top of page
Search

DisplayPort->HDMI dongles/adapters – active vs passive re-visited

Writer: goldenstateservicesj
goldenstateservicesj
Oct 7, 2014
4 min read

[Can’t decide what Surface docking hardware to buy? Click for a comparison of many popular docking options]

Skip down to the bottom for recommendations on which DisplayPort->HDMI adapters to use with your premium laptop like Surface. Or read the full article to understand the difference between so-called “passive” and “active” adapters and why it is typically better to go with a brand-name active adapter than a generic brand.

If you’re a big nerd and read spec sheets or Wikipedia pages, you may know that DisplayPort signaling is 3.3 volts while HDMI/DVI signaling is 5 volts so some active conversion is always required. The word “passive” doesn’t even appear in the VESA DisplayPort interoperability guide. So why do some manufacturers market their adapters as “active” vs “passive” when they are *all* active? I think this is because of the following phrase in the old VESA interop guide:

 “…an implementation guideline of a cable adapter with a built-in active protocol converter between a DisplayPort Device and a dual-link Device is covered.”

I think implementers picked up on the word “active” and figured that since there is a spec for an adapter with an “active protocol converter”, any adapter without that converter ought to be called “passive”. Maybe it’s because of my electrical engineering background, but I find this highly misleading; to me, “passive” means resistors, capacitors, and inductors without integrated circuits. All these adapters include powered integrated circuits and that makes them active.

The VESA interop guide describes a few approaches for these adapters:

The “Type-1” adapters which are commonly sold as “passive” rely on the DisplayPort source (like a video card) supporting DisplayPort++ (aka Dual-mode). When certain pins are pulled up to 3.3 volts or down to ground through specific resistor values, the physical DisplayPort pins are programmed to output the HDMI/DVI TMDS protocol instead of the LVDS DisplayPort packet protocol. But the TMDS protocol is still output at 3.3 volts which won’t work with a DVI/HDMI monitor designed for 5 volts. So the type-1 adapters include active electronics to level shift the 3.3 volt signaling to 5 volts. The signaling is limited to 165MHz bandwidth so screen refresh rates and resolutions are limited too. Aside from the level shifters, additional electronics are needed to regulate power – typically the 3.3 volts supplied from the DisplayPort connector on pin 20 is boost-converted to 5 volts. There is also a circuit to handle the DDC communications as well. So there is a lot of stuff inside your “passive” DisplayPort adapter. It’s not just a cable:

[Picture courtesy Wikipedia]

Type-2 adapters also pass-through the existing TMDS signal and level shift it from 3.3v->5v, but include more advanced integrated circuits and are able to operate at up to 300MHz with HDMI thus enabling 4K@ 30Hz output per HDMI 1.4 spec. Depending on the vendor, these adapters may be marketed as “active” or “passive” so you really need to read the fine print to understand the specs of the adapter.

Finally there are truly “active” adapters that do not rely on DisplayPort++/Dual-Mode at all. Instead they are able to decode the LVDS DisplayPort packet protocol in real-time and re-encode it into HDMI/DVI compatible TMDS protocol. DisplayPort->Dual-link DVI-D adapters are true “active” adapters. As are “EyeFinity” certified adapters and any adapter with HDMI 2.0 support. Some active adapters need to be powered by a USB port, however, most adapters available today are powered by pin 20 on the DisplayPort source which supplies 3.3 volts at up to 500mA. These truly active adapters are more expensive but allow more flexibility with refresh rates and resolutions depending on which integrated circuit is used inside.

So which adapter should you buy?

I’d get an “active” adapter that supports HDMI 2.0 and runs at ~600MHz to have the most flexibility. The main reason is that some video cards and laptop docking stations just don’t support DisplayPort++ so the type-1/type-2 “passive” adapters just won’t work or will fail to work with some monitors. On AMD EyeFininty 6-output video cards, for example, only two of the outputs support DisplayPort++, so you can only use two type-1/type-2 adapters with the remaining ports needing “active” adapters. Even if you aren’t using 4K resolution or high refresh rates, the fact that active adapters are tested at higher signaling rates is a little peace-of-mind towards their reliability with longer cables or interference-prone environments. You can certainly save a few dollars getting a lower-end adapter or get a bulk rate deal for off-brand adapters on E-bay, Alibaba, or Amazon Basics, but whenever you have active electronics inside, quality *is* an issue. I would certainly go with a name brand on these adapters like Accell, Apple, Club3D, Plugable, Startech, Surface, Tripp Lite, etc. I always give my customers the reverse advise for truly passive cables. I would not recommend “Monster” brand cables with oxygen-free copper, gold plating, or anything like that since almost everyone has great results with generic AmazonBasics and Monoprice budget cables.

Here are some good active HDMI 2.0 and DVI adapter/cable options to consider:

And a few even more premium HDMI 2.0b or HDMI 2.1 adapters for DisplayPort 1.4 enabled systems:

References:

 
 
 

Comments

bottom of page