Cables make the AV world work but there are several rules, regulations and recommended practices when it comes to using them. Matt Murray breaks it down. 

For audio and video equipment connectivity, conventional wisdom has held that cabling should never be longer than required when linking devices to one another.

At the dawn of the electrical age, the first universally mass-produced consumer entertainment product was the radio. Collecting waves launched into the atmosphere, these early devices were exclusively tube-based and “the wireless” as it was colloquially termed, exclusively and economically brought entertainment into 1920s homes.

On a side note, the gramophone, an interchangeably used term including the phonograph, was a device for people of means, and it would not be until the late 1930s that the shellac disc would be standardised into something resembling, and eventually becoming, the vinyl in use today.

Shields up

Consideration for shielding equipment and associative connecting cabling from the multiplying dissonant symphony of radio frequency interference, which was beginning to envelop the planet, was yet to be undertaken. As more knowledge surrounding the necessity to protect devices, signal paths and humans accumulated, specific construction techniques evolved and materials were developed to ward off penetration from the relentless bombardment of ever-present EMI and RFI.

Even today, given what is known regarding the massive vulnerability that exists for all industrialised nations to an electromagnetic pulse detonation, adequate protections largely remain untaken.

A question of balance

Bell Laboratories was responsible for early noise reduction attempts in cabling, having developed balanced audio technology for early analogue telephone line use, reducing the susceptibility to external noise caused by the growing shroud of electromagnetic interference. It wasn’t until the late 1970s that cable design philosophy was again revisited, this time through the lens of a semblance of science.

Initially, it was significantly less bench time spent with ‘scopes, and more trial and error through careful listening, however, methodologies developed, and best practices became commonplace.

Unless shielded, any length of wire acts as a possible antenna for a range of frequencies based on its length. As the fidelity of products steadily increased, shielding became a prominent component and cable construction feature, along with materials composition, copper and silver quality, and for cables, a plethora of methods for winding various internal wire diameter paths before applying the cable jacketing.

Perturbations? Huh?

More recent designs have intently focused on ensuring signals inside a cable remain there, while those scattered spuriously about the atmosphere and swirling inside equipment racks, are prevented from penetrating cabling to contaminate their internal signal paths. This design philosophy even extends to AC mains cables. Though many scoff and dismiss such cables proverbially as “snake oil”, such makers may be actually out in front of new, augmented regulations intending to squash radiated electromagnetic emissions for Class A and Class B multimedia equipment (MME).

EN 55032/55035 & CISPR/CISPR 25 sounds like encrypted coding by an international spy ring. CISPR, or the Comité International Spécial des Perturbations Radioélectriques (International Special Committee on Radio Interference) was established as part of the International Electrotechnical Commission (IEC) to set electromagnetic interference standards for electrical and electronic devices. Therefore, European Norm 55032/55035 regulates electromagnetic emission and immunity requirements for all MME.

Class A equates mostly to commercial gear, whereas Class B, with more stringent limits, centres on residential equipment… oh, and by the way, connected cables (the documents use the terms: Conducted Emissions and Conducted Immunity). These standards work to ensure that such equipment and interconnecting cables do not interfere with other MME, regulating electromagnetic compatibility (EMC).

Can one industry have too many acronyms?

EMI and EMC compliance testing verifies correct device operation in noisy electromagnetic environments, such as most AV installations. So, those seemingly python-sized AC mains power cables, rather than marketing-driven, are uniquely designed to prevent EMI from radiating out of the jacketing, and at the same time, eliminate stray EMI from leaping onboard making the power supplies in connected equipment work harder to strip radiated noise out. The same technology is applied to those too-stiff-to-bend interconnects. Exotic? Maybe. Effective? Absolutely.

The long and short of it

HDMI cables are susceptible to the same afflictions as other cabling, adding their own host of scream-inducing, spirit-crushing, interconnective taunts. As mentioned at the onset, disciplined integrators are influenced into limiting cable lengths to what’s only physically sufficient to connect two devices, and with HDMI in particular, there should never be a candidate for excess.

An enigma that exists with HDMI cables, but is thus far little discussed industry-wide, involves lengths that are too short to correctly function as the format dictates. I’ll give you a moment to reread that last sentence…

In April 2002 the HDMI founders, beginning development of HDMI 1.0, made a decision that HDMI cables should be a minimum of 2m in length.

It was determined that HDMI would be an essentially modified derivative of the existing DVI standard and not newly conceived, instead adopting the DVI format for video transmission but sending audio and data during the HDMI video stream blanking intervals. The average length of a DVI cable from a desktop computer to a monitor was 2m. At that time, the HDMI founders used the 2m benchmark to establish the equalisation design point for HDMI ports driving HDMI cables. The equalisation would compensate for the loss brought about by the effects of resistance, capacitance, impedance and inductance occurring as the signal passed through the cables.

As HDMI progressed, bandwidth became greater and demand for longer cable lengths intensified. Home theatre blossomed as digital images grew larger, projectors dropped in price and resolution quadrupled. To push signals farther down the line, HDMI transmitter and receiver chip makers made determinations about “average HDMI cable lengths”, increasing equalisation amounts in an attempt to combat increasing signal degradation over distance.

As the pitfalls that plagued HDMI deployment began to fall by the wayside, attractive automation features from the days of component video started to reappear. HDMI matrix switchers populated equipment racks, with tiers of Satellite and streaming sources awaiting their turns at distribution.

The big short

As 4K made headway, in many parts of the world, streaming was the only means to deliver this content. Apple TV, at least in the United States, became a staple in every installation. Following their instincts, integrators positioned the diminutive, palm-sized devices close to their AVRs, matrix switchers and video processors, connected with HDMI cables as short as 0.5m.

As you might come to guess, the equaliser circuitry in HDMI 2.x and later HDMI output devices presents too “hot” of a signal at the input of HDMI receiving devices, due to the short cable being unable to provide the anticipated level of attenuation that would be associated with a two or three meter HDMI cable. Attempts at HDCP authentication become “scrambled” in the cable as handshake timing cannot be determined and HDCP is unable to gauge whether the signal is making an attempt at venturing off onto the internet, unprotected. The HDCP confirmation process is designed to occur within 7ms from initiation. Any longer, and the transmitting device suspects the receiving device is not in an area determined as “local”, thus shutting down attempts at handshaking.

A longer cable, 2m, or ideally 3m, provides some or all the anticipated resistance, capacitance, impedance and inductance, plus the added pathway from the additional length facilitates the time quotient and attenuation to complete HDCP protocols.

In 4K systems, all HDMI cables should be certified for 18Gbps. Passive Ultra High Speed 48Gbps cables will also work well. It has been discovered that 8K active optical cables may have compatibility issues in 4K, 18Gbps systems. 18Gbps chips do not have the buffer speed necessary to support 8K AOC cables.

And while speaking of enigmas, who would have thought that a better-performing cable could be detrimental to ultimate performance?

Money for nothing and your checks will clear

This cable regimen should be applied between all connected HDMI devices: Source components to switchers, switchers to AVRs, outputs to HDMI or HDBaseT extension transmitters and from extension receivers into displays, as this will greatly contribute to system stability. You may have to spend a bit more time and be creative when dressing racks, but eliminating countless truck rolls contributes to profitability and client satisfaction.

It isn’t written on the Rosetta Stone that HDMI cables under 2m won’t work. Millions have for years – and continue to do so – in installations with early HDMI versions. For the reasons above, however, continuing to install HDMI cables under 2m is tempting fate, particularly as bandwidths rise. You can only keep your fingers crossed for so long.

Great cables will treat you great

At a minimum, use HDMI cables certified as Premium High Speed for installations, as these cables meet existing EMI standards. Certified Ultra High Speed HDMI Cables exceed the latest international EMI standard.

As HDMI are digital signals, the old 0s and 1s argument arises when it is believed that cable construction has little to do with signal integrity (or performance) as many naively shrug that if the 0s and 1s get there, you have a signal. Such luck may better serve you at a casino. You may now add HDMI cables of less than 2m to that list which includes a rabbit’s foot and a four-leaf clover. Chookas!

The standards listed above and their newest implementations will eventually come to play a part in weening out inexpensive, poorly constructed and more importantly, poorly shielded HDMI cables from the marketplace. With HDMI 2.1b and its higher bandwidths up to 48Gbps, HDMI cables with inferior build quality will not likely pass compliance testing.

Make sure you procure products from licensed adopters or the manufacturers you buy from use licensed adopters as their source. This ensures their products have undergone certification testing. Be aware that no cables are certified for HDMI 2.1b; they must use the entire name of Ultra High Speed HDMI Cable.