The Latest HDMI Standard

The Latest HDMI Standard

Getting a 3D cinema experience isn't new, but films such as Avatar have shown how the latest technology has added greatly to the realism of the effect, even if we still have to wear those 3D glasses for now! But the real benefit will come in being able to have that 3D experience on the TV in our home. The larger flatscreen, high-definition TVs already give us a picture size and quality closer to the cinema experience, but adding that third dimension demands quite a leap in the underlying technology, and fundamental to the success of HDTV and 3DTV is the High-Definition Multimedia Interface (HDMI).
 
In the past, we could rely on good quality SCART, coaxial, phono and digital optical cables to deliver the audio and video signals to, and between, the various components in our entertainment system - the decoder, TV, hi-fi etc. These cables moved signals that were mostly analogue or single source digital, but the coming of digital HDTV changed all that. Not just the high definition picture but multi-channel digital surround sound now dictated that the bandwidth requirements became extremely high. It presented a terrific opportunity but also a severe test for the manufacturers of the players and TVs, and not least for the suppliers of cables, connectors and switching devices. Any signal losses down the line would become critical since digital signals don't degrade gracefully, as with analogue - in simple terms, the picture doesn't just go a little fuzzy, it breaks up big time! The requirement was for a high quality, high bandwidth, single cable solution to transmitting these digital signals.
 
HDMI was pioneered by Silicon Image but is now developed by a group of the leading electronics manufacturers. HDMI provides a very high-speed digital signal transmission medium capable of transmitting signals at up to 5Mbps (5,000,000 bits per second), but even the first versions of HDMI didn't have 3D TV in mind. That was added with the development of the HDMI 1.4 specification (the latest version at time of writing), which also includes integrated data connectivity as manufacturers aim to stay one step ahead of the trend towards the fully-wired digital home.
 
The two major advantages of a digital signal over analogue are no signal compression and no digital-to-analogue signal conversion. When any one of these two take place, some degree of signal loss is unavoidable and so keeping things in the digital domain is the first important step. But even a digital signal can become imperfect. HDMI is a standard that provides the basis for fast, high-bandwidth digital signal transmission, but as with most technologies, how that standard is applied will result in quality differences between brands and individual products.
 
There are many factors that pose problems to a signal being transmitted through any cable, be it analogue or digital. However, due to the high transmission speeds of a digital signal, it is even more delicate and easily affected than analogue. Since the core technology of HDMI relies on TMDS (Time Minimized Differential Signaling) - a way to transmit huge amounts of data over a twisted pair - impedance (resistance) and interaction between the twisted pairs plays a more significant role in the performance of the cable than ever. I spoke with Joe Carri of UK-based specialist cable developer IXOS and he explained some of the key points in designing an HDMI cable. In order to produce a cable that will not alter the timing of the digital signals, IXOS engineers its cables with PC-OFC (High Grade Pure Crystal Oxygen-Free Copper), a copper with fewer impurities than standard Oxygen-Free Copper, thus creating less resistance. Triple layer shielding also minimizes both radio frequency (RF) and electromagnetic (EM) interference from both internal and external forces. IXOS also applies lower than industry standard percentages for allowable tolerances on engineering and manufacturing imperfections, and there's no cost-cutting on the connector used, which in the case of IXOS are designs approved by Silicon Image.
 
One of the most testing issues caused by the extremely high bandwidth requirement of HDTV and 3DTV is the effect on cable length. Manufacturing a cable to pass 720p/1080i is relatively straightforward for a specialist manufacturer and IXOS have cable lengths of up to 20m that give no noticeable difference in the picture quality, but with twice the resolution, 1080p demands that twice as much information needs to get through. In the case of IXOS, every HDMI cable will pass 1080p up to 7.5m (almost 25 feet), and this is verified at the factory by testing every single cable using a standardised Eye Diagram Test. This test analyzes the quality of a digital signal at the far end of the line by showing a repetitively sampled oscilloscope measurement that gives a clear visual depiction of the signal's behaviour at it passes through the cable.
 
The speed at which the industry has moved on from 720p/1080i has been rapid, and what was originally hailed as "high definition" was quite quickly surpassed by the 1080p resolution of the movie-making industry. Manufacturers in the consumer electronics market all had to up their game. Some were ahead of the curve, some a little behind. Those ahead of the curve use research and development to help them recognise potential problems that would hinder further performance improvements. IXOS's Joe Carri highlighted FEXT (Far End Cross Talk) as one example. FEXT has been a common problem with the HDMI connector. Because of the connector's small size and multiple solder points, exposing the contacts without insulation within a very tight area, the impedance of the cables is altered at the point of contact. When the impedance of a cable changes, crosstalk will occur between two or more wires of different impedances and this can significantly reduce the quality of the picture and sound. Minimising FEXT by stabilizing the impedance at the solder points is therefore extremely important, and the company's patented solution was incorporated at a very early stage in the development of their own HDMI cables.
 


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