HIGH DEFINITION VIDEO CABLE
Nowhere has cable manufacturing made greater strides in manufacturing consistency and quality than in digital video coaxial cable
. It is safe to say that 20 years ago, these cables would have been impossible to make. The technology didn't exist to make them!
This, in a nutshell, is why older analog cables cannot effectively carry digital signals. Analog cables lack almost every requirement to carry digital signals. Table 1 is a short list of what you should expect from a digital cable.
CABLE PARAMETER
REQUIRED FOR DIGITAL
ANALOG CABLE PERFORMANCE
Resistance
High-purity, larger size.
High-purity, size not critical.
Bandwidth
HD is 1.5 GHz, 1080p/50 is 3 GHz.
Analog PAL is 5 MHz.
Skin Effect
Huge percentage of signal on skin.
Tiny percentage runs on skin.
Impedance
75 ohm
75 ohm
Impedance Tolerance
+/- 3 ohms or better
+/- 7 ohms or worse
Velocity
83%
66%
Return Loss
-21 dB to 4.5 GHz
Unspecified
"Safe" Distance RG-6
HD: 111m. 1080p/50: 76m
Kilometer (based on EQ recovery)
Table 1
There are simple things like the bandwidth. Today's HD-SDI bandwidth (1.5 GHz) is 300 times the bandwidth of the analog signal (5 MHz). It is extremely unlikely that any analog cable (much less connector, patch panel or other device) has been tested anywhere near the correct digital bandwidth, or are close to 75 ohms at those frequencies..
A huge percentage of the digital signal will travel on the skin of the conductor, so a lot of time is spent making sure that this conductor has a mirror finish on the outside. No such preparation is done for the analog cable.
The insulation ("dielectric") around the conductor affects the attenuation, even at moderately high frequencies. The loss on foamed (digital) cable can be more than 3 dB better at 1 GHz compared to solid plastic (analog) cable. At the third harmonic of the occupied bandwidth (2.25 GHz for HD, 4.5 GHz for 1080p/50), analog cables are rarely tested up there, so their performance is unknown.
However, it should be stressed that these distance are "safe" distances, based on a SMPTE formula. This formula can be found in the SMPTE 292M standard for high-definition video. The formula says that, when the signal strength has dropped by 20 dB at half the clock, then that is as far as a cable can "safely" go. The clock for HD is 1.5 GHz, so half of that (750 MHz) is the same as the occupied bandwidth of the video signal, often called the "Nyquist Limit". Many box and chip manufacturers say that they can go much farther than these safe' numbers. With modern top-of-the-line chips, you can often double these distances. This simply proves that the safe' distances are intended where you are using older equipment, or chips that are not top-of-the-line. Then these distances might be more realistic.
Table 2 shows a list of common cables, with the distance they can go for HD and 1080p/50 signals.
Data Rate:
1.5 Gb/s
3.0Gb/s
Spec:
SMPTE 292M
SMPTE 424M
Cable Part Number
HD-SDI
1080p/50
Ft.
m
Ft.
m
179DT
109
33
76
23
1855A*
209
64
154
47
1855P
190
58
127
39
1505A*
308
94
215
66
1505F
225
69
150
46
1506A*
267
81
91
28
1694A*
364
111
250
76
1694F
286
87
192
59
1695A
323
98
217
66
7731A
546
166
364
111
7732A
427
130
289
88
7732LL
515
157
354
108
*Bundled versions of individual cable follow the same distance formulas.
Table 2
Once you pass these safe distances, what can you do? The first option is to test the signal. This can be done in a number of ways. You can look at eye patterns, for instance. The problem is that eye patterns are not as accurate as many designers and installers think. For instance, there maybe a completely closed eye in the analyzer and yet EQ'ing and reclocking will bring back a robust and completely useable signal.
The reason for this is that the clock recovery in the test equipment is not the same as the clock recovery in actual broadcast gear. In the test equipment clock recovery must work at a huge range of frequencies. In broadcast equipment, it only needs to recover one clock frequency (or perhaps two). Therefore, the clock recovery in HD or 1080p/50 equipment can be much more accurate, and allow longer cable runs, than any test gear might tell you.
The point is that there is a digital cliff, when the bit errors increase so rapidly that the signal can go from fully acceptable to unplayable in only a few feet of cable. Wherever that "digital cliff" is, the wise thing is to stay away from it. The only question is, how far away from the cliff do you want to be? 10 meters? 20 meters? 50 meters? And perhaps this is a paranoia question rather than a technical question.
A simple, and perhaps more accurate test, is to put an HD signal on each cable, and plug the other end into an HD monitor. Good HD monitor can shift the image area to the side so you are looking at the black retrace area between frames. Any bit errors (ones that have been incorrectly read as zeros, or vice versa) will be very apparent on this part of a monitor. The black area will flash white!
Now we take that paranoia number the distance you want to stay away from the digital cliff. Let's say you chose 20 meters for this example. Obtain a 20 meter piece of cable of the same type used in your installation. Put connectors on each end and, at one end, put a feedthrough (female-to-female) adaptor. Be sure that the connectors and the feedthrough are good out to the bandwidth of HD signal (at least 2.25 GHz for HD, and as high as you can find ideally 4.5 GHz for 1080p/50).
Now put this extension cable in the circuit you are testing. It doesn't matter if it's added at the source or destination. You're just making that cable 20 meters longer. Now look at the black retrace area. If you see nothing, no white speckles, then you are at least 20 meters away from the cliff!
If you do see speckles, check the connectors and all the passive devices you are going through (patch panels, patch cords, adaptors, bulkheads, feedthroughs) and make sure they are 75 ohms through the bandwidth of the HD signal. If everything checks out, then either the cable is too small (change to a larger cable) or you are going too far. You can move the equipment in the racks or, the ultimate fix, change to fiber optic cable. Fiber is an expensive option but, if you have very long runs, certainly more than 300 metres, that is pretty close to the limit of copper cables, even with the best chips.
HIGH DEFINITION VIDEO CABLE
By: Dixon
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