Feb 1, 2003 12:00 PM
Protect Yourself from Fiberglass
Allan Soifer's December 2002 “Contractor's Toolbox: A World of Hurt” was a most interesting and enlightening article emphasizing the cautious attitude needed when handling fiber-optic cables, in which it is so easy to get the glass pieces in the skin, in the eye, and so on. So what am I missing with the photograph, which shows a pair of hands splicing a fiber-optic cable? Because it seems that the person is not wearing any protective gloves, whose need is stressed in your article.
Thanks for the keen eye, and you certainly make a good point. The article did stress that wearing gloves when splicing fiber-optic cable is important, and the image does not reflect that point. As they say, a picture is worth a thousand words, and we apologize for presenting a conflicting message.
— Mark Mayfield
More Shield-Grounding Questions
I would like to add something to Bill Whitlock's answer to Y. Bravda's question regarding grounding (“The Buzz: Feedback,” November 2002).
When dealing with unbalanced audio wiring, consumer audio products with RCA connectors, or mono ¼-inch phone-type plugs, it is good practice to not use the RCA connector terminated cables that come with the CD players and the like. Make a new cable using 2-conductor, shielded cable. Solder one conductor to the tip of the RCA, the TS, or whatever connector you're using and solder the other conductor and the shield to the ground side of the connector at one end of the cable only. In other words, the tips connect together through the cable, and the other conductor also connects at both ends, but the shield is connected at only one end, not both. The signal circulates through the conductors while the shield protects and carries no audio current. The longer that the unbalanced line has to be, the more important the grounding technique described becomes. The end of the cable that is grounded should be plugged in to the source output and not the input side of your circuit. Noise in the line is measurably lower with this method. Balanced is best, but when you cannot balance a circuit, the technique I described will almost always help. I used scraps of high-quality 2-conductor shielded cable to connect up my home sound system.
Your notion about the superiority of shielded twisted-pair (balanced cable) with the shield grounded at one end only is widespread and promoted by many high-end cable manufacturers. Although grounding a shield at one end only is preferred practice in balanced interconnections, the situation is quite different for unbalanced connections. Virtually all noise “picked up” by unbalanced cables is because of tiny currents (actually power line leakage) that flows in the grounded conductor. Because of the resistance of the grounded conductor, a noise (hum or buzz) voltage is created over the length of the cable. This voltage is directly added to the signal at the receive end. This is called common-impedance coupling and has nothing whatsoever to do with shielding. Shielding (a conductor that completely surrounds the signal conductor) can only prevent capacitive coupling from nearby electric fields, and that is rarely a real problem. In fact, many high-end interconnects are simply twisted pairs with no overall shielding at all. What is important to reduce common-impedance coupling in unbalanced interconnections is to reduce the resistance of the grounded conductor as much as possible. Therefore, when shielded twisted-pair cable is used for an unbalanced interconnection, ground one inner conductor and the shield at both ends. That parallel connection reduces the resistance of the grounded conductor. Note that many standard coaxial cables intended for video use have heavy braided copper shielding whose resistance may be even lower than the parallel-connected shielded twisted pair.
— Bill Whitlock
Nathaniel Hecht's column (“Line Out: A Dirty Little Secret,” December 2002) on heavy metals pollution has me concerned. The specter of millions of pounds of lead and thousands of pounds of mercury, selenium, cadmium, and other toxic heavy metals entering the environment unmediated by recycling laws or precautionary common sense presents a grim picture of the future, but what concerns me even more is Hecht's use of the term bleeding-heart environmentalist to characterize his concern. When pieces of the polar ice caps the size of Connecticut are breaking off, when global fisheries are collapsing, and when 85 percent of biologists confirm that we are moving into the sixth major planetary biota extinction, portraying environmentalists as dewey-eyed idealists is reckless, at best.
So what can people in the industry do to stave off what may seem inevitable? Look at our actions to evaluate how we affect the environment. The disposal of old-style CRTs is a good starting place. Hecht's suggestions of consolidating devices and specifying less hazardous and less wasteful products are all good, but people can become much more proactive. We can insist that existing industry-supported legislation that puts remanufacturing surcharges on recycled materials be abandoned, and we can insist that the providers of all new products take responsibility for reusing the materials that are in their obsolete products. We can insist that all packaging materials be part of the reuse stream, and we can promote gleaning industries that reclaim copper, gold, lead, glass, and plastics from all of the detritus of our work. We can also hold our legislators to a higher environmental standard and insist that they stop pandering to self-serving, short-term profiteers.
There may be some cost associated with this, and we will all just have to take part in this cost, from the manufacturers on through to the clients. In the short term, this may mean a few points above current product costs, but in the intermediate term, we will find that recycling and sustainable industrial practices are cheap and profitable, and in the long term, we will find the survival of our species on this planet is far less expensive than the reckless alternatives we have pursued to this point.
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