EoW November 2009

technical article

by a single manufacturer. Mixing cables and manufacturers makes the situation even more unpredictable. Testing requires an HD source. It doesn’t matter what the source is as long as it produces the appropriate HD or 1080p/60 output. Also required is an HD or 1080p monitor. A professional broadcast-quality display has one valuable feature: the image can be shifted so that the centre of the monitor is the black ‘retrace’ area between images. When looking for bit errors, these will show as flashes of pixels, the most recognisable being a black pixel turning white (where 0 is misread as 1). A normal video image can often hide these bit errors, especially if the image is ‘busy’, so the black inter-frame area is the best choice. Then it must be decided how far to be from the digital video cliff. Whether ten, twenty or fifty feet, once a number is chosen the tester will take a piece of cable of that length, matching the coax cable used in the installation. Assuming for this example that a distance of 50 feet has been chosen, the tester will take a piece of cable that is 50 feet long. Connectors should be attached at each end and, at one end, a female-to-female adaptor. This adaptor must have low return loss at the highest frequency required to pass. It is a male-to-female ‘extension cable’. The installer simply adds this extension cable to any existing cable under test between the HD source and the monitor – it can be added at the source or desti- nation end, it is simply extending the original installed cable. If, after a few seconds, small white flashes cannot be seen in the black inter-frame area, then the cable length is at least 50 feet short of the cliff. If flashes are discernible, it could mean that the cable under test is damaged, the connectors poorly attached, or the wrong connectors or cable used. It could mean that the cable under test is simply too long for that signal. There is a choice of solutions: check the cable and connectors, replace it temporarily to test if it is creating the problem. Move equipment in the rack to change the length of cable, or change to a larger cable with lower loss.

basic error correction until the error rate becomes greater than the chip can handle. This means that the digital image is perfect until that critical distance where the data, the image, can no longer be resolved. The chip rapidly goes from perfect picture to no picture in only a few feet of cable. This is commonly called the digital cliff. The real concern is that an installer or user could be inches away from the cliff, and not know it. By simply inserting a patch cable, even a patch cable specifically made for digital signals, the user might push that signal over the cliff. In the SMPTE 292M standard, there is a formula to determine the maximum distance on any given cable. It simply states than when the signal drops 20 dB at half the clock frequency, that is the distance limit. Table 3 shows some common cable sizes with this calculated distance for HD. Also shown is the maximum distance for SMPTE 424M, running 1080p/60, again 20 dB loss at half the clock. However, the distances in Table 3 are based on a formula, not on real-world applications. The real-world distances are obviously very chip-dependent and really good chip sets would perform over longer distances than those shown [note4] . The distances in Table 3 are approximately halfway to the digital cliff with an average chip set. Thus, a user could probably double these distances before reaching the cliff. The numbers shown are, therefore, ‘safe’ numbers, designed to keep an installation operational even when there are flaws, a poor connector or two, a bent cable or an older device with yesterday’s chips. Testing distance If it is decided not to rely on Table 3 , or similar distance charts, then cables must be tested. Given that a high-quality network analyser can cost $60,000 or more, most installers are content to use a chart. There are, however, ways to test HD and 1080p/60 for little or no cost and determine, in an approximate way, where the cliff is. To be most effective, an installer should use a single cable type, produced

Return loss

Match Reflected

-10 dB

90%

10%

-15 dB

96.84% 3.16%

-20 dB

99%

1%

-21 dB

99.21% 0.79%

-23 dB

99.5% 0.5%

-25 dB

99.68% 0.32%

-30 dB

99.9% 0.1%

-35 dB

99.97% 0.03%

-40 dB

99.99% 0.01%

Table 2 ▲ ▲ : Return loss versus match

A major improvement in chip design in the last two decades has been the ability to withstand, and continue operating with, large amounts of reflected signals. The ideal passive device would be exactly 75 ohms with no reflection at all, but this is not possible. This is where the return loss guarantee, previouslymentioned, becomes a serious requirement to maximise the HD performance of cable, connectors and similar components. One should also note that the SMPTE return loss requirement (–15 dB or 3.16% reflection) is extremely generous. Installers are cautioned that component manufacturers who claim to meet the SMPTE standard are suggesting only that their components are no better than the minimum requirement, not a very positive start to an installation. Translate to 1080p/60 When converting to 1080p/60, the clock is doubled to 1.5 GHz, and the third harmonic raised to 4.5 GHz. Memory space, tape, disc, hard-drive size is essentially halved to store these images, audio, and metadata. The standards for this are contained in SMPTE 424M. Return loss minimum, under this extended specification is –15 dB to 1.5 GHz and –10 dB to 3 GHz [note2] . Components should be tested to 4.5 GHz and some return loss guarantee should be assigned. One cable manufacturer now routinely tests many of its HD cables to this new HD standard with a guarantee of –23 dB from 5 MHz to 1.6 GHz and –21 dB from 1.6 GHz to 4.5 GHz. A similar guarantee should be sought for all passive devices. Cable distance Digital signals have a significant problem with distance. Receiving chips can perform

Table 3 ▼ ▼ : Cable distance by cable type and signal

Cable

Diameter

HD distance 1080p/60 distance

7731A RG-11

0.405"

550 ft

360 ft

1694A RG-6

0.275"

400 ft

270 ft

1505A RG-59

0.235"

310 ft

220 ft

1855A 'Mini'

0.159"

260 ft

150 ft

179DT 'Micro'

0.100"

110 ft

80 ft

69

EuroWire – November 2009