HDMI to Cat 5/6: Are 1- or 2-Wire Solutions Best?
Going from an extravagant HDMI cable to Cat 5/6 seems pretty incredible. Jeff Boccaccio explains how this transition can be done reliably.
By Jeff Boccaccio | 05.15.2012
Going from a very extravagant HDMI cable to a Cat 5 or 6 seems pretty incredible. So how can this work and work reliably?

Cat 5 and Cat 6 have two special attributes that make much of this possible. One is that HDMI video is differential and performs best with twisted pair topology. The second is that Category cable uses the same 100-ohm impedance that HDMI uses.

But that doesn’t explain how we get the integrity of the DDC, CEC, HotPlug, Voltage and A/V down category cable, does it?

HDMI connectors have 19 pins/19 wires - how do you get 19 wires down to one or two Category cables that only have eight wires within each? Let’s run the numbers.

HDMI Wire Specification:
  • 4 Twisted Pairs to support video (total of 8 wires)
  • Grounds for each pair (total of 4)
  • Power (1)
  • HotPlug (1)
  • DDC (I2C) for EDID and HDCP (2)
  • Ground for DDC (1)
  • CEC (1)
Add it up and you get 18, not 19, wires. We only need 18 wires to successfully transport the entire HDMI envelope without Ethernet, which still leaves us with a two-wire deficit.

In most cases, all five grounds become one common ground as they enter and exit most HDMI electronics, which saves us four wires. Just one catch: the massive amount of ground (shield) included within a standard HDMI cable provides a huge amount of AC noise immunity and a whole lot more current capability than the one wire typically used by ATDs (Alternative Transmission Devices). Some products are weak in this area by using the remaining two wires as a Com Port (IR or RS232), limiting the ground return current.

We examined three attempts that successfully accomplish a category HDMI solution, and common to all three systems is some form of equalization to restore the high-frequency data. This is lost over distance and is used pretty much by everyone.

Two-Wire Category Cable Solutions

Pros:
  • Designated wires for each channel
  • No complex conversion process
  • Extended frequency response for Rev 1.4 applications (3D, Deep Color)
  • Single power supply
  • Correction for cable deficiencies such as re-clocking and DDC errors
  • Increased reliability
  • Improved power management
  • Less expensive
Cons:
  • Uses two Category cables
  • Limited on additional features such as IR and RS232
  • Vulnerable to EMI
In a two-wire system things are straightforward. Having the wires needed to transport the HDMI data offers a robust connection, provided the necessary engineering and countermeasures are included.

One-Wire Category Cable Solutions - Multi-tasking

Pros:
  • Extended frequency response for Rev 1.4 applications (3D, Deep Color)
  • Correction for cable deficiencies such as re-clocking and DDC errors
  • Increased reliability
  • Improved power management
Cons:
  • Distance limitations to 100 feet
  • Limited on additional features such as IR and RS232
  • Requires two power supplies
  • Vulnerable to EMI
  • More expensive
Reducing this to only one Category cable gets more complicated. One method is to combine all the signal data by multi-tasking each wire while at the same time adding in the necessary countermeasures for cable deficiencies that have been discovered since ATDs were first deployed. The second way is to radically transform the entire HDMI signal data and then reconstruct it back to HDMI.

Combining all the signal data has its advantages. With newer chip sets the frequency response, supply voltage, DDC (EDID and HDCP) integrity, and time correction can all be corrected supporting a very reliable transport mechanism.

One-Wire Category Cable Solutions - Reconstructed

Pros:
  • Extended frequency response for Rev 1.4 applications (3D, Deep Color)
  • Extra features
  • Longer distances
  • Improved power management
Cons:
  • Increased risks by using two operating systems
  • Requires two power supplies
  • Power hungry and can run hot
  • More expensive
  • Vulnerable to EMI
Reconstruction also has its pros and cons. The thought of tearing down the entire interface only to rebuild it at the display can be questionable. However, they have some extra features and impressive distance claims. But now on top of HDMI, we have no clue how these new systems work, nor have we learned all their quirks.

None of these systems rectifies the AC interference issues that just about everyone has experienced. A shielded category cable will eliminate most of the AC interference out there as long as the user has an ATD that supports a shield drain connection within the female RJ45 jack.

There is no perfect system. Some work and others don’t. Both two-wire or one-wire solutions offer better reliability than Category systems from the past.


Jeff Boccaccio, President, DPL Labs
Jeff Boccaccio, president of DPL Labs, can be reached at jeff@dpllabs.com.