EoW November 2009

technical article

If an engineer doesn’t want to carry a 50 ft cable, cable clones are available. This is a small device that mimics a given length of cable. It most commonly has a male and female BNC at the ends of a cylinder, and the type of cable and the length are marked on it. Consumer variations In the consumer world, things are not as simple and elegant as in the professional world. The reason is content. In the professional world, it is assumed that the production house, broadcaster, or other content producer, owns the content outright. There are no legal complications. In the consumer world, most often the user does not own the content and is not the copyright holder. Those who do own the copyright are concerned about loss of content protection and loss of revenue and, therefore, they require a delivery system that assures that content will not be stolen. The solution to this is a system called DVI (Digital Video Interface) and its latest version HDMI (High Definition Media Interface). DVI allows for both analogue and digital signals to be delivered from a source device (cable box, disc player, for example) but for digital delivery, there is a complex electronic ‘handshake’ between devices, called HDCP, (High-Definition Copy Protection) assuring that both source and destination devices are legal. The HDMI cable version supports digital signals only, not analogue. Inside an HDMI cable there are nineteen wires between source and destination devices including three shielded data pairs, one shielded clock pair, and seven more wires for HDCP, power, and future applications. 1080p/60 in the home Just as in professional systems, signal delivery comes in various data rates depending on the image size and image quality desired, from SD-SDI (called simply ‘standard digital’ in the consumer version) to HD-SDI (called HDTV in the consumer version). These fall under specific stan- dards, controlled by HDMI LLC, and shown in Table 4 . Notice the immense data rate required for 1080p delivery. This includes the embed- ded 5.1 surround-sound and 12-bit or 16-bit deep colour that will be available in future sources and displays, compared to the current standard 8-bit per colour. These cables are severely distance-limited and the standards do not address the subject of distance.

Standard

Clock pair

Data rate per pair Total data delivery

Category 1 Version 1.3a 1080i/720p Category 2 Version 1.3a 1080p/60 Category 2 Version 1.3a 1080p/60

74.25 MHz

742.5 Mbps

2.2275 Gbps

148.5 MHz tested at 165 MHz

1.485 Gbps

4.455 Gbps

340 MHz

3.4 Gbps

10.2 Gbps

Table 4 ▲ ▲ : Consumer HDMI standards

Notes

There are two ways of testing these cables and, if they pass either test procedure, then they pass no matter what that length might be. Long HDMI cables Since the data is split between three pairs and recombined, there are two specifications aimed at accurate delivery of the parts at the other end. One is delay skew, which addresses the difference (skew) in the delivery time (delay) of each pair. The other is the length of each wire in each pair, called ‘intra-pair skew’. This is a new specification for many cable manu- facturers and test facilities and has not appeared before in other pair cables, such as category data cables. A process called ‘bonded-pairs’ – where the wires in each pair are stuck together without glue – dramatically improves intra-pair skew, and makes the delay skew easier to control and predict. At the time of delivery of this paper, there is only one manufacturer of HDMI cable in the United States. All other cables are made offshore in Asian countries [Note 3] . Putting nineteen wires inside a tiny connector is difficult, which is why raw cable and installer-friendly connectors are not available. There is pressure to change this, and field installable connectors might eventually appear. However, such connectors must pass 10.2 Gbps, high data rates, just for the existing 1080p signal. Any impedance variation (return loss) would show up instantly, making HDMI connectors even more problematic; and 1080p/60 is not the end of the line. There is significant discussion about 1440p and even higher resolutions. These discussions also include improving the frame rate, also called the refresh rate, of 60 frames per second. Proposed rates of 85 frames, 100 frames, even up to 120 frames, sometimes described as 120 Hertz, makes signals even larger, and the delivery of those signals even more complex. n

Note 1: Network analysers are easily available beyond 3 GHz; however, they are 50 ohm input/ output devices, so matching networks must be obtained to use this equipment at 75 ohms at any higher frequency or bandwidth. These matching networks must be tested and verified (be traceable) to NIST (the National Institutes of Standards and Technology) other- wise the results of the testing are not verifiable or repeatable Note 2: Since SMPTE 292M establishes a return loss of –15 dB at the second harmonic of the clock (750 x 2 = 1.5 GHz) it is consistent that a new limit for 1080p/60 be set as the second harmonic of that clock (1.5 GHz x 2 = 3 GHz), with a limit of –10 dB. However, it should be stressed that any component should easily pass this –15 dB/–10 dB requirement, and any part that does not exceed this minimum should be rejected. Note that –10 dB return loss is equivalent to 10% reflection, a significant amount Note 3: USA-manufactured HDMI cable is just cable, as no connectors are available in the USA or anywhere else outside Asia. The US-produced cable is shipped to Asia to have connectors attached. To be completely accurate, these finished cables are made from US– and Asian–manufactured parts and assembled in Asia Note 4: At NAB two years ago, a major Canadian chip manufacturer demonstrated a prototype chip driving professional 1080i along 2,000 feet of RG-6 (1694A) with a perfect eye pattern at the end. The formula states 400 feet, further emphasising that these distance are safe distances and real-world applications could go much further. If one wishes to go further than the calculated distance, then those cables should be individually tested for bit errors or eye patterns This paper was first presented at the 57 th IWCS and is reproduced with the permission of the organisers.

Belden San Francisco, California, USA Email : shlampen@aol.com Website : www.belden.com

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EuroWire – November 2009