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Displayport: A Better Video Interface

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Re: Displayport: A Better Video Interface

#141

Post by chrisco255 »

_kb wrote: post #124
pid-1 wrote: post #48

Sorry if that sounds stupid, but why isn't Ethernet used for this sort of high bw signaling?

Because (consumer) ethernet is low bandwidth compared to whats needed for video. DisplayPort 2.0 provide a 80Gbps link. HDMI is 48Gbps. Both already use DSC which is a mezzanine compression as this is already saturated for immediate needs. Ethernet is absolutely used for media transport outside of the consumer space. Particularly where there's a need to distribute signals further than what DP or HDMI can provide. The…

The current Ethernet spec supports up to 1000 Gbps, but I think 40 Gbps is about as fast as you can find on the market right now. They could build multi Gbps wifi routers too, it's just not common in the consumer market.

Re: Displayport: A Better Video Interface

#142

Post by hot_gril »

Aaargh20318 wrote: post #132
hot_gril wrote: post #116

Earlier quoted context omitted.

Makes sense to move the windows if you close a laptop lid, but not if you turn off a monitor. You can't disconnect the laptop screen, but you can easily disconnect the monitor. I have a cheap laptop that doesn't understand a turned-off monitor as unusable, and it's actually way nicer that way.

Why is it nicer? What is the use case for having unreachable windows?

Not having your windows get messed up when you turn the monitor on/off, which is especially relevant for TVs and projectors. On the flip side, what's the use case for connecting to a powered-off monitor and not using it? If I don't want to use a monitor, I won't connect it.

Re: Displayport: A Better Video Interface

#143

Post by TazeTSchnitzel »

The amazing thing about DVI, and by extension HDMI, is that it's just VGA but digital, with all the timing and synchronisation complexity that implies. Recall that DVI can have both an analog (VGA/DVI-A) and digital (DVI-D) signal in the same cable. They aren't independent; they share some pins and have the same timing. You could have made a CRT monitor that used DVI-D just by adding a DAC, though I'm not sure if anyone ever did.

DisplayPort does away with all that legacy. I assume the hardware to implement it is much simpler and more reliable.

Re: Displayport: A Better Video Interface

#144

Post by NavinF »

tshaddox wrote: post #123

> However, I’d like to tell you that you probably should pay more attention to DisplayPort – it’s an interface powerful in a way that we haven’t seen before. Lines like that really show just how long it can take for standards to get on the radar of mainstream tech culture. I remember hearing about and being excited about DisplayPort's move to packetized digital video in college in 2008, and seeing the first Macs with…

> impression that it has been well-known and commonplace for hobbyist and enthusiast PCs for well over 10 years

It is. For a long time DP was the only standard that could do variable refresh rate. Even today all high end monitors have DP while the cheapest monitors only have HDMI.

Re: Displayport: A Better Video Interface

#145

Post by ianburrell »

xienze wrote: post #101
aidenn0 wrote: post #90

Earlier quoted context omitted.

Sounds like the real question is why don't people use DP for p2p networking links...

There’s nothing magic about DP cables, they’re just typically a short-ish length, which is why they can be high bandwidth and relatively cheap. Once you get to a decent length, the cables get really expensive because you have to put optical transceivers on either side. I have a 25 foot DP cable which cost over $100, for example. So you could do a short length, high bandwidth Ethernet cable, I’m sure. But the reason w…

The commonality of connector is important. Ethernet has stopped using twisted pair cables for higher speeds. The SFP transceiver means can use short DAC copper cable or fiber optics for longer distances. The DAC cables that would compete with DisplayPort are pretty cheap.

Also, a big portion of the cost of networking is in the switch that can handle high bandwidths. My guess is that 25G DisplayPort switch would be just as expensive as 25G SFP one.

Re: Displayport: A Better Video Interface

#146

Post by tverbeure »

londons_explore wrote: post #64
pid-1 wrote: post #48

Sorry if that sounds stupid, but why isn't Ethernet used for this sort of high bw signaling?

It should be. As soon as video data is in packets, it should be routable over a network like any other data. Give up the custom connectors, custom cables, etc. Need a wireless display? No worries, we can route the data over wifi too. Need 5 displays? Thats what an ethernet switch is for. Screen mirroring? We have multicast. Need a KVM? Well you can probably write a few scripts to change which computer your screen get…

They write that DP uses packets, but it's still an isochronous connection with guaranteed BW allocation.

For video timings with high pixel clocks, the bandwidth used to transfer a signal is very close to what's theoretically available. There's no way you'd be able to do that reliably over something like an Ethernet cable.

There's nothing sinister about how video standards are designed. From DP1.0 to DP1.4, which spans more than 10 years, all changes have been incremental and most just a matter increasing data rates while the coding method stayed the same.

They need a system that's very high and guaranteed bandwidth, high utilization, very low bit error rate, and cheap.

Even today, a 10Gbps Ethernet card will set you back $90. And that will carry less than half the data that can be transported by DP 1.4 which is old school by now.

Re: Displayport: A Better Video Interface

#147

Post by wtallis »

crazygringo wrote: post #119

> Just like most digital interfaces nowadays, DisplayPort sends its data in packets. This might sound like a reasonable expectation, but none of the other popular video-carrying interfaces use packets in a traditional sense – VGA, DVI, HDMI and laptop panel LVDS all work with a a stream of pixels at a certain clock rate. Funny, a stream of data at a constant rate makes much more sense to me intuitively than packets,…

> Funny, a stream of data at a constant rate makes much more sense to me intuitively than packets, specifically for uncompressed video.

Sure, until you start trying to design the transceivers and realize that supporting two or three fixed standard data rates is a lot simpler than supporting a continuously-variable clock speed. Every other high-speed digital interface operates at just a few discrete speeds: SATA/SAS, PCIe, Ethernet, USB.

The fact that DVI and HDMI were such a shallow digitization of VGA's racing-the-beam meant features like variable refresh rate (Gsync/Freesync) showed up far later than they should have. If we hadn't wasted a decade using CRT timings (and slight modifications thereof) to drive LCDs over digital links, it would have been more obvious that the link between GPU and display should be negotiated to the fastest data rate supported by both endpoints rather than the lowest data rate sufficient to deliver the pixels.

Re: Displayport: A Better Video Interface

#148

Post by Joker_vD »

Aaargh20318 wrote: post #53
LoveMortuus wrote: post #33

Earlier quoted context omitted.

But when I turn off my displays that are on HDMI or DVI or VGA, the computer still detects them like normal, like they're connected, because they are. I don't understand in which case the 'act like it's physically disconnected' behaviour would be more desired than what we had with all the standards before. I have read that some DisplayPort displays do have an option in the settings to disable this behaviour.

> But when I turn off my displays that are on HDMI or DVI or VGA, the computer still detects them like normal, like they're connected, because they are. But since they’re off, that doesn’t make any sense, now you can’t reach any windows on those monitors. I think the macOS behavior make the most sense, move them so you can access them, but move them back once the display is turned on.

Or, you know, don't do any of those silly dances and just let the windows stay where they are the whole time between turning the displays off and on?

Reminds me of that problem with video driver update on Windows when the screen is momentarily resized down to 1024x768 resolution and then instantly goes back to 2560x1440: all the non-maxed windows get shrunk down and shifted to the upper-left corner (so they would be visible on a 1024x768 screen) and then they just stay like this. It's totally useless and actually quite annoying.

Re: Displayport: A Better Video Interface

#150

Post by hot_gril »

Perz1val wrote: post #65
Aaargh20318 wrote: post #15

Earlier quoted context omitted.

Sounds like an OS problem. When I do that it moves all open windows to the remaining display, but when I turn the other display back on it just moves them back. No special software installed for this, just stock macOS Ventura.

It's not an OS problem, that's how DP is supposed to work and that's in fact stupid

Is this specific to DP? I thought HDMI was the same.
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