How does HDMI ARC work?

[MCH]

Hello, i know this is not an easy question but just in case there is someone out there with some experience with HDMI ARC.

I am thinking on building a multichannel HDMI to i2s audio extractor hat for raspberry pi. The idea is to capture multichannel audio from HDMI and send it to a raspberry pi 5, that has multichannel i2s input and output capabilities. Member @mdsimon2 has tested and confirmed it works:

Multichannel audio on a Pi will get a whole lot easier and cheaper!

I have to admit the multichannel I2S input is pretty cool and was much more plug and plug than I ever expected, although that is largely due to folks way more knowledgeable than me. I did some more testing today and really think an Apple TV + HDMI to I2S Extractor + RPi5 + CamillaDSP +...

audiosciencereview.com

I am going to use the HDMI transceiver IC from this multichannel HDMI extractor that i have:

https://www.aliexpress.com/item/1005002564331114.html?spm=a2g0o.order_list.order_list_main.318.6e101802Pz44tV

This extractor uses a semicom EP92A3E chip that is ARC capable. The web page of the chip says:

However, the ARC pins of the chip are not connected and the ARC function in the extractor is not advertised and not functional.

My tv is a smart tv and i have all the services i need as built in apps, i would like to use its ARC HDMI to plug directly to the extractor without needing any other boxes. Unfortunately, i am not aware of other affordable HDMI multichannel extractors that are ARC capable.

What i would like to do, as i am going to transfer the chip from the extractor to the diy hat anyways, is try to activate the ARC function. From an schematic of a different device using this same chip i have this relevant section:

pins 117 and 118 (not connected in my extractor) seem to carry the ARC signal/code/whatever. and go to the HDMI output socket:

Now my question, for anyone that understands how ARC works, is:

Do you guys think that if i replicate these connections in my diy PCB i will have any chances of having the ARC function working, or it is not that easy? (note that there is no MCU in my circuit, the transceiver chip comes programmed from factory. It is unknown to me if it comes programmed with all its functionality by default)

If this is the case, why the people selling these extractors just don't do the same and have one more selling point?

I am aware of the answer "try and see", that is a fair one. The problem is that this is a project that will be time consuming and complex for me. If i know beforehand that the chances of having ARC working are very little or none, i will probably decide not to move forward and save a lot of time and effort.

thank you for any clue or direction where i can research more. When i type "how HDMI ARC works" in google, as you can imagine, i get hundreds of answers that have nothing to do with this.

 

[mdsimon2]

I am still in the early days of learning about HDMI, but I do not think ARC supports anything more than 2.0 if you are using uncompressed PCM. ARC can do compressed multichannel formats but then you need something to decode it. If you want uncompressed multichannel you need eARC.

Michael

 

[MCH]

I am still in the early days of learning about HDMI, but I do not think ARC supports anything more than 2.0 if you are using uncompressed PCM. ARC can do compressed multichannel formats but then you need something to decode it. If you want uncompressed multichannel you need eARC.

Michael

Ouch!!
That definitely answers my question. Thanks a lot

 

[boXem]

Lattice has a chip handling eARC. Good luck getting the specs...

 

[MCH]

Lattice has a chip handling eARC. Good luck getting the specs...

In my little experience so far anything with "hdmi" on it has zero chances to have datasheet or any other information available. This is for me a game of reverse engineering, find schematics on service manuals and crossing fingers.

 

[boXem]

In my little experience so far anything with "hdmi" on it has zero chances to have datasheet or any other information available. This is for me a game of reverse engineering, find schematics on service manuals and crossing fingers.

I went a little further and contacted them. Summary, sales tell you to ask support, support tells you to ask sales...

 

[MCH]

I went a little further and contacted them. Summary, sales tell you to ask support, support tells you to ask sales...

Does this mean you are looking into HDMI? Interesting....

 

[MCH]

Well, if someone is interested, indeed Lattice has both transmitter and receiver earc chips and they even sell an eval board that could be perfect for my use case, if it wasn't 899 euros, auch! comes with a CD, maybe there is some software to program it or detailed information. The white paper (attached) is interesting.

https://www.digikey.lu/en/products/detail/lattice-semiconductor-corporation/CP9437/7672116

 

[boXem]

Does this mean you are looking into HDMI? Interesting....

Don't extrapolate too much out of this.

 

[stigger]

@MCH so, were you able to get anywhere with eARC, or is the idea on hold for now?

Also, have you considered just plugging the RPi into the TV as a USB OTG gadget? In my case it doesn't work: the TV detects it as "Bluetooth Headphones", and doesn't route any sound there. Still, it heavily depends on the TV and is worth a try.

 

[MCH]

@MCH so, were you able to get anywhere with eARC, or is the idea on hold for now?

Also, have you considered just plugging the RPi into the TV as a USB OTG gadget? In my case it doesn't work: the TV detects it as "Bluetooth Headphones", and doesn't route any sound there. Still, it heavily depends on the TV and is worth a try.

eARC? No. Too difficult, too expensive, and no info available to the DIYer that I know of.

What I do plan to finish is a multichannel hdmi to i2s hat for rpi5, ai it seems chances are low to have a commercial solution any time soon. I plan to populate the ARC (not eARC) circuit but I don't know if it will work, just to see if it does. But I have two other projects before that...