Hacker News Reader

Comment by lisper | Hacker News Reader

> An HDMI input would be great.

There is a reason that is difficult to do: the licensing terms for HDMI require manufacturers to put DRM in place to make it impossible (or at least not straightforward) to record from an HDMI source. It's nearly impossible to meet that requirement on a computer.

View on HN

You're thinking of HDCP, not HDMI. HDMI recorders exist (Elgato built an entire brand off selling them for console game streaming) but they are not allowed to speak HDCP because gestures vaguely at DMCA 1201.

In the specific case of the iMac:

- USB-C video outputs are all either DisplayPort[0]. HDMI Forum was promoting an HDMI USB-C altmode but it was never implemented. USB-C HDMI adapters are all DisplayPort adapters that additionally tunnel DVI traffic (because HDMI 1.x is actually single-link DVI with a funny connector and extra bits).

- Apple used to support Target Display Mode for video input until the Retina iMac a decade ago. They dropped Target Display Mode because their 5K display panels were using eight lanes of DisplayPort, a configuration that doesn't exist[1] outside of Apple's custom display controllers. Every other 5K display (and even some 4K ones) were instead using multiple cables, with some extra (poorly-supported) metadata to tell the graphics card to treat both halves of the display as one.

Why Apple hasn't brought TDM back now that we have enough bandwidth for 4.5K over standard cables is beyond me.

[0] Or MyDP, aka "the funny Nintendo Switch altmode"

[1] Think of it like a "32-lane PCIe port". It's technically possible but no host or device silicon supports a link that wide.

> USB-C HDMI adapters are all DisplayPort adapters that additionally tunnel DVI traffic

Not tunnel, convert: these are active adapters that take in a DisplayPort signal from the computer and re-encode it as a DVI/HDMI output signal. The computer doesn't need to know or care that there's HDMI happening downstream, because it's still just outputting DisplayPort signal. It's not like Thunderbolt where DisplayPort traffic is tunneled inside a different signalling mode that the host machine supports.

This is why USB-C to HDMI adapters are unidirectional (at least, all the mainstream commodity ones). You'd need a different converter chip to do the conversion in the other direction.

Yes, that's right. Thanks for pointing that out.

https://www.makeuseof.com/hdcp-vs-hdmi-whats-the-difference/

HP does HDMI-in in somo of its AIOs: https://www.hp.com/us-en/shop/pdp/hp-eliteone-840-g9-all-in-...

It's not impossible, but it has to be a completely separate signal chain that bypasses the PC. (Well, OK, it's theoretically possible to do it other ways, but it would be really hard.)

DisplayPort doesn't have that same restriction, right?

I don't know, but probably not. It's a legal requirement for HDMI, not a technical one. It's actually not hard to build an HDMI recorder, you just can't legally sell one. But bootleg recorders are easy to find.

They could design it in such a way that the hdmi input port bypasses the whole 'computer' and goes straight into the display board. If it's plugged in the 'computer' could either not boot or just run headless, i'd prefer the former (just to save on energy costs).

Couldn't you "just" wire the HDMI port to the panel control logic? That is, the HDMI-in doesn't connect to the computer part of the iMac, just to the display circuitry.

Edit: nevermind, I see you addressed this in another comment.

Reply on news.ycombinator.com