the colorspace's available? I know that the "pixel format" option was NOT available in CCC until I used their dongle. So, yet another reason to use theirs and not a generic.

[vkristof]

Originally Posted by mslide

I'm still hoping that someone will crack open one of those dongles and either prove or disprove my theory. You can try to buzz out the connector, but it won't prove whether or not an EEPROM is inside it. If it is there, it just hangs off of the I2C bus, which would be passed through anyways as that I2C bus is part of the DVI and HDMI standard. That's how the EDID info is sent. I'd be surprised if they simply used a pull-up resistor to tell if it's plugged in.



I agree that it could just be done in software. I don't really know why some would require the dongle where as some other cards, or IGP mobos, can use a generic adapter. Maybe ATI initially wanted to cover their butts, so they came up with the dongle so there was no possibility of damaging a DVI monitor and then later realized it wasn't a big deal?

Disclaimer: I don't own one of these cards, so take what I say with a grain of salt. I really wish an engineer, who worked at ATI, would speak up

edit: I had more written, but Walford's post contains the same solution I was going to suggest (hdmi male to dvi female adapter in between the ati dongle and your cable).

This is a function of AMD/ATI's lack of public documenation. Intel, blesss their profit-making souls, puts a LARGE amount of their documentation online. That's how you can find out the reason behind some of Intel GMCH oddities (IGP defeated when add-in PEG card used, etc.)

AMD/ATI does NOT. I've complained to various AMD Apps engineers who appear at trade shows and they typically act chagrined.

Note that ATI used to peddel that DVI to component dongle, so the new dongle might be a corporate philosophy carryover. Companies can do many dumb things becasue of institutional reasons.

Note that the EDID in HDMI monitors also contains the audio configuration information, so the graphics chips SHOULD know whether the far-end end point really can accept audio embedded in the TMDS signal.

[topdogzero]

I have a 4850 from Palit that came without the dongle, Palit has since sent me one but I still can’t get audio over HDMI, the dongle that Palit sent me has no makings on it and the colour is light gray. Did Palit send me the wrong dongle because my HDTV is detected as a DVI device in CCC.

[RADCOMj1]

you may need to enable Audio over HDMI in the sound config.properties. Also helps if you use the latest drivers.

[topdogzero]

Sorry the colour of the dongle is black, and I should give more info. Hdmi is selected in audio properties, using 9.2 drives and also tried realtek ATI audio driver since they made the audio codec chip on the card, when I use this driver it says cable not plugged in. Dongle I received has marking HDMI on it and a circle with a arrow going through it. No ATI makings and no rev numbers or letters.

[jdubbs23]

So I have a weird problem where if I use the ATI HDMI dongle, then I can set my pixel format to limited and get correct 16-235 grayscale (with clipping of BTB/WTW), but if I use the Asus Xonar HDAV supplied DVI-HDMI cable, then I cannot even get to the Pixel Format in CCC, and I get double compression of the video grayscale (something like 32-219). This is all wiht 9.4 drivers.

Now I can set the pixel format to limited in CCC (when using the dongle) so that I get correct greyscale levels, and then switch to the DVI-HDMI cable, and retain those levels, however I lose this upon a restart.

I would really like to use the DVI-HDMI cable due to space constraints in my TV cabinet (the HDMI dongle plus an HDMI cable is like 4 inches from the back of my case which is just too much). I would hate to have to pull my HTPC out of the cabinet and run through the procedure above to get correct grayscale every time I restart my PC.

Is there anything I can do? Besides wait around for another driver release from ATI?