2. Convenient, yet confusing
HDMI offers one connection that provides the most incredible audio and video experience to consumers yet. With each advance, HDMI delivers sharper, more vivid images with brighter and deeper colours, and the highest-quality surround sound available. A "one-cable" solution. Convenient.
However, HDMI causes more confusion than any other connector type, with different cable and component standards, interoperability problems, and false information about the quality and performance of HDMI cables. Moreover, many electronics and cables do not perform to the standard, so many combinations of cables and components don't work. Additionally, cable performance suffers over long runs, resulting in degraded picture quality – from dropped pixels, snow, streaks across the screen, to total picture drop out. Confusing?
The HDMI cable confusion
One of the most bandied-about misconceptions is that all HDMI cables are the same – so just buy the cheapest cables you can find and they'll work for you. That is simply just not true. The truth is:
- There are different levels of High Definition digital video, and different standards for HDMI-enabled components, hence there are different levels of HDMI cables to connect them.
- High Definition (720p/1080i), as we know it today, is delivered at a 2.2 Gbps (giga-bits per second) throughput rate. Although high compared to standard DVD at .81 Gbps, it is miniscule compared to the throughput requirements that will achieve the best performance from High Definition in the near future, which will require 10.2 Gbps and more. Previous generation HDMI cables developed to meet the original 2.2 Gbps standard are not certified for the new standard and may not pass the full bandwidth required to provide the best performance from these new products.
- HDMI marks the first time that a cable connection standard has created a "superhighway" for digital content, with components yet to come. Even some of the original HDMI 1.0 cable from 2002, may not work with the latest sources and displays and sources of today (1080p, 12 bit colour), and will surely strain to deliver the quality picture and sound content of tomorrow.
- Unlike computer data, which is tolerant of data transmission losses, video data is not. Accurate transmission is essential. Most HDMI-compliant cables can transfer older versions of High Definition (720p/1080i) data rate over short lengths, but have problems maintaining the same performance over longer lengths as well as the higher data rates required of 1080p.
- HDMI cables are very sensitive to construction variances. With HDMI's super-high data rate and microscopically tight tolerances, mishandling of cables during an install can significantly affect performance. Look for better shielded connectors, mesh jackets for durability, special winding techniques, nitrogen gas dielectrics to minimize loss, and CL 3 jacket ratings, to ensure maximum performance with every installation. These features all provide high practical use value.
- A number of manufacturing factors such as cable construction, precision cable twisting, low loss dielectrics contribute to better performance in digital.
- A new term has come to HDMI cables. With all of the confusing numbers of Megahertz, 60 Hz refresh rates, as well as Gigabytes per second to rate cables, Silicon Image, co-founders of HDMI, have determined that speed is a good way to combine all of the various parameters. Standard Speed and High Speed are the two cable categories, but there is a wide spread of speeds that each of these cover.
In addition to the aforementioned points, we tend to assume, when thinking about wires and cables, that when a signal is applied to one end of a wire, it arrives instantaneously at the other end of that wire, unaltered. If you've ever spent any time studying basic DC circuit theory, that's exactly the assumption you're accustomed to making. That assumption works pretty well if we're talking about low-frequency signals and modest distances, but wire and electricity behave in strange and counterintuitive ways over distance, and at high frequencies. Nothing in this universe – not even light – travels instantaneously from point to point, and when a voltage is applied to a wire, a wave of energy starts propagating down that wire which takes time to get where it's going, and which arrives in a different condition from that in which it left. This isn't really important if you're turning on a reading lamp – but it's very important in high-speed digital signaling! There are a few considerations that start to cause real trouble:
- Time: electricity doesn't travel instantaneously. It travels at something approaching the speed of light, and exactly how fast it travels depends upon the insulating material surrounding the wire. As the composition and density of that insulation changes from point to point along the wire, the speed of travel changes.
- Resistance: electricity burns up in wire and turns into heat.
- Skin effect: higher frequencies travel primarily on the outside of a wire, while lower frequencies use more of the wire's depth; this means that higher frequencies face more resistance, and are burned up more rapidly, than lower frequencies.
- Capacitance: some of the energy of the signal gets stored in the wire by a principle known as "capacitance," rather than being delivered immediately to the destination. This smears out the signal relative to time, making changes in voltage appear less sudden at the far end of the wire than they were at the source. This phenomenon is frequency-dependent, with higher frequencies being more strongly affected.
- Impedance: if the characteristic impedance of the cable doesn't match the impedance of the source and load circuits, the impedance mismatch will cause portions of the signal to be reflected back and forth in the cable. The same is true for variations in impedance from point to point within the cable.
- Crosstalk: when signals are run in parallel over a distance, the signal in one wire will induce a similar signal in another, causing interference.
- Inductance: just as capacitance smears out changes in voltage, inductance--the relationship between a current flow and an induced electromagnetic field around that flow--smears out changes in the rate of current flow over time.