A QUESTION OF BALANCE

ARTICLE

A QUESTION OF BALANCE
Voltage, Current, Video, and Cables

by Peter H. Putman, CTS

Video. Coaxial cable. The two seem to go together like soup and sandwich. In fact, we've been using coaxial cable with video signals for so long that it seems both made their appearance at the same time. You can just imagine the big black cables snaking away from huge image orthicon TV cameras, covering a day baseball game in the late 1930s.

Well, coaxial cable may be the most expedient way to move video (and audio) signals around, but it ain't necessarily the best, nor is it the only way! To understand why, let's step back a little bit and review some basic electrical physics.

BACK AND FORTH

Every analog audiovisual signal we pump through an installation - audio, video, computer - is essentially an alternating current (AC) signal. The frequency may vary, as will the intensity. But aside from those differences, what we're doing is switching and moving a lot of AC voltages! By using tricks such as bandpass/band reject filters, we're able to send more than one of those signals at a time.

Stop and think about this for a moment: An AC signal is really two different DC signals with opposite phases. For one half of the cycle, half the voltage is present. In the other half of the cycle, the rest of the voltage is present. And so we go along, swinging merrily from "peak to peak" from source to load.

To move a DC signal, you really don't need anything more than a pair of wires - one for the positive (+) side, and one for the negative (-) side. Since an AC signal is really an oscillating DC signal, it isn't any more demanding about cabling than its direct-current cousin - two wires are fine. And that's how RF energy was first handled back in the early days of the century - using a pair of wires spaced a precise distance apart to couple energy from a transmitter to an antenna.

Open wire transmission is indeed the lowest-loss method of moving AC signals from point to point. For many years, TV sets used 300-ohm ribbon wire or 450-ohm ladder line to connect antennas to TV sets. In this world of cable television, that's hard to believe! But open-wire line is still the best way to move RF signals, since the lowest-loss dielectric available is still air. There are, however, the usual caveats:

First, the cable can't get excessively wet or covered with snow, ice, or semi-conducting fluids. Next, it can't rest on or come in close proximity to metal objects, such as gutters, flashing, windows, ducting, screens, and pipes. Finally, open-wire line doesn't like to be bent at severe angles - gentle, long-radius curves are all it can tolerate.

It's easy to see why coaxial cable came into existence. We run cables alongside plenty of metal objects, often burying them in hostile environments. We put all kinds of twists and turns into cable (not to mention splices) and even tie it into loops. That kind of path is a nightmare to negotiate with ladder line.

But there's a problem. The AC voltages in coaxial cable want to travel as if they are still flying along open-wire transmission lines. Electrically, that's not a big issue - one leg flows in the center conductor, and the other flows in the outer shield. The problem occurs when these AC signals arrive at their "load", which could be a speaker, antenna, resistor, splitter, or an amplifier, and one side is grounded. Most AC loads are designed to function as balanced components - most antennas use a dipole radiator as the driven element.

Connecting that unbalanced, grounded transmission line (coaxial cable) directly to a balanced load can result in all kinds of problems, such as currents flowing back down the shield to the source. These stray currents create standing-wave problems, resulting in a mismatch from the cable to the load. The result can be ringing or ghosting, not to mention unwanted radiation of the signal from the coaxial cable which is now working as part of the load.

ON THE LEVEL

The solution is to use a small transformer to convert from an unbalanced to a balanced signal, and vice-versa. This transformer is known as a balun, and there are literally millions of them in operation around the world every day, matching everything from telephone lines to transmitters. Baluns are used both to sort out the flow of AC signals and make the necessary impedance transformation between coaxial cable, which has a low impedance, and balanced loads, which have higher impedances.

With a balun, I can now send AC signals of any kind over a pair of wires and make the necessary transformations at either end to effect a clean transfer of AC signals. Want to connect a 200-ohm balanced load to a 50-ohm cable? Get a 4:1 balun. 75 ohm coax to 300 ohm twin lead (a common CATV connection twenty years ago) also requires a 4:1 balun. While this is a very common turns ratio, you can also wind 2:1, 6:1, 8:1 and even 10:1 baluns if necessary.

At RF frequencies, the use of a balun is imperative for proper matching of transmitters and antennas. If not, unwanted current flow and mismatches can actually increase interference to adjacent electronic equipment through stray coupling into power lines, audio cables, and antennas. But what about baseband video? If everything in the AV installation uses BNC or RCA jacks, what's the big problem?

Well, here's a scenario you may come across. Let's say you've been asked to retrofit an existing training room with video and audio playback. However, running new coaxial lines will be a tedious, costly job requiring structural modifications. After some technical "nosing around", you discover there are a considerable number of unused "twisted-pair" lines already in place, such as Cat 3 or Cat 5 structured UTP cable. Great - what good are they?

With the use of a balun, those seemingly superfluous wires can be converted into open-wire transmission lines to carry audio and baseband video from one point to another with little difficulty. As long as the pair of wires runs straight without encountering any splices or interruptions, you're in business. Simply add another balun at the destination to convert the signals back to regular coaxial cable.

The point to understand here is that if you have achieved a true balanced/unbalanced transformation, your video signal won't even notice. The balun prevents an impedance "bump" from occurring at the point of transition, although there will be a little signal loss in the transformer. This can be minimized by using high-efficiency, high"Q" cores made from advanced ferrites and ferrite composites.

Got audio as well? You can mix them with RF-modulated video signals on the same line by using different windings on the balun. For a 600-ohm audio pair, the transformation might need to be 4:1 or 6:1. Special baluns with isolated windings act like isolation transformers to eliminate the possibility of common-mode current flows - also referred to as "ground loops" which result in hum and rolling interference bars.

By adding inductors and capacitors to the balun, we can multiplex RF-modulated video and audio on the same line pair using low-pass, high-pass and bandpass filters to "sift out" the RF from the audio. This fifty-year-old trick was (and still is) used to couple RF from AM radio transmitters into AC power lines and use those lines as low-efficiency, limited radiation broadcast antennas in college dormitories.

But what about the problem with bending and close-coupling of open-wire lines? The cause is a phenomenon known as "skin effect". As AC signals increase in frequency, they have an increased tendency to move along the surface or "skin" of a conductor, instead of through it. This is why signal attenuation is so much higher for a given piece of coaxial cable than comparable open-wire line at RF frequencies - the coaxial dielectric is absorbing more and more of the energy, leading to the use of pure air dielectrics at microwave frequencies.

With baseband video, "skin effect" is much less pronounced, although there is still some signal loss due to capacitive and inductive coupling into adjacent wires. In addition, you will see greater signal attenuation using a balun to match 4 MHz baseband video into Cat 3 or Cat 5 wire than if you coupled into pure open-wire transmission line with an air dielectric, or even conventional RG-59/RG-6 coaxial cable.

How much signal loss? That will be largely dependent on the quality of the wiring and the distance of the run, although one manufacturer figures about 6.6 dB per 1000 feet @ 1 MHz for 24-gauge Cat 3 wire. Contrast this number with Belden 8281, a workhorse RG-59/U video cable that exhibits 2.5 dB loss per 1000 feet @ 1 MHz., or Belden 9085, a 300-ohm open-wire transmission line that attenuates 1.4 db for every 100 feet - but at 100 MHz!

Theoretically, you could run RF-modulated video at higher frequencies down Cat 3 or Cat 5 wire, but you would have to expect great losses at the receving end, not to mention "ghosting" (signal cancellation due to phasing and standing-wave errors) and high-frequency rolloff. So baseband video is the best way to go with a "hybrid" coax-to-structured wiring system.

BALUNS TO GO

One company is already offering products for coax-to-Cat3/5 transitions. Intelix of Middleton, WI has four different balun products available that require nothing more than plugging them in. Since baluns are passive devices, they don't require any electrical power to operate which means you can put them in out-of-the-way locations.

Intelix' V1 is perhaps the simplest product, looking like a humped box with a BNC plug on one end and RJ45 jack on the other end. It's used for connection of one composite video signal (two required for a full transition). A companion product, the V3 Video Balun, accepts a RGsB component signal and converts it to an RJ45 jack. For multiplexing, Intelix offers the V2A2 A/V balun for two-way transmission of composite video and audio signals, while the V1A2 combines a single BNC composite plug jack and two RCA jacks for stereo audio. Again, both baluns convert to RJ45 terminations.

According to Intelix' specifications, Cat3/5 wire pairs have a characteristic impedance of 100 ohms @ 1 Mhz. That would require a 6:1 balun for audio and about a 1.5:1 match for video, although a straight 75 ohm-to100 ohm connection will result in only 4% reflected power coming back at the video source. Intelix also claims a bandwidth of 10 MHz for their baluns and audio frequency response of 20 Hz to 22 KHz.

How about signal attenuation? Under ideal circumstances, Intelix advises a maximum cable run of 1300 feet for sending color composite video down Cat 3 wire and 2000 feet down Cat 5. Black and white signals (and audio) can safely be strung as far as 2000 feet on Cat 3 pairs and 2500 feet on Cat 5. However, things become a little more restricted for RGsB signals, which can only be sent about 300 feet along Cat 3 pairs and 500 feet on Cat 5. (It would appear that Y/C video could be ported along two of the three inputs on a V3 Video Balun, but Intelix provides no information on such a connection.)

Is it worth using Cat3 and Cat 5 cable exclusively for video wiring? That approach sure would make cabling a lot simpler, but if you had a choice it would be prudent to stick with conventional coaxial cable and plenum wire whenever possible. Still, one advantage of a balun would be in a temporary hook-up, such as running a decoded video conference signal to multiple classrooms.

Video from desktop PCs that is scan-converted to a composite signal could also be sent along structured lines, an approach that might come in handy in media classrooms where wire pulls are already in place and can't be enlarged. You could also feed video from a remote camera back to a video recorder or server via structured wire.

The truth is, if you have any constant-impedance twisted-pair wire - even older plenum cable - and you know the characteristic impedance (available from the cable manufacturer), you can wind a balun to match it to coaxial cable and use that twisted-pair as a transmission line for baseband video. As we saw earlier, AC signals don't care what they travel along if we keep the transitions orderly and smooth. If the signal loss figures are acceptable, feeding video through a balun to structured wiring might just be your ticket for a retrofit installation.

Copyright ©1999 Peter H. Putman/Primedia Intertec. This article originally appeared in the May 1999 issue of Sound & Video Contractor.


						ARTICLE
					A QUESTION OF BALANCE Voltage, Current, Video, and Cables


					by Peter H. Putman, CTS Video. Coaxial cable. The two seem to go together like soup and sandwich. In fact, we've been using coaxial cable with video signals for so long that it seems both made their appearance at the same time. You can just imagine the big black cables snaking away from huge image orthicon TV cameras, covering a day baseball game in the late 1930s. Well, coaxial cable may be the most expedient way to move video (and audio) signals around, but it ain't necessarily the best, nor is it the only way! To understand why, let's step back a little bit and review some basic electrical physics. BACK AND FORTH Every analog audiovisual signal we pump through an installation - audio, video, computer - is essentially an alternating current (AC) signal. The frequency may vary, as will the intensity. But aside from those differences, what we're doing is switching and moving a lot of AC voltages! By using tricks such as bandpass/band reject filters, we're able to send more than one of those signals at a time. Stop and think about this for a moment: An AC signal is really two different DC signals with opposite phases. For one half of the cycle, half the voltage is present. In the other half of the cycle, the rest of the voltage is present. And so we go along, swinging merrily from "peak to peak" from source to load. To move a DC signal, you really don't need anything more than a pair of wires - one for the positive (+) side, and one for the negative (-) side. Since an AC signal is really an oscillating DC signal, it isn't any more demanding about cabling than its direct-current cousin - two wires are fine. And that's how RF energy was first handled back in the early days of the century - using a pair of wires spaced a precise distance apart to couple energy from a transmitter to an antenna. Open wire transmission is indeed the lowest-loss method of moving AC signals from point to point. For many years, TV sets used 300-ohm ribbon wire or 450-ohm ladder line to connect antennas to TV sets. In this world of cable television, that's hard to believe! But open-wire line is still the best way to move RF signals, since the lowest-loss dielectric available is still air. There are, however, the usual caveats: First, the cable can't get excessively wet or covered with snow, ice, or semi-conducting fluids. Next, it can't rest on or come in close proximity to metal objects, such as gutters, flashing, windows, ducting, screens, and pipes. Finally, open-wire line doesn't like to be bent at severe angles - gentle, long-radius curves are all it can tolerate. It's easy to see why coaxial cable came into existence. We run cables alongside plenty of metal objects, often burying them in hostile environments. We put all kinds of twists and turns into cable (not to mention splices) and even tie it into loops. That kind of path is a nightmare to negotiate with ladder line. But there's a problem. The AC voltages in coaxial cable want to travel as if they are still flying along open-wire transmission lines. Electrically, that's not a big issue - one leg flows in the center conductor, and the other flows in the outer shield. The problem occurs when these AC signals arrive at their "load", which could be a speaker, antenna, resistor, splitter, or an amplifier, and one side is grounded. Most AC loads are designed to function as balanced components - most antennas use a dipole radiator as the driven element. Connecting that unbalanced, grounded transmission line (coaxial cable) directly to a balanced load can result in all kinds of problems, such as currents flowing back down the shield to the source. These stray currents create standing-wave problems, resulting in a mismatch from the cable to the load. The result can be ringing or ghosting, not to mention unwanted radiation of the signal from the coaxial cable which is now working as part of the load. ON THE LEVEL The solution is to use a small transformer to convert from an unbalanced to a balanced signal, and vice-versa. This transformer is known as a balun, and there are literally millions of them in operation around the world every day, matching everything from telephone lines to transmitters. Baluns are used both to sort out the flow of AC signals and make the necessary impedance transformation between coaxial cable, which has a low impedance, and balanced loads, which have higher impedances. With a balun, I can now send AC signals of any kind over a pair of wires and make the necessary transformations at either end to effect a clean transfer of AC signals. Want to connect a 200-ohm balanced load to a 50-ohm cable? Get a 4:1 balun. 75 ohm coax to 300 ohm twin lead (a common CATV connection twenty years ago) also requires a 4:1 balun. While this is a very common turns ratio, you can also wind 2:1, 6:1, 8:1 and even 10:1 baluns if necessary. At RF frequencies, the use of a balun is imperative for proper matching of transmitters and antennas. If not, unwanted current flow and mismatches can actually increase interference to adjacent electronic equipment through stray coupling into power lines, audio cables, and antennas. But what about baseband video? If everything in the AV installation uses BNC or RCA jacks, what's the big problem? Well, here's a scenario you may come across. Let's say you've been asked to retrofit an existing training room with video and audio playback. However, running new coaxial lines will be a tedious, costly job requiring structural modifications. After some technical "nosing around", you discover there are a considerable number of unused "twisted-pair" lines already in place, such as Cat 3 or Cat 5 structured UTP cable. Great - what good are they? With the use of a balun, those seemingly superfluous wires can be converted into open-wire transmission lines to carry audio and baseband video from one point to another with little difficulty. As long as the pair of wires runs straight without encountering any splices or interruptions, you're in business. Simply add another balun at the destination to convert the signals back to regular coaxial cable. The point to understand here is that if you have achieved a true balanced/unbalanced transformation, your video signal won't even notice. The balun prevents an impedance "bump" from occurring at the point of transition, although there will be a little signal loss in the transformer. This can be minimized by using high-efficiency, high"Q" cores made from advanced ferrites and ferrite composites. Got audio as well? You can mix them with RF-modulated video signals on the same line by using different windings on the balun. For a 600-ohm audio pair, the transformation might need to be 4:1 or 6:1. Special baluns with isolated windings act like isolation transformers to eliminate the possibility of common-mode current flows - also referred to as "ground loops" which result in hum and rolling interference bars. By adding inductors and capacitors to the balun, we can multiplex RF-modulated video and audio on the same line pair using low-pass, high-pass and bandpass filters to "sift out" the RF from the audio. This fifty-year-old trick was (and still is) used to couple RF from AM radio transmitters into AC power lines and use those lines as low-efficiency, limited radiation broadcast antennas in college dormitories. But what about the problem with bending and close-coupling of open-wire lines? The cause is a phenomenon known as "skin effect". As AC signals increase in frequency, they have an increased tendency to move along the surface or "skin" of a conductor, instead of through it. This is why signal attenuation is so much higher for a given piece of coaxial cable than comparable open-wire line at RF frequencies - the coaxial dielectric is absorbing more and more of the energy, leading to the use of pure air dielectrics at microwave frequencies. With baseband video, "skin effect" is much less pronounced, although there is still some signal loss due to capacitive and inductive coupling into adjacent wires. In addition, you will see greater signal attenuation using a balun to match 4 MHz baseband video into Cat 3 or Cat 5 wire than if you coupled into pure open-wire transmission line with an air dielectric, or even conventional RG-59/RG-6 coaxial cable. How much signal loss? That will be largely dependent on the quality of the wiring and the distance of the run, although one manufacturer figures about 6.6 dB per 1000 feet @ 1 MHz for 24-gauge Cat 3 wire. Contrast this number with Belden 8281, a workhorse RG-59/U video cable that exhibits 2.5 dB loss per 1000 feet @ 1 MHz., or Belden 9085, a 300-ohm open-wire transmission line that attenuates 1.4 db for every 100 feet - but at 100 MHz! Theoretically, you could run RF-modulated video at higher frequencies down Cat 3 or Cat 5 wire, but you would have to expect great losses at the receving end, not to mention "ghosting" (signal cancellation due to phasing and standing-wave errors) and high-frequency rolloff. So baseband video is the best way to go with a "hybrid" coax-to-structured wiring system. BALUNS TO GO One company is already offering products for coax-to-Cat3/5 transitions. Intelix of Middleton, WI has four different balun products available that require nothing more than plugging them in. Since baluns are passive devices, they don't require any electrical power to operate which means you can put them in out-of-the-way locations. Intelix' V1 is perhaps the simplest product, looking like a humped box with a BNC plug on one end and RJ45 jack on the other end. It's used for connection of one composite video signal (two required for a full transition). A companion product, the V3 Video Balun, accepts a RGsB component signal and converts it to an RJ45 jack. For multiplexing, Intelix offers the V2A2 A/V balun for two-way transmission of composite video and audio signals, while the V1A2 combines a single BNC composite plug jack and two RCA jacks for stereo audio. Again, both baluns convert to RJ45 terminations. According to Intelix' specifications, Cat3/5 wire pairs have a characteristic impedance of 100 ohms @ 1 Mhz. That would require a 6:1 balun for audio and about a 1.5:1 match for video, although a straight 75 ohm-to100 ohm connection will result in only 4% reflected power coming back at the video source. Intelix also claims a bandwidth of 10 MHz for their baluns and audio frequency response of 20 Hz to 22 KHz. How about signal attenuation? Under ideal circumstances, Intelix advises a maximum cable run of 1300 feet for sending color composite video down Cat 3 wire and 2000 feet down Cat 5. Black and white signals (and audio) can safely be strung as far as 2000 feet on Cat 3 pairs and 2500 feet on Cat 5. However, things become a little more restricted for RGsB signals, which can only be sent about 300 feet along Cat 3 pairs and 500 feet on Cat 5. (It would appear that Y/C video could be ported along two of the three inputs on a V3 Video Balun, but Intelix provides no information on such a connection.) Is it worth using Cat3 and Cat 5 cable exclusively for video wiring? That approach sure would make cabling a lot simpler, but if you had a choice it would be prudent to stick with conventional coaxial cable and plenum wire whenever possible. Still, one advantage of a balun would be in a temporary hook-up, such as running a decoded video conference signal to multiple classrooms. Video from desktop PCs that is scan-converted to a composite signal could also be sent along structured lines, an approach that might come in handy in media classrooms where wire pulls are already in place and can't be enlarged. You could also feed video from a remote camera back to a video recorder or server via structured wire. The truth is, if you have any constant-impedance twisted-pair wire - even older plenum cable - and you know the characteristic impedance (available from the cable manufacturer), you can wind a balun to match it to coaxial cable and use that twisted-pair as a transmission line for baseband video. As we saw earlier, AC signals don't care what they travel along if we keep the transitions orderly and smooth. If the signal loss figures are acceptable, feeding video through a balun to structured wiring might just be your ticket for a retrofit installation. Copyright ©1999 Peter H. Putman/Primedia Intertec. This article originally appeared in the May 1999 issue of Sound & Video Contractor.