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Mix Magazine

This installment of The Bitstream column appeared in the June 2002 issue of Mix Magazine.

The Bitstream

This column discusses copper wiring and infrastructure, along with some high school physics…

Cut A Check, We’ll Hook It Up!

Summer is here and what better time to work on your studio tan than right now? After all, who wants to be outside in the heat when you can be so cool in the studio…Besides, now’s the time for expansion and upgrades so I’m gonna take a look at that stuff in the walls, floor and ceiling that moves your product throughout the facility. We’re talkin’ wire, baby! So mundane, so misunderstood yet, sooo essential.

There’s every imaginable grade and size of wire out there but I’m going to focus on the essentials for pushing digital electrons around the place. Now, you didn’t know there was such a thing as a digital electron, did ya? Well, frankly there isn’t but there certainly is digital wire or, to be more precise, wire designed to specifically carry a particular digital signaling and PHY standard such as AES/EBU audio, Gigabit Ethernet or Serial Digital video.

For a moment, cast your addled mind back to high school physics, where we learned that there are three basic electrical components: resistors, inductors and capacitors. If you were on 420 patrol during that period of your life, refer to Pedant In A Box…it’s quicker than summer school! Anyway, wire is a great conductor at low frequencies but exhibits increasing impedance as signal frequencies rise. That’s why long runs of wire, such as for tie lines, should employ better grades of low capacitance cable.

Signal frequency is important when connecting digital devices since, unlike analog audio where the highest frequency of interest is about 80 kHz on a good day, digital audio requires at least 100 times the bandwidth to ensure safe transmission of the data. As an example, the folks at Gepco spec their AES/EBU–specific, wide bandwidth, extended distance multi–pair products at a 12.3 MHz bandwidth, “complaint with the 1999 revision of the AES3 standard for transmission of digital audio at sampling rates up to 96 kHz.” Also, any product intended for AES/EBU, whether from Belden, Canare, Gepco, Mogami, Mohawk or West Penn/CDT must also provide the controlled 110 Ohm impedance that the AES/EBU PHY spec requires.

“So…” you may ask, “what’ll happen if I use Billy Bob’s El Cheapo brand of mic cable instead of AES spec cable?” Maybe nothing or maybe the signal won’t arrive at the receiver with enough strength to be decoded reliably. If you’re desperately in need of a temporary run of wire, a good rule of thumb is to try a double length of the cable in question and if that appears to work, using half that length should serve reliably. Again, only in a pinch since, unless you apply test gear to the wire in question, you have no idea how close you are to the cliff where digital stops working. Also, as AES/EBU decreases in signal strength, the effect of jitter rises proportionally, which is not a good thing.

If you find that you often need ad hoc audio wiring around your facility, a good alternative to patch and pray is AES baluns or impedance matching transformers in conjunction with standard 75 Ohm video cable. Baluns, available from Canare, ETS and Graham Patten, are passive widgets that match the impedances between the AES/EBU Type I PHY or balanced physical interface and the unbalanced coaxial cable. They also provide the necessary XLR to BNC connector conversion and Canare offers one that even provides level matching as well. Since coax is designed for the wide bandwidth of analog video, it works well in a pinch for digital audio. The issue of controlled impedance also means that you should never use connectors or patch bays for digital audio that were designed for analog applications since the screwy impedance through such old school assemblies will seriously compromise the signal integrity.

For networked data transmission, requirements are even more stringent. With Gigabit Ethernet becoming increasingly common, especially in Mac–centric shops where their towers all ship with it standard, the issue of cabling is rather significant. The most common Category 5 cabling choice is UTP or unshielded twisted pair and old school UTP works fine for 100BaseT runs within most studios. However, 1000BaseT brings new restrictions on crosstalk and signal loss to ensure full throughput so new patching and wiring systems have been developed for the latest network standards. For short runs, enhanced Category 5 or Cat5e works great but, Cat5e cable isn’t designed for GigE on long wire runs and cannot support multiple patch bays, punch blocks or other impedance discontinuities. For those situations, Category 6 is needed and that stuff cannot be installed by a DIY kinda guy. For new installs, I recommend you look into having a contractor provide your data cabling infrastructure. They can assess your needs, specify and install the stuff, then test, benchmark and document the whole lot before you sign off on the job.

I should mention here that, since TCP/IP is designed to traverse a hostile path where packet loss is a routine fact of life, you can hook up any old UTP and get some data through your system. What you won’t get is the specified data throughput and I’m not even talking about managed switches versus dumb hubs or any such high level stuff. In the world of high speed data carriage, less than spec means poor performance.

All this copper we’ve been discussing has to go somewhere and, though dedicated wiring troughs in or under the floor are common for new build–outs, not everyone can take that route. These days, wiring troughs designed for retrofit installs have gotten much better than the two piece, bent sheet metal crap that I contended with in Ye Olden Days. Companies such as HellermannTyton, with their InfoStream raceway, and Hubbell, with their MediaTrak products, are less expensive, install faster and make changes easier than older, two piece designs. Modern raceways are designed to maintain the minimum bend radius required for copper and fibre and are available in multichannel configurations, so you can lay your audio and data cables in separate channels. When you lay, pull or dress your own cable, be sure to go easy on the wire ties. They should not be so tight that they dent the cable jacket, this will cause impedance discontinuities, reflections and reduced throughput.

One last thing…if your remodeling plans call for a remote machine room/closet, you should consider smart power management which will save you precious time and money. Modern “power strips,” from Pulizzi Engineering, Server Technology and Western Telematic include an Ethernet port to allow remote management and sequenced power–up capabilities. So, when upgrading your creative crib, expend some time and spend a bit of cash on your wiring. You’ll be happy you did!

Pedant In A Box

Ridgemont High, Part Deux…

Resistors

All room temperature electrical conductors have some amount of “resistance” to electron flow. A substance with massive amounts of resistance can be thought of as an “insulator.” Silver, gold, copper and aluminum are good electrical conductors while most plastics, glass, dry air and dry paper are all good insulators. By the way, pure water is a good insulator but dissolved impurities improve the conductance considerably.

Theoretically, resistors are frequency–blind. A good conductor acts as a “short circuit” for the easy flow of electrons and an insulator blocks electron flow, regardless of frequency. Complex signals, like analog and digital audio are “alternating current” or AC signals. This means that electrons flow back and forth in opposite direction at some instantaneous frequency. For instance, the wall outlets here at Seneschal nominally provides 110 volts of electrical potential or pressure and about 10 amperes of current or flow rate before the circuit breakers trip. Now for a key point: All real word electrical circuits behave differently when AC signals are applied, providing different resistance relative to their DC behavior. This differing, frequency–related AC resistance is referred to as “impedance.”

Capacitors

When two conductors are separated by a small amount of insulation, they act as a “capacitor,” capable of storing electrical potential or voltage. Imagine a “jelly roll” of two sheets of aluminum foil (the dough) separated by plastic food wrap or waxed paper (the jelly). Sounds yummy, doesn’t it? Well, even better is that, if you hook a wire or “terminal” to each sheet of foil, you can store electricity inside the jelly roll. The better the intervening insulation, the longer it will hold the “charge.” [“free” electrons – OMas] This “capacitive effect” is produced when opposing charge, polarity–wise, builds up on each “plate,” the foil in our jelly roll. Since the electrons are prevented from flowing across the insulated gap between plates as voltage is applied, the electrons pile up along the outer surface, unable to jump across the chasm of low electrical conductivity.

You (a “bag” of salty water) act as a capacitor when you shuffle around in your slippers (an insulator) on a rug, pulling charge off of the carpet fibers, across the insulator and onto your outside conductive surface (your salty skin). Touch a good conductor and, Ouch!, you allow all those spare electrons to flow away in a momentary spark.

Back to the above mentioned impedance…when an AC signal is applied to a capacitor, the amount of charge stored varies depending on the frequency. At zero Hertz or DC (direct current), a capacitor will charge up quickly but not let any electrons flow once it’s “full.” So, for DC, it basically acts as an insulator after a momentary spike to charge it. For AC however, capacitors act more and more like a conductor as frequency increases. At some relatively high frequency, a capacitor will act as a good conductor or “short circuit” as if the insulation weren’t even there…go figure. So remember: for capacitors, impedance drops as frequency increases.

Inductors

On to inductors, which are nothing more than a length of wire wound into a coil. All conductors have magnetic fields that wrap around them whenever electricity flows through them. That’s why voice coils move relative to the permanent magnet in a loudspeaker when signal is applied. When you coil up a wire, the individual magnetic fields of each strand reinforce each other to collectively produce a larger strength field…it’s that old “sum of the parts is greater…” thing. Impedance–wise, inductors are the compliment of capacitors, acting opposite to them when you apply AC signal. At low frequency, it’s just a wire so it exhibits low or no resistance. As signal frequency increases, it’s harder and harder to build a magnetic field of one polarity, then collapse it and build another of the opposite polarity. So, at relatively high frequencies, inductors act as an “open circuit” or insulator. Remember: for inductors, impedance rises as frequency increases.

These three building blocks, resistors, capacitors and inductors are all the components you need to build a passive equalizer, a box that changes the amplitude of an analog signal based on frequency. These three building blocks are also present, to some degree or other, in every piece of commercially manufactured wire. This is especially true of cable with two of more conductors since those conductors have insulation between. So, cable inherently behaves differently to AC signals of differing frequency. Since the birth of “hi fi,” this fact has allowed consumer audio manufacturers to sell some laughably goofy products to uninformed consumers. Though the effect is very, very small in short lengths, it makes a difference in long cable runs or when multiple connectors are present.

Bio

OMas, heralding the return of the fog to his City By The Bay, now wonders what became of Adrian Barbeau…This column was created while under the influence of EMI Classics’ DVD-Audio release of Holst’s The Planets and Badly Drawn Boy’s The Hour Of Bewilderbeast, which I wish was released on SACD.