Friday, November 13, 2009

The great 3G cable shoot-out

Thanks to Simon Hillman for tabulating them in a way that makes trends clear. The link in the title is the directory with the A1 plots.

The tests based on a pathological signal are more extensive and test for six cable types - three HD and three SD. Since we were trying to spot trends due to cable length we feel this is the most informative of the sets of data. Simon re-did a subset using only the three HD types and 1080/50P colour bars to test for jitter which (as you'd expect) doesn't vary to any degree with length but you can see the relative damage barrels and U-Links do.

The conclusions that spring out are;

  • At 3G using coax specified for HD 60m seems to be the workable cable length before attenuation becomes an issue and the eye closes below 400mV.

  • SD coax goes about half the distance - this seems counter-intuitive as most SD coax has a notional analogue bandwidth (+/- 6dBs) of 360Mhz - three octave less than HD coax. Clearly the signal recovery in the WFM8300 is at play.

  • The variation between the best (most expensive) and worst cable at HD before the signal becomes sub-optimal (i.e. worse than 3dBs attenuation) is less than 10m with Belden 1694 coming out on top.

  • The 8300 was still able to recover a signal at 150m with Belden but only 120m with the Draka DC DVC13C. At these length the mean time between corrupt video frames would be unacceptable.

  • The Condufil 1694-equivelent tracks the more expensive Belden cable very well.
As mentioned this was really a test of run-lengths for a practical guide to cabling TV facilities. In the bulk of the tests we used the correct BNC crimp connectors and the proper tools for the brands of BNCs (attached by an experienced wireman) - We did try and provoke jitter by mixing up connectors with cable but it seemed to make scant difference. It does seem for 3G HD video the newer style 4.5Ghz are to be preferred over the original HD-type cable.

In the end I suspect that these results will represent the best possible world as Tektronix gear is known to drive a coax line optimally and has excellent return loss on its inputs. Other manufactures are less so and if our experience with 1.48G 4:2:2 HD is anything to go by the massive variation in the quality of line-drivers and receivers will make these results meaningless. Nobody (particularly in these hard economic times) builds a facility with only Sony and Tektronix equipment.

Many thanks to Tom & Lee at Tek for the loan of the equipment and advice and Simon and Graham at Bryant for providing the various cable types, ends & tools. Please note all original information and test results are the property and copyright of root6

update: After IBC 2010 I had dinner with a couple of the guys from Condufil and they told me that the trick with high frequency coax is the consistency of the dialectric. The best examples use nitrogen to inflate the dialectric foam whereas budget cable is manufactured with a mechanical extrusion method. In their words this if the difference between Belden & Condufil vs Draka. This seems to be borne our by our tests.

No comments: