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Comment by leohonexus | Hacker News Reader

This is similar to how a (cheap) DisplayPort-to-HDMI adapter worked, the adapter itself is completely passive.

Many devices with DisplayPort output support multimode, which allows the device to switch from outputting DP packets to outputting TMDS-based HDMI or DVI signals. You'll sometimes see a ++DP logo on these ports.

This is also why you don't see a cheap HDMI(source)-to-DisplayPort(sink) adapter, the smarts just aren't built into the source device.

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> the adapter itself is completely passive

I think it was not completely passive, but simple level conversion; DisplayPort is AC-coupled signal, HDMI is DC-coupled.

Signal level and coupling are separate things.

> Signal level and coupling are separate things.

sure, but the part is called a level shifter. MAX9406 DisplayPort to DVI™/HDMI Level Shifter

https://www.analog.com/en/products/max9406.html

There is a difference between DP+ and DP++. DP+ uses external level shifter, DP++ uses passive adapter and the card switches different transmitter into the path internally.

> I think it was not completely passive, but simple level conversion; DisplayPort is AC-coupled signal, HDMI is DC-coupled.

Wait, I don't think that's accurate. I've worked with HDMI/DVI and both are AC-coupled, that is, you put a 10 or 20 nF cap in series on the TMDS lines if you're driving directly from an FPGA's digital IO pins.

Cheap-ish, luckily. I was quite relieved to find a reasonable ($50ish) active HDMI-DP converter. I have an old Dell monitor that only does HDMI 1.3 so to drive its full resolution you need to use its Displayport (or dual link DVI) input. And modern devices generally don't have either. The gadget solves the problem and gets me my 2560x1440 resolution.

if it uses 'power' (aside wire resistance) is not passive, e.g. I'd consider resistors, capacitors, coils - passive, even I'd agree about diodes, even though they'd add latency. DP->HDMI requires some electronics (a level shifter) as bare min.

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