DisplayPort 1.4/2.0 length for my needs?

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deestroying
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DisplayPort 1.4/2.0 length for my needs?

Post by deestroying » 15 Aug 2022, 21:21

Going crazy trying to get a concrete answer but what I gathered so far is:

Direct from DisplayPort.org - Tested and support to run at 15 meters.
CableMatters - "DisplayPort 1.4 cable can carry a 1080p signal about 15 meters, but a 4K signal has a much shorter 2 meter length limit due to the high bandwidth usage"
Dell - "length that Dell recommends for optimal performance is 1.8 meters (around 6 feet)"

I just purchased the Acer AOPEN 25XV2Q and want to purchase a quality DP cable to replace the one that comes in the box. I am using a monitor arm (Ergotron) so that eats up some length in cable management alone. TLDR; 390hz 1080p monitor, how long of a DP 1.4/2.0 cable can I run without having issues?

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Re: DisplayPort 1.4/2.0 length for my needs?

Post by Chief Blur Buster » 17 Aug 2022, 00:58

If you need super long lengths... go optical.

An optical DP or HDMI cable can bypass this distance limitation -- I've got an 8K capable optical HDMI cable from an Amazon supplier that sells them in 100 to 300 foot lengths, I got the 30 foot length since that's all I needed. Google "Optical HDMI" or "Optical DisplayPort", or search on Amazon/eBay.

The prices are getting affordable for optical HDMI now (~$50) but still expensive for optical DisplayPort - about 2x-3x more expensive per foot for optical DisplayPort.

But you positively simply absurdly totally utterly shatter the distance limitation, throw those recommendations out of the window, with these ~100 to ~300 foot (~30 to ~100 meter) 8K-capable optical HDMI or DisplayPort cables... They can send 8K to a monitor in the next city block over!

This is achieved by using miniaturized optical transceivers built inside the existing connector housings of regular HDMI/DP connectors, powered by the port electricity supply (55 milliamps in the case of HDMI). So they work with existing ports.

The catch? Not too many -- three main ones are the cost, the unidirectional nature, and occasionally fragility (with some of the brands). One thing though, these optical video cables are almost always unidirectional, so accidentally connecting in opposite direction = no signal, so that becomes a gigantic oops for in-wall installations (so verify first!). And be more gentle with these sometimes-fragile electronics-filled copper connectors (the optical transceiver chips), too -- electronics-filled connector ends are more prone to damage from accidental exposure to static electricity damage... Use common sense and don't roughly/continually attach/detach while gaming on a shaggy carpet... You'll be fine.

Much thinner cable too, another perk.
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deestroying
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Joined: 17 Jul 2022, 18:44

Re: DisplayPort 1.4/2.0 length for my needs?

Post by deestroying » 17 Aug 2022, 15:47

Thank you for such an informative reply Mark! I wish it would make (financial) sense to go optical in my scenario but a 10ft. cable will be more than enough for my new desk setup. If we're not talking super long lengths, is it all up to the individual cable if it will perform as expected? Can a DP 2.0 cable (I see one right now for the same price as a DP 1.4 from the same brand) send a 390hz 1080P signal 10ft with no sweat? I'm just afraid of this flickering that is reported with the original DP cable that comes in the box.

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Re: DisplayPort 1.4/2.0 length for my needs?

Post by Chief Blur Buster » 17 Aug 2022, 19:45

390Hz over 10feet is achievable using copper DisplayPort, but you must be selective: Get a DP cable capable of 8K over 10 foot.

What you want to think about is its bitrate, not the refresh rate. Much like how 4K 60hz is exactly the same pixels per second as 1080p 240Hz. It's 4x the number of pixels at 1/4th refresh rate.

Likewise
- 8K 30Hz cable can transmit 1080p 480Hz -- 16x more refresh rate at 1/16 pixels.
- 8K 60Hz cable can transmit 1080p 960Hz -- 16x more refresh rate at 1/16 pixels.

Most 1920x1080 signal is actually about 2200x1125 pixels when you include the hidden overscan data just beyond edges of screen (100 year old sync technology junk, that digital preserved from the legacy analog era).

Both MUSE (1980s Japan HD) and ATSC (HDTV) standardized a 45 line VBI, so 1080+45 = 1125. Most monitors use roughly that size (although it varies from ~1090 to ~1160 depending on manufacturer, we'll use 1125 for standardization simplicity's sake). These numbers are made visible in a Custom Resolution Utility, if you need the exact math stuff.

So you pretend 1920x1080 is actually a 2200x1125 signal. Multiply 2200x1125 and you're getting a number near 2.5 million

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So that's 2.5 million pixels per refresh cycle, times 24bit color (2.5 x 24 = 60) leading to about 60 million bits per second per refresh cycle.

Multiply that 60 by 390 (=23400) and you need just under 24000 megabits/sec or, rather, 24 gigabits/second.

Add a 20% error margin for overheads (Modern DP is a micro-packet technology now like USB-C is) and we're looking at a cable that can transmit at least 28 gigabits per second for 1080p 390Hz 24-bit.

Incidentally, 8K 60Hz (non-DSC) requires just a hair under 60 gigabits per second. So any cable capable of delivering 8K 60Hz uncompressed, will easily do 1080p 390Hz uncompressed.

So even 8K 30Hz (~30 gigabits/sec without overheads) uses more bandwidth than 1080p 390Hz (28 gigabits/sec including overheads)!

A DisplayPort 2.0 with UHBR enabled (all lanes 4x20GBps) is 80 gigabits per second.

You can probably make do with just 40 gigabits per second, which is what some DisplayPort connections might automode down to, if the DP transceivers behind the ports on both ends can't successfully modem an 80 gigabit link over that distance.

1080p390, even at 10-bit color and the worst overscan porches in the industry, fits fully within the 40 gigabits/sec budget with a massive safety margin. So, yes, should be able to do 40 gigabits per second over 10 foot (4 lanes x 10 Gbps each), no problem, with a cheap-but-good MonoPrice DisplayPort 2.0 cable.

Since bleeding edge 8K is (alas) slightly more common (business displays, etc) than 1080p 390Hz, reviewers that report any success at 8K (30Hz and up) are valuable defacto confirms of 1080p 390Hz=works.

Verdict: 1080p 390Hz is not a problem over copper over 10 feet, if you get any decently-made 8K-capable cable
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