Differential Audio

Most professional audio facilities use differential audio or balanced audio within their plants.  The main reason for this is noise rejection, which was discovered by the early pioneers of wired telephony back in the late 1800’s.  Balanced audio is created by generating two audio signals that are 180 degrees out of phase using either a transformer or an active device.  These are usually labelled High and Low, + and – or something similar.  Those two audio signals are then transmitted across some distance and recombined at the far end, again by a transformer or some active device.

Noise rejection, differential signaling. "DiffSignaling" by Linear77 - Own work. Licensed under CC BY 3.0 via Wikimedia
Noise rejection, differential signaling. “DiffSignaling” by Linear77 – Own work. Licensed under CC BY 3.0 via Wikimedia

When an interfering signal is picked up, it is transmitted along both sides of the balanced audio circuit until the signals are recombined.  During the re-combining process, and common mode interference is cancelled out, as it becomes 180 degrees out of phase with itself during the re-combining process.

Differential signaling is used in analog audio, digital audio (AES/EBU), HDMI, Display Port, USB, Ethernet, POTS lines, ISDN, T-1/DS-1, E-1, etc.   It is a fairly simple concept, but one of the basic building blocks in broadcast studios.

When a studio project was completed at a disused studio/transmitter site location, a certain amount of RFI was being induced on the studio microphones by the unassociated FM transmitter in the next room.  The problem with microphone level audio is the relatively low level of  microphone output, which requires a good deal of amplification.  The amplifiers in this console have active balanced inputs, which might not be exactly 180 degrees out of phase.  In this installation, microphone level audio was run about 20-25 feet on standard microphone cable then it was converted to Cat 6 cable before going into the console.  It may have been better to use the shielded Cat 6 cable for the longer runs as it likely has better common mode rejection than standard mic cable. Another option might have been Star Quad cable.  However, none of those things were done.

Western Electric was the manufacturing arm of Bell Telephone.  In their day, they made some really good equipment.  One such piece is the WE-111C repeat coil.  These can be configured for either 600/600 ohms, 600/150 ohms, 150/150 ohms,  or 300/300/300/300 ohms impedance ratios.  Since this is microphone level audio 150/150 ohms is the appropriate setting.

WE 111 repeat coil, one of the best such transformers ever made

Over the years, I have found many of these transformers discarded at various transmitter sites and studios. There are five microphones feeding this console. I mounted five of these coils in a sturdy metal enclosure and wired them with RJ-45 jacks to be compatible with the Studio Hub wiring equipment used in this studio installation.  I also grounded each unit to a piece of copper strap, which is connected to a grounding lug on the side of the unit.

Western Electric 111C repeat coils mounted in box
Western Electric 111C repeat coils mounted in box

I swept the coils from 20Hz to 20kHz:

WE 111C coils, 20Hz sweep
WE 111C coils, 20Hz sweep
WE 111C coil 20kHz sweep
WE 111C coil 20kHz sweep

This shows a 0.4 dB difference from 20 to 20,000 Hertz, thus they are all nearly flat which is a pretty cool feat of engineering.  I would estimate the age of these transformers is between 50 to 60 years old.

These coils isolate each microphone from the microphone preamp in the console.  This completely eliminated the FM RFI and solved the problem.

The Answer to Ailing Copper

I don’t know how things are in your neck of the woods, but here in the Northeastern US, our old copper TELCO networks are on their way out.  This is a problem for broadcasters who still rely on POTS lines (Plain Old Telephone Service) for transmitter remote controls, studio hot lines, etc.  The vast majority of my transmitter site access is through dial up remote controls.  There are a few locations that have web based remote controls, but to be honest; the phone part of my smart phone still gets a lot of use.  There are several locations where the old copper is just failing outright and not through a lack of effort by the repair techs.  Generally, the copper pairs get wet and develop a loud hum, which makes the remote control unit either hang up or become unresponsive to touch tone commands.

The best course of action is to get some type of VOIP line installed.  Here is the rub; many transmitter sites are nowhere near a cable system.  Several times, I have contacted the cable company to see if they will provide a VOIP phone line at a certain site.  The response is usually; sure, we can do that!  However, it will cost  you (insert some ridiculous amount of money) to extend the cable to your transmitter site.

LAN extensions to the transmitter site are a useful for a number of reasons.  More and more transmitters are equipped with web interfaces as are processors, UPSs, transmitter remote controls, security cameras,  etc.  LAN extensions can also be used for backup audio in case of STL failure.  Finally,  an inexpensive ATA (Analog Telephone Adaptor) and DID line can replace a POTS line for a lot less money.  One example; voip.ms has the following plans as of this writing:

Plan type Per month per DID number (USD) Incoming call rate (USD) per minute Outgoing call rate (USD) per minute
Per minute $0.85 $0.01 (USA) $0.009
Unlimited $4.25 $0.00 $0.009
Toll Free (800) $0.99 $0.019 $0.009

Any of those plans surely beats the standard TELCO rate of $40-50 per month per line.

Design criteria for a wireless LAN system needs to take into account bandwidth, latency and reliability.  Each VOIP phone call takes anywhere from 28-87 Kbps depending on the protocol being used.  If the wireless LAN is being used for other things such as back up STL service, access to various GUI’s, etc then the total bandwidth of all those services need to be considered as well.  Do not forget ethernet broadcast traffic such as DHCP requests, ARP, SMB, etc which can also take up a fair amount of bandwidth.

For LAN extensions, I have been using a variety of equipment.  The older Moseley 900 MHz LAN links still work, but are slow in general.  The Ubiquiti gear has proven to be both inexpensive yet reliable, a rarity to be sure.  There are several links to various transmitter sites running on various types of Ubiquiti gear, usually without problem.  One simply needs to remember to log into the web interface once in a while and make sure that both ends have all the firmware updates installed.  They are cheap enough that a couple of spares can be kept on the shelf.

The following diagram shows how I replaced all of the copper pots lines at various transmitter sites with VOIP:

Diagram of LAN extensions to various transmitter sites
Diagram of LAN extensions to various transmitter sites

List of equipment:

Nomenclature Amount Use New or used
Ubiquiti Rocket M5 3 AP and station units New
Ubiquiti AirMax 5G-2090 90 degree sector antenna 1 AP point to multi-point antenna New
Ubiquiti Rocket Dish 5G-30 2 Station antennas New
Ubiquiti ETH-SP-G2 3 Lightning protection New
Trastector ALPU PTP INJ 6 Lightning protection out door units New
Cambium PTP-250 2 Point to Point link Existing/Used
Motorola Canopy 900DA PCDD 1 AP point to multi point Existing/Used
Motorola Canopy 900DA PCDD 2 Station Existing/Used
Microwave Filter #18486 diplexer 3 Diplexer 900 MHz ISM band and 944-952 STL band Existing/Used
Cisco SPA122 ATA 9 Dial tone for remote controls New

The main studio location has the gateway to the outside world. This system is on a separate subnet from the automation and office networks. From that location a point-to-multipoint system connects to the three closest transmitter sites.  This setup uses the Ubiquiti Rocket M5’s with various antenna configurations.  Then, from one FM transmitter site, there is an existing 5.8 GHz path to another set of transmitter sites.  This uses Cambium PTP-250s.

The next hop rides on the STL system, using Motorola Canopy 900 MHz radios and Microwave Filter Company #18486 dilpexers.  These are long paths and the 900 MHz systems work well enough for this purpose.  The main cost savings comes from reusing the existing STL system antennas which negates the cost of tower crews to put up new antennas and or rent on the tower for another antenna.

There is a smaller sub system many miles away that is connected to the outside world through the cable company at the AM transmitter site.  Unfortunately, due to the distances between the main studio and those three stations, there was no line of site shots to these sites available on any frequency.

When installing the 5.8 GHz systems, I made sure to use the UV rated, shielded cable, shielded RJ-45 connectors and Lightning Protection Units (LPUs).  Short cuts taken when installing this equipment eventually come back in the form of downed links and radio heads destroyed by lightning.

Regardless, I was able to eliminate seven POTS phone lines plus extended dial tone service to two sites that previously did not have it before.  In addition to that, all of the transmitter sites now have Internet access, which can be useful for other reasons.  All in all, the cost savings is about $310.00 per month or $3,720.00 per year.

A tale of five signals

I am currently finishing an interesting project involving putting up two translators on a diplexed AM tower which also holds a mobile phone/data tenant as well.  All-in-all, this seems to be a very efficient use of vertical real estate.

WMML WENU tower, Glens Falls, NY
WMML WENU tower, Glens Falls, NY

The AM stations are WMML and WENU in Glens Falls, NY.  The AM stations are diplexed using a Phasetek diplexor/ATU.

Diagram showing WENU/WMML tower with W250CC/W245DA antenna installed
Diagram showing WENU/WMML tower with W250CC/W245DA antenna installed
Diplexor diagram, WENU/WMML Glens Falls, NY
Diplexor diagram, WENU/WMML Glens Falls, NY

The translators are W250CC and W245DA which are using a NICOM BKG-77/2 two bay 3/4 wave spaced antenna mounted at 53 meters AGL.  The translators use a Shively 2640-04/2 filter/diplexor which as a broad band input port in addition to the translator input ports.  Since these translator signals are only 1 MHz apart, the higher power Shively filter was installed because it has better rejection characteristics.  The broadband input port allows the NICOM antenna to be used as a back up for any of the three FM stations; WKBE 107.1, WNYQ 101.7, or WFFG 100.3.  Two transmitter sites for those stations are mountain top locations which are very difficult to get to in the winter time.  Having a backup site available takes some of the pressure off during storms or other emergencies.

Shively 2640 -04/2 filter for W250CC and W245DA

The NICOM FM antenna was mounted on the tower when W250CC went on the air in October of 2016.  When it was installed, the base impedances for both AM stations were measured.  For some reason, WENU 1410 KHz seems to be more sensitive to any changes on the tower, thus the WENU ATU needed a slight touch up.  When working on diplexed AM systems, it is also important to make sure that both trap filters, which are parallel resonant LC circuits, are tuned for maximum rejection of the other signal.  During this particular installation, nothing was added to the tower and no change in the base impedance for either station was noted.

Shively Filter, connected to transmitters and antenna
Shively Filter, connected to transmitters and antenna

As a condition of the construction permit, measurement of spurious emissions of all stations sharing the common antenna needed to be completed to ensure compliance with FCC 73.317(b) and 73.317(d).  I made careful measurements of the potential intermod products between the two translator frequencies.  This measurement was completed with my TTI PSA6005 spectrum analyzer.

The primary concern here is mixing products between the two transmitters. Both transmitter are BW TXT-600 with low pass filters before the output connector. There are three frequencies of interest;

  1. (F1 – F2) + F1 or (97.9 MHz – 96.9 MHz ) + 97.9 MHz = 98.9 MHz
  2. F2 – (F1 – F2) or 96.9 MHz – (97.9 MHz – 96.9 MHz) = 95.9 MHz
  3. F2 + F1 or 97.9 MHz + 96.9 MHz = 194.8 MHz

That, plus harmonic measurements out to seven or eight harmonics of the fundamental frequency should be enough to demonstrate compliance with FCC out of band emissions standards. Being that this site has LTE carriers, it is very important to measure the harmonics in those bands. Mobil data systems often use receiver pre-amps, which can amplify harmonics from the FM band and make them look out of compliance. Having a base set of reading to fall back on is always the best course in case the “out of tolerance” condition gets report to the FCC.

Measurements on these frequencies must meet the emissions standards outlined in FCC 73.317 (d), which states:

Any emission appearing on a frequency removed from the carrier by more than 600 kHz must be attenuated at least 43 + 10 Log10 (Power, in watts) dB below the level of the unmodulated carrier, or 80 dB, whichever is the lesser attenuation.

Harmonic frequencies to be measured:

Harmonics for 96.9 MHz fundamental Harmonics for 97.9 MHz fundamental Comments
193.8 195.8
290.7 293.7
387.6 391.6
484.5 489.5
581.4 587.4
678.3* 685.3* US LTE Band 71
775.2* 783.2* US LTE Band 5
872.1* 881.1* US LTE Band 5
969.0 979.0

*Frequencies that fall within the mobile data LTE bands. Traces where recorded and saved for these frequencies.

All of that information, once compiled is attached to the FCC form 350-FM, which, once filed grants Program Test Authority.

BW TXT-600 V2 translator transmitters
BW TXT-600 V2 translator transmitters under test and measurement

Installing a satellite dish

This is a replacement dish for the Comtech dish destroyed in a downburst event a few weeks ago.  The first part of the job entailed placement of the new dish down on the ground.  The town code enforcement officer was much happier with this idea than mounting it up above roof level along back the building as the old one was.  Of course, this is possible due to the shift in satellites last year to AMC-18.

Finding a good spot on the radio station property was fairly easy.  The studio is located in a business district, thus the side yard requirements where zero feet, which is great.  The building inspector required that we dig a test hole to see what type of soil was there.  It turned out to be fill.  That required the footing design be changed somewhat and stamped by a licensed engineer.  Not a major problem.

Satellite mount pole, waiting pre-pour inspection
Satellite mount pole, waiting pre-pour inspection

The footing is 36 inches wide by 7 feet deep.

A little bit of water in the bottom of the hole
A little bit of water in the bottom of the hole

The mounting pipe has flanges welded to the side of it to prevent it from spinning in the concrete.

Footing poured and cured
Footing poured and cured

After the pour, we let the concrete set up over the weekend.

New dish bolted together
New dish bolted together

The dish is assembled and waiting for lift.  We used a back hoe to lift the dish onto the mounting pole, unfortunately, I was not able to take a picture as I was on a ladder attaching the dish to the pedestal with U-bolts.

Viking 1374-990 3.7 Meter R/O dish installed
Viking 1374-990 3.7 Meter R/O dish installed

Here it is installed and aimed at AMC-18. I used the Satellite Buddy, which makes the aiming job much easier. Once the signal is acquired, I like to peak the Eb/No on the West Wood One carrier, which seems to be the most sensitive to any type of change.

Viking 1374-990 3.7 Meter satellite dish, back view
Viking 1374-990 3.7 Meter satellite dish, back view