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Comment by Matthias247 | Hacker News Reader

Very interesting article.

Can here anybody maybe even explain a little bit more about the video (Displayport) alternate mode? As far as I understand now both USB3 and Thunderbolt support it, but they support it with a different Displayport standard. How will that work if I plug in a future monitor with USB-C? Will there first be some negotiation in which both devices clarify whether to use USB oder Thunderbolt. And then another one in which the alternate mode is set? Or is displayport directly available on some dedicated pins of the cable and if yes, would it be the same for both cases? Or is displayport somehow modulated/multiplexed on the remaining data stream, and in a different fashion for USB3 than for Thunderbolt?

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"Alternate Mode" means "using the same USB Type-C pins for other protocols". That protocol can be HDMI or DisplayPort or Thunderbolt or even analog audio!

The USB consortium specifies using HDMI 1.4b and DisplayPort 1.3 (and MHL 3.0) on the Type-C port. So non-Thunderbolt machines have these specs as a maximum.

Thunderbolt 3 can also pass video signals, and Intel specifies different versions of the protocols: HDMI 2.0 and DisplayPort 1.2. Again, these are the maximums.

So if you have a USB Type-C monitor (not Thunderbolt) or a native USB Type-C to video cable, you're limited to HDMI 1.4b (1x4K at 30 Hz) but instead you might be able to use DisplayPort 1.3 (1x 4K at 120 Hz). If your video card, connector, cable, and display supports it, of course.

If you have a Thunderbolt-native monitor, you might be able to do 2x DisplayPort 1.2 (4K at 75 Hz or 5K at 30 Hz) or 2x HDMI 2.0 (4K at 60 Hz and much more). If your video card, cable, and the Thunderbolt controller support it.

Essentially, you can pass video directly over the port (in USB Alternate Mode) or over Thunderbolt. Then there are external video adapters that use USB or Thunderbolt data. But that's not in scope of your question.

Thanks for the answer. Also to fragmede.

That means from USB-C I would either go to thunderbolt or directly to displayport AM. But how would it go on from Thunderbolt mode to video (displayport 1.2 version that intel specified)? Would Thunderbolt again dedicate some pins for it or is the signal somehow multiplexed into a big thunderbolt stream which carries everything?

Thunderbolt multiplexes that in, yes.

Although, more accurately, it’s actually just a PCIe connection – you can even connect a GPU via thunderbolt via USB-C

Best explanation I found was from the following forum post which was discussing why the 4K LG only has USB2 ports on the back. Will quote it here. http://forums.macrumors.com/threads/apple-teams-up-with-lg-f...

With the new displays, its the 4k one that only supports USB 2.0 speeds, the 5k one has "Three USB-C (USB 3.1 Gen 1, 5 Gbps)".

Looks like the two displays are actually using different technologies: the 4k one connects via "plain" USB-C, which means it uses USB-C's "DisplayPort Alternate Mode". USB-C has 4 pairs of high speed data wires plus 1 pair for "legacy" USB-2 signals. In DisplayPort Alternate Mode, some or all of the high-speed pairs are physically dedicated to DisplayPort signals. Unfortunately, Intel's USB-C/TB3 controller only supports DisplayPort 1.2, so to get 4k at a decent refresh rate takes over all 4 of those pairs just to run the display, leaving only the legacy USB-2 channel for other uses.

As for 5k, you can't do that with just 4 DisplayPort 1.2 data pairs - remember the Dell 5k display needs two DisplayPort 1.2 cables. However, Thunderbolt 3 works in a different way: rather than physically allocating wires to DisplayPort alone, the displayport data gets moshed together with the rest of the Thunderbolt signal so the display & peripheral data shares the same physical wires - the DisplayPort connection is a "virtual" one. Plus, Thunderbolt 3 has enough bandwidth to provide two virtual DisplayPort cables (8 virtual "wires") down a single physical cable, so it can can drive a 5k display and still fit some high-speed USB data around the edges.

That said - it probably doesn't make sense to hang your USB3.0 RAID array off the same pipe as a bandwidth-hogging 5k display. If you're gonna go 5k and want seriously fast external storage you can probably forget the whole single-cable docking thing until Thunderbolt 4 rolls around (...by which time you'll probably want a 25k display wall...)

USB-C DisplayPort mode can support 5k by using DisplayPort 1.3's higher data rate (although, again, that only leaves the legacy USB 2) but not when the computer's usb-c controller and GPU only support DisplayPort 1.2 - and that currently includes anything with an Intel USB-C controller (which is part of their Thunderbolt 3 controller). Complaints on a post card to Intel.

Fantastic explanation. Right on!

No one can say at this point what your hypothetical future monitor will use. There's a standard, of course, but there's more than one of them, of course.

There's DisplayPort, HDMI (yes, it's different from DP), and now Thunderbolt 3 Alternate Mode for USB Type-C, and then also since it's still USB, there will also be video over USB (aka the displaylink chipset).

Alternate Mode is feature of USB Type-3 to allow a bunch of pins to be used as a host-device negotiates, keeping USB control pins intact, so Thunderbolt 3 is just Thunderbolt using the AM pins after negotiation.

I'll bet that if Apple starts making 5k-iMacs-without the-iMac (aka monitors) again, that in true Apple fashion they'll run video over Thunderbolt-only, just like the now-discontinued thunderbolt only displays did. This means there's a high likelyhood possibility they won't work with PCs, despite having a working cable.

Thanks!

Regarding Apple monitors: Apple seams to be partnering with LG now instead of producing it's own monitors: https://www.computerbase.de/2016-10/lg-ultrafine-4k-5k-monit... . Sorry, I have only the german article here, but there's maybe also somewhere one in english. The infos on the connector are also somewhat vague. Maybe it's the small model using USB3 alternate mode and the bigger 5k one thunderbolt mode, which makes it make only. For the expected apple customers it probably doesn't matter as long as it works with the MBP. However for other potential users (which do not seem officially supported) it would be quite interesting.

I'm not an expert but my understanding is closer to your dedicated pins option. Basically everything that connects over USB-C starts with the USB-C handshake, after that alternative modes allows USB-C to get out of the way and (most of) the pins of the cable can be used for whatever the devices want.

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