More lower level monitor notes

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

More DisplayPort notes

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Cable length

It seems many cables are ≤1m because most desktop setups don't need more.

Specs say 2 or 3 meter max.

This is a rather cautious figure, meant as an "everything should work".

Lower resolutions have lower data rates - and lower sensitivity to issues. But don't count on more than 10m, optimistically 20m.

Active cables could go up to 15-30m, at somewhat higher rates/resolutions, in a more compliant and thereby less fragile way. (Note that while passive cables do not have a direction, active cables do).

If you want more than that, you probably want fiber DP instead of copper DP (at high enough rates and distances, the issue is capacitance and sort of unavoidable).

Note that each wall-socket should be counted as loss, and you can estimate each as equivalent to a 1-meter loss. This can matter if if you are the one responsible for putting DP into wallsockets.

Bandwidth

Note that your average monitor will probably just work on most any cable - making 1080p fail on a short cable would take effort.

You would only care about these numbers if you use a combination of 4K, deep color, 120fps, and 3D, e.g.

3840×2160@120fps requires HBR2,

5120x2880@60Hz require HBR3

multiple displays on a single port can be done

A single lane (differential pair) in a DP cable can be run at one of four speeds,

• 1.6GBit/s raw, ~1.3GBit/s data in RBR (reduced bit rate)
• 2.7GBit/s raw, ~2.1GBit/s data in HBR mode
• 5.4GBit/s raw, ~4.3GBit/s data in HBR2 mode (introduced in DPv1.2)
• 8.1GBit/s raw, ~6.5Gbit/s data in HBR3 mode (introduced in DPv1.3)

Cables made for higher speeds will often be 4-pair, which means:

• 4-pair HBR is ~8 Gbit/s (~10Gbit/s raw minus 20% coding overhead)
• 4-pair HBR2 is ~17 Gbit/s (~21Gbit/s raw minus 20% coding overhead)
• 4-pair HBR3 is ~25 Gbit/s (~32Gbit/s raw minus 20% coding overhead)

Dual-mode

A Dual-mode DisplayPort output (a.k.a. DP++) supports the use of an adapter to single-link DVI or HDMI. (a passive adapter in that the output itself already speaks the right thing, the adapter only changes voltages)

Without this, only an active adapter (something that does a conversion between different video formats inside it) would work.

Other conversion formats (e.g. to VGA) always require this.

More HDMI notes

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

On HDMI cables

tl;dr:

• a cable certified for a speed will always support everything that keeps under said speed

and yes, this needs to be said, to the golden cable crowd

• fancy cables don't mean better images. When signals are digital, it either works or it is broken.

A High Speed cable can cost, order of magnitude, 1-2 Euro per meter. Paying more is often a waste of your money

• most cables are High Speed (except for some from early days)

Cable features and specs

There are two distinctions in cables:

• with or without ethernet

There are currently few devices that can use it.

• Standard Speed, High Speed, Premium High Speed, and 48G (see below)

...and, spec-wise, nothing more.

Whether a cable is in spec is purely about bandwidth, not about features. if both ends support a feature, and the cable supports the bandwidth, that feature will work. If one side does not support it, that feature will not work. period.

So products like "3D HDMI cable", "ARC HDMI cable" (and even technically "HDMI 1.4 cable") do not exist. This is marketing for consumers who want to buy the more expensive thing because surely it's better.

The shorter the cable, the fewer problems it has to deal with, so a good number of short (< 3m / 10ft) standard-speed cables may actually do higher speeds than they are certified for, and may e.g. do 1080p60 perfectly, even if the specs don't guarantee that.

Your devices don't know what your cable is labeled at, so it will either work, or not. (There is a very narrow zone where you get some errors, or maybe semi-dramatic degradation. You'll probably rarely or never see this)

Cable speed

The way higher-speed communication works, the amount of Gbps a cable could support relates mostly to cable length and wire gauge.

This is basically relevant for prefabricated cable, because either it's in spec, or not.

To avoid buyers having to get lost in details, there are two standardized certifications since HDMI 1.3(verify) (around 2006), and some added later:

• Category 1, a.k.a. Standard, tested at 74.5 MHz

most cables quickly moved to high speed, because Standard isn't enough for 1080p60

• Category 2, a.k.a. High Speed

enough for almost everything

• "Premium High Speed" (since 2015) ensures 18Gbps

• "48G" (since 2017) which ensures 48Gbps

The last two are mostly relevant if you have ≥4K, deep color, 3D type things -- and even then it will probably only need it if it does multiple of these things at the same time, which is not common.

So if a particular cable is certified, it is guaranteed to work at the related speed. (...there are also non-certified cables out there, and you can argue about which made-in-china specs are truer than others, but that's another issue).

So what needs what speed?

Standard speed cables will do 720p, 1080i, and probably 1080p/24 but various devices have no need for the last. This is enough for cable/satellite (1080i), game consoles that go up to usually 720p or 1080i, and things like 720o Apple TVs.

It does not do 1080p/60, quite relevant in that computer monitors have done at least that for years, which is a good part of why most HDMI cables are High Speed.

High speed supports resolutions like

1080p/60 (including Blu-ray, and computer monitors)

and WQXGA (2560x1600) and 2160p24 (see also 4K)

usually those resolutions combined with 3D or Deep Color.

some 120Hz / 240Hz screens, when they are actually post-procesing 60-or-less fps, rather than rendering and transferring that rate

How much better are the pricier, Gold Plated Ultra Monster cables?

They do not affect picture quality or speed.

It's true that HDMI chooses not to checksum its video, it's realtime send-and-forget. So in theory you can get interference affecting display, but the margin between 'works fine' and 'is broken' is much thinner than it was in analog.

In most cases something that would interfere wouldn't be selective on pixels - it'd extremely easily keep the entire thing from working at all.

So it's still hard to think of pathological cases where this even could make a useful difference.

The only case I can think of is that extra shielding may help in environments with stray RF so strong that most of your cabling would have these issues - and even then you've got a wider problem that you should probably fix at the source.

Some of these fancy cables could sustain much higher speeds than they are certified for, meaning you won't have to buy a new cable in 5 years when upgrading to a 4K 120fps 3D TV.

But you won't know what speed, you're buying something expensive now for something that isn't even guaranteed to work later.

Ferrite beads are usually there to meet emission standards, so on a HDMI cable they may help avoid influencing nearby analog signals. But do almost nothing to protect your HDMI signal.

http://www.bluejeanscable.com/articles/hdmi-cable-information.htm

http://www.bluejeanscable.com/articles/speed-rated-hdmi-cables.htm

Splitter notes

DisplayPort splitter notes

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

When connecting multiple monitors with shared cables, the specs broadly give two ways to do it, SST and MST.

SST (Single Stream Transport)

only one video stream

The OS will probably see the splitter device as if it were a single monitor.

(this is also sometimes use as an active repeater, or in a box that also converts to another connector)

Hardware-wise

the name SST turns up around SST splitter boxes, where it implies all monitors are showing the same framebuffer

MST (Multi-Stream Transport)

requires DP ≥1.2 (but most things are)

distinct video streams on the same cable,

where the source (the GPU) configures distinct video streams for each connected monitor.

How many

depends on the resolution, and the version of DP, and even the cables.

While MST can do 63 logical streams(verify),

high resolutions mean you run out of bandwidth with as few as two monitors.

for more everyday resolutions don't assume more than 4 (maybe 8),

and you'll have trouble finding MST hubs with more than 4 outputs.

Hardware-wise

MST usually implies a MST hub - an external box with multiple outputs

technically, daisy chaining monitors is part of the MST spec(verify), where instead of a hub, the display controller inside such a monitor passes 'the rest' of the signal down an output port (with negligible latency)

On names of boxes

It should be noted that when you buy a 'DisplayPort splitter', you might get either

• SST splitter
• MST hub
• MST hub that fall back to SST in certain conditions, but may not allow control over when that happens.

Shops and even some manufacturers aren't consistent about communicating this.

"If you have a MST hub and you configure mirroring in the OS, is it just one stream?"

It seems possible, but you cannot assume that.

Also, it would be hard to tell. Windows basically doesn't tell you at all, linux may reveal it at lower level.

Note that most of the time, if it functions, it may not matter how it does it.

But sometimes it does matters, in terms of on-the-wire behaviour, bandwidth, or synchronized timing between monitors, in which case you might even want to force SST behaviour with a specific external device.