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What is a transmission line, anyway? There are broad definitions, such as "the conductive connections between system elements which carry signal power"


Jan 1, 2000 12:00 PM,
Bill Whitlock

What is a transmission line, anyway? There are broad definitions, such as”the conductive connections between system elements which carry signalpower” (ANSI/IEEE Std 100-1977 Dictionary of Electrical and ElectronicTerms, page 739), under which every cable becomes a transmission line. Mostengineers, myself included, would narrow the definition considerably. Thefollowing is the most concise, accurate and simple definition I haveencountered: “A transmission line consists of an arrangement of electricalconductors by means of which electromagnetic energy is conveyed, overdistances comparable with the wavelength of the electromagnetic waves, fromone place to another. Transmission lines differ from simple electricalnetworks in that their inductance, capacitance, and resistance are notlumped but are distributed over distances such that the time required forelectrical energy to travel from one part to another has to be taken intoaccount. A uniform transmission line has what is called a `characteristicimpedance’. This is the impedance that would be measured at the end of sucha line if it were infinitely long. The importance of this characteristicimpedance lies in the fact that if any length of line is terminated in animpedance of this value, then all the energy flowing along the line isabsorbed at the termination and none is reflected back along the line.”(Radiotron Designer’s Handbook, F. Langford-Smith, Amalgamated WirelessValve Company, Sydney, 1953, pages 890-891.)

The first widespread users of balanced transmission lines were the earlytelephone companies. The earliest systems had no electronic amplificationyet needed to deliver maximum audio power from one telephone to another upto 20 miles (32 km) away. It is well known that, with a signal source of agiven impedance, maximum power will be delivered to a load with the same,or matched, impedance. As stated above, it is also well known thatreflection and standing wave effects will occur in a transmission lineunless both ends are terminated in its characteristic impedance. Note that,as implied in the definition by “distances comparable with the wavelength,”these effects occur only at frequencies where the time it takes the signalto propagate from one end of the transmission line to the other issignificant. Because propagation time through 20 miles of line issignificant even at voice frequencies, equipment at each end had to matchthe line impedance to avoid severe frequency-response errors because ofreflections and standing waves. Early Bell engineers knew this, but thesystem impedance of the earliest telephone systems was dictated by themiles of open wire pair transmission lines strung along poles, whichalready existed for telegraph use. These lines typically used AWG#6 wiresspaced 12 inches (305 mm) apart, which made their characteristic impedanceexactly 600 V. For those who are interested, the formula for thecharacteristic impedance of such an open two-wire transmission line in airis Z = 276 log (2D/d), where d is the diameter; D is the spacing of thewires (in same units of measure), and Z is in ohms. Therefore 600 V becamethe standard impedance for these balanced telephone lines and all earlytelephone equipment in general.

Interference, mostly from the AC power lines frequently running parallel tothe phone lines for miles, was largely eliminated through balancedoperation of the lines. Balanced (for noise rejection) andimpedance-matched (for power transfer) transmission lines were clearlynecessary for acceptable operationof the early telephone systems, which hadno amplifiers. Later, as the telephone network grew, amplifiers, filtersand “hybrid” transformers were added to enable long-distance transmission.Proper operation of these components depended critically on rather precise600 V impedances. Because other audio systems were rare, telephoneequipment and practices were adopted by radio broadcasters and, later, byrecording studios and audio professionals in general. Passive filters andEQs, step attenuators and amps having 600 V input and output impedanceswere widely used well into the 1960s, and some of this equipment is stillused today.

In modern professional audio, the goal of signal transmission systems is todeliver maximum voltage, not maximum power. Thus, devices need low outputimpedances and high input impedances. This practice is the subject of a1978 I.E.C. standard requiring output impedances to be 50 V or less andinput impedances to be 10 kV or more. Sometimes called “voltage matching,”it minimizes the effects of cable capacitances and allows an output todrive multiple inputs simultaneously with minimal level losses.

Nor do modern systems need to terminate audio cables. For a 20 kHz audiosignal in typical cable, a wavelength is about 40,000 feet (12,200 m). As Ihave said in a previous column on long lines, these effects begin to becomesignificant only when the physical length of the line becomes about 10% ofa wavelength at the highest frequency of interest. For audio signals intypical cable, this is about 4,000 feet (1,220 m). Therefore, with rareexceptions, such as telephone equipment interfaces, the use of matched 600V sources and loads in professional audio is generally unnecessary andusually degrades performance.

First, driving a 600 V terminated input rather than a 20 kV (i.e.,high-impedance or bridging) input to a level of +22 dBu requires anadditional 23 mA of peak current from the line driver. Because the linedriver IC may already be taxed near its limit driving cable capacitance(see S&VC, October 1999, page 78), the termination may cause high-frequencycurrent limiting, making cymbal crashes sound ugly. Second, if the drivingequipment actually has a 600 V output impedance, driving a 600 V load willthrow away half the signal, reducing headroom and S/N ratio by 6 dB. Thenotion that impedance matching and termination are required for modernaudio equipment and ordinary audio cabling is simply an old idea whose timehas long past.

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