EoW November 2009

technical article Maximising ultra-high definition video cable By Stephen H Lampen, multimedia technology manager, Belden

Abstract With the arrival of digital video distribution, one sub-set, high definition has become the key format. In the professional broad- cast world, this is called HD-SDI and is commonly transmitted in two formats, 1080i (an interlaced format) and 720p (a progressive format). While producing excellent results, this is by no means the end-point in the search for ultimate picture resolution and quality. The next step is 1080p, a progressive format. This is also available in a consumer format. This paper will outline why 1080p was desirable, how such signals are carried, and how installations can accommodate the higher bandwidth and data rate of these ultra-high definition signals. Also included is an analysis of 1080p signals as they relate to consumer delivery systems such as HDMI cables. Introduction With the arrival of digital video, there was always the intent toprovide a highdefinition wide-screen version. For broadcasters this eventually became two formats: interlaced 1080i, and progressive 720p; both produced a high-resolution image with a 16x9 aspect ratio. However, higher definition formats were also considered, such as 1080p/60 (in Europe and other countries this would be 1080p/50). A general comparison between formats is shown in Table 1 . Master format 1080p/60 can serve three functions: first, as a master format between 1080i and 720p, where these two original formats are not very compatible. When shooting in one format, it is quite possible that a customer will want the material in the other format. Conversion between 1080i and 720p, or vice versa, is problematic, especially with rapid picture movement, leading to pixelisation, streaking, and other undesirable effects. By shooting in 1080p/60, the image can be converted into either 1080i or 720p with no side effects. This makes such content doubly saleable in the marketplace.

Format

Line

Data rate

Clock

1080

Interlaced

1.5 Gbps

750 MHz

720

Progressive

1.5 Gbps

750 MHz

1080

Progressive

3 Gbps

1.5 GHz

Table 1 ▲ ▲ : High definition formats

Extended life By shooting in 1080p/60, this material will also be available when 1080p/60 becomes a distributed format. It will also allow footage to be used for theatrical presentations, approaching (and in some cases surpassing) the resolution of earlier film-based product. Thus, 1080p/60 offers the ability to take a product, such as an episodic television drama, and increase its saleability by seamlessly offering it in 1080i or 720p formats with no loss of image quality, and also allowing that product to be shown in 1080p, on Blu-Ray or other ultra-high- definition delivery systems, or even in theatrical venues. Thus, the cost of moving up to 1080p/60 can be easily justified when looking only slightly into the future. Extending the life of the product can bring extended revenue. The cost The bit stream for 1080p/60 is double that of 1080i or 720p so tape, disc or server space will be more expensive as more space will be required for a given project. As with any new technology, the cost of hardware will be greater for the new higher-bandwidth equipment. In the long run, however, this is easily justified by the extended life and greater versatility of the product produced. It might be expected that the cost of interconnection, the cost of the cable in-between, would also increase. Surprisingly, that is not the case; professional cable, tested and verified for 1080p/60, is currently available at the same price-point as SD-SDI or HD-SDI products. Some of these cables are less expensive than previous professional analogue coax cables.

For the consumer, however, there is HDMI and, for some applications, the predecessor DVI; both cables are for the specific delivery of digital signals in standard definition 4x3, high-definition 16x9, or 1080p/60. These consumer cables carry a significant price increase compared to simple analogue coax cables. One emerging format, DisplayPort, may usher in a completely new distribution scheme – digital television delivered over the Internet. High definition cable Professional broadcast coaxial cable intended to carry high definition signals, such as 1080i or 720p, must carry some- thing beyond the clock to assure a robust bit-stream. The SMPTE (Society of Motion Picture and Television Engineers) 292M standards suggest a minimum level of testing out to the second harmonic of the clock. As seen in Table 1 , the clock for both 1080i and 720p is 750 MHz, so the second harmonic is 1.5 GHz. However, that is the minimum bandwidth. What would be the ideal bandwidth? The answer is an infinite bandwidth. Since square waves are made of the ‘clock’ or ‘fundamental’ frequency with harmonics added to the clock, the clock with an infinite number of harmonics would form an ideal data bit stream. This would produce a pristine square wave. Of course, it is impossible to generate, interconnect, record, or manipulate an infinite number of harmonics, so a maximum level of harmonics must be established. While the minimum is the second harmonic (1.5 GHz), many manufacturers set the maximum at the third harmonic (750 x 3 = 2.25 GHz). Still others set it at the fourth harmonic (750 x 4 = 3 GHz).

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EuroWire – November 2009