Questions about data wire shielding

[TechTree]

Hello, I've been splicing a HDMI cable which has 19 wires in total I've soldered all the data cables apart from the 5 shielding wires. I'm a little confused because as far as I know the wire shielding is meant to act as a faraday cadge to stop interference. But when I don't connect the shielding wires the cable doesn't work at all, why would this be, if it's only meant to stop interference?

Here are a few images of HDMI cables:

There are individually shielded wires wrapped in foil within the wire shielding. Does each wire shielding need to be separate, or could I just connect all the shielded wires together into one. If so what would be the consequences?

Say I connected all the shielding wires individually like the image above, would it be okay if the foil surrounding the shielded wires touched the foil of another shielded wire? Or would I need to completely isolate the foil, wrapping each shielded foil wire in insulation tape?

Any help is appreciated!
Thanks

 

[CWatters]

HDMI signals are high frequency. So any discontinuity in the cable impedance caused by a missing screen may cause reflections ringing and signal distortion.

 

[CWatters]

Why are you splicing an HDMI cable?

 

[gneill]

Why are you splicing an HDMI cable?

+ me too

The effort involved in dealing with soldering and insulating umpteen wires and ensuring shield integrity on multiple pairs seems to me to be an exercise in frustration compared to simply purchasing a new cable of appropriate length. Time vs cost.

 

[Klystron]

Thanks for posting diagrams. HDMI's rather new to me but in the general case of coaxial transmission lines doesn't the shield also provide the outer conductance path for transmitting RF?

As posted above most physical modifications to shield mesh lead to impedance changes that affect the signal.

 

[jrmichler]

HDMI signals are high frequency. So any discontinuity in the cable impedance caused by a missing screen may cause reflections ringing and signal distortion.

Learning experience #umpty thousand: I have a 4K resolution (3820 X 2160 pixel) monitor. One "high speed HDMI" cable would only allow 2K resolution, another apparently identical "high speed HDMI" cable did give 4K resolution. I don't know the actual frequency, but I now see HDMI cables marked 18 gHz. If that is indicative of the frequencies involved, I can easily believe that an HDMI cable full of splices will fail miserably.

 

[CWatters]

I've had problems at lower resolutions with feed through connectors and 5m cables.

 

[Baluncore]

But when I don't connect the shielding wires the cable doesn't work at all, why would this be, if it's only meant to stop interference?

The impedance of a balanced twisted pair is influenced by a close outer shield. Because the shield, s, does not lie on the zero equipotential plane between the conductors, 1~2, it changes the capacitance of the line. So there are three parallel transmission lines 1~2, 1–s and s–2. Where the screen is missing you should expect a step change in impedance sufficient to reflect significant energy.

You will need to wrap the foil back around the twisted pairs with a tapered overlap, which will be difficult without more foil, and then reastablish continuity of the outer braid.

 

[davenn]

don't know the actual frequency, but I now see HDMI cables marked 18 gHz.

I assume that should have been GHz

Maybe you mis-read and meant 18 Gbps gigabits per sec ?

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

Data Cable, HDMI, and Bitrate
HDMI is very unlike other methods of routing video signals between consumer devices. Most methods--component, composite, s-video, and so on--are analog connections, while HDMI is digital. An HDMI signal rides primarily upon four twisted-pair lines, one of which carries clock pulses and the other three of which carry the colour components and sync information of the signal; as with any digital transmission, these signals are carried not as continuously varying voltages representing a range of values, but as positive and negative voltage pulses representing binary bits -- ones and zeros -- which, when decoded in accord with the HDMI signalling protocol, reveal the values of the red, green and blue components of the video signal.

For this reason, HDMI cable really has more in common with familiar data cables, such as CAT5, than with other video cables such as component video cable. And how HDMI cable behaves depends very heavily on exactly what we demand of it in terms of bitrate. The bitrate (or, to use the more colloquial terminology above, "speed") of an HDMI signal is determined by the resolution, the frame rate, and the colour depth of the signal. At the low end, a standard 480p signal, with 480 x 720 resolution, 60 frames per second, and 8-bit colour will run about 270 Mbps (Megabits per second) through each of the signal pairs of the HDMI cable. A 1080i signal (1080x1920, 30 full frames/second) or a 720p signal (720x1280, 60 full frames/second), using 8-bit colour, will run 742.5 Mbps through each of those pairs, almost triple the 480p rate. A 1080p signal, with 60 full frames per second of 1080x1920 video, at 8-bit colour, doubles that to 1.485 Gbps. But the HDMI interface now also allows for the possibility (implemented, as of this writing, on very few devices) of "deep colour," colour depths which require 12 or 16 bits instead of the standard 8. Not surprisingly, going from 8 bit to 12 bit colour increases the bitrate by 50%, while going from 8 bit to 16 bit doubles it, so that a 16-bit 1080p signal runs the signal pairs at 2.97 Gbps. The HDMI spec document calls for the interface to be able to support data rates at least up to 3.4 Gbps per signal pair; this doesn't correspond to any resolution currently in common use.

from Wiki

Version 2.1[edit]
HDMI 2.1 was officially announced by the HDMI Forum on January 4, 2017,[79][80] and was released on November 28, 2017.[120] It adds support for higher resolutions and higher refresh rates, including 4K 120 Hz and 8K 120 Hz. HDMI 2.1 also introduces a new HDMI cable category called Ultra High Speed (referred to as 48G during development), which certifies cables at the new higher speeds that these formats require. Ultra High Speed HDMI cables are backwards compatible with older HDMI devices, and older cables are compatible with new HDMI 2.1 devices, though the full 48 Gbit/s bandwidth is not possible without the new cables.

Additional features of HDMI 2.1:[121][120]

• Maximum supported resolution is 10K at 120 Hz
• Dynamic HDR for specifying HDR metadata on a scene-by-scene or even a frame-by-frame basis
• Display Stream Compression (DSC) 1.2 is used for video formats higher than 8K with 4:2:0 chroma subsampling
• High Frame Rate (HFR) for 4K, 8K, and 10K, which adds support for refresh rates up to 120 Hz
• Enhanced Audio Return Channel (eARC) for object-based audio formats such as Dolby Atmos and DTS:X
• Enhanced refresh rate features:
  • Variable Refresh Rate (VRR) reduces or eliminates lag, stutter and frame tearing for more fluid motion in games
  • Quick Media Switching (QMS) for movies and video eliminates the delay that can result in blank screens before content is displayed
  • Quick Frame Transport (QFT) reduces latency
• Auto Low Latency Mode (ALLM) automatic latency setting to the lowest ideal latency

Video formats that require more bandwidth than 18.0 Gbit/s (4K 60 Hz 8 bpc RGB), such as 4K 60 Hz 10 bpc (HDR), 4K 120 Hz, and 8K 60 Hz, may require the new "Ultra High Speed" or "Ultra High Speed with Ethernet" cables.[80] HDMI 2.1's other new features are supported with existing HDMI cables.

The increase in maximum bandwidth is achieved by increasing both the bitrate of the data channels and the number of channels. Previous HDMI versions use three data channels (each operating at up to 6.0 GHz in HDMI 2.0, or up to 3.4 GHz in HDMI 1.4), with an additional channel for the TMDS clock signal, which runs at a fraction of the data channel speed (one tenth the speed, or up to 340 MHz, for signaling rates up to 3.4 GHz; one fortieth the speed, or up to 150 MHz, for signaling rates between 3.4 and 6.0 GHz). HDMI 2.1 doubles the signaling rate of the data channels to 12 GHz (12 Gbit/s). The structure of the data has been changed to use a new packet-based format with an embedded clock signal, which allows what was formerly the TMDS clock channel to be used as a fourth data channel instead, increasing the signaling rate across that channel to 12 GHz as well. These changes increase the aggregate bandwidth from 18.0 Gbit/s (3 × 6.0 Gbit/s) to 48.0 Gbit/s (4 × 12.0 Gbit/s), a 2.66x improvement in bandwidth. In addition, the data is encoded using a 16b/18b scheme as opposed to the 8b/10b scheme used by previous versions, which let's a larger percentage of the bandwidth be used for data rather than DC balancing (88.8% compared to 80%). This, in combination with the 2.66x bandwidth, raises the maximum data rate of HDMI 2.1 from 14.4 Gbit/s to 42.66 Gbit/s, approximately 2.96x the data rate of HDMI 2.0.[122][123]

The 48 Gbit/s bandwidth provided by HDMI 2.1 is enough for 8K resolution at approximately 50 Hz, with 8 bpc RGB or Y′CBCR 4:4:4 color. To achieve even higher formats, HDMI 2.1 can use Display Stream Compression with a compression ratio of up to 3:1. Using DSC, formats up to 8K (7680 × 4320) 120 Hz or 10K (10240 × 4320) 100 Hz at 8 bpc RGB/4:4:4 are possible. Using Y′CBCR with 4:2:2 or 4:2:0 chroma subsampling in combination with DSC can allow for even higher formats.[121]

 

[jrmichler]

Maybe you mis-read and meant 18 Gbps gigabits per sec ?

Yes, exactly. From BestBuy: Connect your visual and audio devices with this 4-foot Insignia HDMI cable. It supports 4K Ultra HD resolution and Dolby TrueHD for clear, colorful pictures and dynamic sound, and its 18 Gbps of bandwidth ensures fast signal transmission.