tech stuff – Page 55 – Engineering Radio

Category 5e cable in AES/EBU applications

I love wire. I know, what a geeky thing to say, but it is true. For no reason that I can explain, I have always been fascinated with wire, cables, and electricity.

Category 5e and up cabling is amazing stuff. Designed for computer applications, it can fulfill a wide variety of roles in radio and television stations, mostly because of its high bandwidth capacity. Category 5e cabling has a 100 MHz bandwidth, category 6 bandwidth is 250 MHz, with 6a (augmented) being 500 MHz. AES/EBU audio uses ATM and requires from 4-26 MHz bandwidth, depending on the sample rate (the highest sample rate is 200 KHz). Category 5e cable has a minimum common mode balance of -60 dB, which makes it nearly impervious to RF, electrical noise, mutual interference, and other noise issues.

Further, each pair in category 5e or 6 cables has a different twist rate, to reduce cross-coupling between pairs.

Here is a chart of electrical characteristics for Cat 5e, Cat6, and Belden AES/EBU cable:

	Category 5e	Category 6	AES/EBU*
Impedance (ohms)	100 +/- 10%	100 +/- 10%	110 +/- 20%
Bandwidth (MHz)	100	250	52
DC resistance 1K Ft (ohms)	27.12	27.42	23.68
Capacitance per Ft (pF)	15	15	12
Velocity factor (%)	72	72	76
Common Mode Balance (dB)	60	63	30
NEXT	-44.3	-53.4	–
Configuration	UTP	UTP	SPT
Wire	Solid	Solid	Stranded
Gauge	24	23	24

* Belden 1800F

Specifications above are for Belden cabling, but are typical for high quality category cabling available from other sources as well.

Although the AES/EBU cable specifications call for 110 ohm impedance cable, that specification is pretty loose, calling for +/- 20%, which means 88 to 132 ohm cable will work well. Category 5e and 6 cable is 100 ohm impedance, +/- 10%, which translates to 90 to 110 ohms, nominal.

Category 5 and 6 cabling can also be used for analog audio, RS232 and RS485 applications. One area of caution, however, is for T-1 or fractional T-1 services. On the DS side (between the smart jack and the CSU), T-1 type service runs 3 volts peak to peak. That is much higher than AES/EBU or ethernet, which run 1 volt peak to peak. As a result, cables in this type application should be 22 gauge or higher to reduce emissions from the cable.

Shielded category cable is available in Cat5 and 5e. The shielding acts to reduce emissions from the cable in low noise environments. It can also act to reduce RF fields around the cable pairs, so long as the proper cable terminations for shielded cable (RJ-45 or more properly 8P2C connectors) are used and installed correctly. The shield must be connected to a ground on at least one end. I know a facility that has all shielded Cat5 cable, but they used standard RJ-45 connectors, so both ends of the shield are floating, which completely defeats the purpose of the shield.

25 pair category 5e cable is available for trunk cabling between studios and the technical operations center. For one studio project, I purchased pre-made cables with RJ-21 connectors on both ends. Those connectors were then plugged into KRONE LSA-PLUS blocks. Cable, connectors and blocks were all 100 ohm impedance, category 5 equipment. Since we did not have to strip any insulation or punch down any wires, we pulled and terminated the studio to rack room trunk cables for five air studios and three production rooms in one morning. This greatly sped up the studio build out process.

The studios and TOC use SAS 32KD (Sierra Audio Systems) audio routers and Rubicon SL consoles, so most of the audio is AES/EBU. There are, however, several analog audio sources that are included in this system, things like telephone caller audio, off air monitors, satellite feeds and remote broadcast sources.

This facility is located about 1 mile away from a 5 KW AM station on 850 KHz. Several concerned people commented on the possibility of RFI on the cabling. In the five years since that project was completed, there have been zero issues with the cabling or the audio quality.

One thing to consider in these installations is the length of the cabling and the sample rate being used across the network. The capacitance per foot is the deciding quality in cable lengths. This is because capacitance, which is the ability to store an electrical charge, will begin to distort the signal (turn it into a saw tooth waveform) in the cable if certain lengths are exceeded. A good way to calculate maximum cable runs is thus:

Most professional AES/EBU devices sample 24 bits per channel, if the sample rate is 48 KHz, the 24 bits x 48,000 Hz = 1,152,000 bits per second per channel. For stereo, as most applications will be, that is doubled to 2,304,000 bits per second, or 2.3 Mbps. There is some overhead in an AES/EBU signal, so, for arguments sake, we will call it 4 MHz.

In this facility, the sample rate is locked at 48 kHz by a master clock. The longest cable length is 145 feet, which adds (15 pF x 145 Ft) up to 2,175 pF capacitance. From the chart above, we know that Cat5e has a resistance of 27.42 ohms per 1000 feet, or 0.02742 ohms per foot. That works out to be 145 feet x 0.02742 ohms = 3.9759 ohms.

To calculate the capacitive reactance, the following formula is used:

X_c= -1/(2π FC)

Where X_c is the capacitive reactance, F is the frequency in Hz and C is the capacitance in Farads.

Therefore X_c = -1 / (2 x 3.1415 x 4,000,000 x 0.000000002) = -19.89 ohms.

The characteristic impedance of Cat5e and Cat6 is 100 ohms. The DC resistance is 3.97 ohms and the capacitive reactance is -19.89 ohms, make the circuit impedance of a properly terminated cable 145 foot cable 84.08 ohms.

The design formula for a low pass filter is thus:

f_c = 1/(2πRC)

Where f_c is the cutoff frequency, R is the resistance and C is the capacitance.

Therefore, f_c= 1/(2 x 3.1415 x 3.9759 ohms x 0.000000002 farads) = 20,014,958 Hz or 20 MHz.

Generally speaking, one should try to keep the capacitance below 2500 pF in a 10 Mbps circuit. Belden datatwist 1212 cable has a 4.0 dB insertion loss and a 23.0 dB return loss per 100 meters (328 feet) at 4 Mhz.

145 feet is well within the limits of this cable for AES/EBU applications.

Further, all cable circuits need to be properly terminated to reduce return loss. Using common impedance wiring blocks, connectors and terminations help keep return loss to a minimum. Stranded wire works better in applications where cabling may move. There are Cat5e and Cat6 stranded cables available.

As data transfer rates approach that of RF, ethernet, digital audio, and RF are going to seem more and more similar. 1000 Base T (1GBT) and 10000 Base T (10 GBT) networks are coming.

The Engineering Room

The rack room, engineering room, technical operations center, frame room, or whatever it is called, is a central part of any radio station studio facility. Normally found in a rack room are things like computer servers, switches and routers, structured cabling, telephone equipment, audio routing equipment, audio processing equipment, satellite receivers, wire termination blocks, microwave transmitters, and receivers, and sometimes even broadcast transmitters and phasors.

This is a rack room for one of the clients we service. This particular facility has 6 FM and 2 AM radio stations. Several stations share the same programming, however, so more accurately, there are four formats, thus four air studios, and four production studios. The station uses AudioVault 100 for its audio playback and automation. It is probably one of the few AV100 systems still in use.

Broadcast Electronics Audiovault 100 system

There are four servers and drive bays mounted in two racks, right next to the 5 KW AM transmitter. Doesn’t seem to be an issue.

TOC AM transmitters and Phasor — TOC AM transmitters and Gates Phasor

The studio is also co-located with one of the AM station’s transmitter sites, which always creates a special set of engineering problems. The antenna array consists of two 90-degree towers phased 210 degrees, which creates a figure eight pattern to the north and south. The studio building is in one of the main lobes of the antenna system. This means extra grounding is needed in the rack room and special attention needs to be paid to things like phone wiring and computer network cabling. This facility uses shielded Cat5E, which seems to take care of any stray RF. Of course, all the audio is balanced and properly terminated.

ATI 416 DA wiring connections — ATI DA 416 wiring connections

For audio distribution, ATI DA416 audio distribution amps are used. These are used to distribute the air signal, the program and audition audio, the satellite feeds, and remote broadcast feeds.

There are several considerations for well designed engineering rooms:

Future wiring work will be needed, there is no way that an engineer can plan for every contingency. It is difficult to plan one or two years ahead, much less five or ten. Keeping the wire conduits, raceways, trays, or troughs accessible is key to a happy existence. This can be done by using overhead trays or raised floors and good cable management.
Ground everything to a single-point ground bus. There is no such thing as too much grounding, so long as everything is bonded together. Be sure to include TELCO and service entrance grounds.
Have direct paths outside to accommodate the STL transmission line, satellite cabling, etc. If those antennas are located on the roof, have roof access in the rack room.
The environment directly impacts the life of the equipment. Keeping the rack room environmentally isolated from the rest of the facility is highly desirable. HVAC systems should be sized for the highest equipment load on the hottest day of the year. Having some type of backup air conditioning is also a good idea.
Leave plenty of room to work behind racks or on the wire wall. Cramped spaces create mistakes.
Make sure there is plenty of light to work, lack of light also creates mistakes.
Reserve some space for future growth. Extra room on the back wall for more punch blocks, and extra space for additional racks is always a good plan.
Keep the wiring neat and documented. There is nothing worse than an undocumented engineering room, it makes life difficult and in many cases, will eventually knock a station or two off the air when a wire gets snipped or pulled out.
Make the room secure. Keep the doors closed and keep unauthorized people out.

There are two conventions of thought when it comes to rack planning. The first is that all equipment should be mounted in close proximity. It is easier to run all the STL transmission lines to one rack, all the satellite cables to another, etc. Then there is the “rack assigned to a station” method, where each station has one rack with all of its processing, STL, EAS, and other equipment in it. I prefer the first method, as it makes the room look more uniform, your mileage may vary.

When done right, engineering rooms can be a great centerpiece to any facility. It is very impressive to take a client through the studio building and end up with “And this is the heart of the facility, all the radio stations run through this room…” Several larger facilities have glass walled rack rooms for just that purpose. It can be a positive attribute when everything is buttoned up.

The first radio station licensed to Albany, NY

Although not the first station in the area, that honor goes to WGY. In fact, RPI licensed WHAZ in 1923, which makes it the second regional station. Starting on 1430 Khz as WOKO in New York City in 1923, the station made a few stops along the way. One of those was on Mt. Beacon from 1928 until 1930. The original transmitter building is still there, although the tower was taken down in 2005 to make way for the DTV stations that moved in. I always wondered why an FM tower on the top of a mountain had a base insulator.

In 1930, WOKO was sold and moved to Albany, NY, becoming the first station licensed to that city. The transmitter site is located off of Kenwood Avenue in the town of Bethlehem, about 4 miles south of downtown Albany. It first signed on with 1 KW, increasing to 5 KW in 1947. This is the original transmitter site, but the towers were redone in the mid-1970s. The towers themselves are 130 electrical degrees (235 feet) tall. Like all AM stations, for years it serviced the community until it was gradually reduced to a satellite repeater, now owned by Disney.

The original transmitter building is in the back, the front was added in the 1970s when the studios and offices colocated with the transmitter. Prior to that, they were in downtown Albany.

Nautel XR6 Medium wave broadcast transmitter — Nautel XR6 medium wave broadcast transmitter

The Harris BC5H transmitter was replaced with a Nautel in 2006. The Harris AM H series transmitter has a pair of transistors on the audio driver board that were unique to that transmitter and no longer manufactured. There are no equivalent replacement parts. Once those transistors fail, the transmitter is done.

I really think that AM could make a comeback, but the following conditions need to be met:

Kill AM HD radio. Kill it dead.
Cut away the dead wood. Those stations that are not making money, have not made money and have no hope of ever turning a profit again. Most of these are owned by large consolidators that cannot yet afford to write off the bad investment. More and more will be spun off and given to MMTC and others. If they can make a go of it, good. If not, then the stations will go dark and eventually surrender their licenses. We have one like that around here that basically turns its transmitter on one day a year to avoid license forfeiture. That should stop, either they use it or lose it.
FM radio will continue to be the investment bank darling, in spite of lower and lower listeners and revenue. This will lead to more and more translators, HD radio, LPFM, and other things being shoehorned into an already crowded band, creating AM-like conditions on the FM band.
Those that can take on the challenge of an AM station should immediately begin looking at reducing maintenance costs. Directional antennas are money holes, if at all possible, get rid of the DA in favor of a single tower closer to town. Duplexing with another AM is a great way to save money and the costs of building a new tower. Using a taller tower, up to 190 electrical degrees, will daytime signal and reduce the radiation angle (vertical) of the tower, thus permitting better PSSA, PSRA, and or nighttime operation.
Local programming. Local sports, local politicians, local bands, local church services, local events, etc. Local.

But anyway…

The Bauer Transmitter

This is a Bauer FB-5000J transmitter, stashed away in the corner of a transmitter site.

Sorry I can’t get a better angle on it, as I said, it is stuck in the corner.

I don’t know what vintage it is, it seems to be from the early 1960s or so as it has a low serial number. It ran as the main transmitter until the Harris Gates BC5H was installed in 1976. The transmitter is in beautiful shape, almost a museum piece. I don’t know if it still has all its original iron, as the modulation transformer may have contained PCBs and been disposed of. Otherwise, it is complete and tuned to 1,460 kHz.

I think the owner might be willing to donate it to a reputable organization, preferably a 501(c)(3).