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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.

Another small market build out

Finishing up another studio build out in an unrated market. There are some engineers who think that small market work is beneath them. That is fine with me, I enjoy it.  Once again, creating a nice, functional, modern facility while not breaking the bank poses some challenges.   I like to take sort of a minimalist simple approach while not compromising good engineering practice.  Another challenge is rebuilding an existing facility.  Each studio needed to be demoed one at a time with the stations playing hop scotch from studio to studio around the work.  There were four studios total plus the rack room.  There were also several other renovations going on at the same time as this project.

Looking at the overall facility, the client decided that one studio would be the main room where multiple guests could be seated, etc.  The other rooms would have guest microphones, but they are smaller rooms and limited to one guest each.  The smaller rooms have AudioArts Air4 consoles while the main studio has an R-55e.

WZOZ console, main studio, Oneonta, N

WZOZ console, main studio (Studio A), Oneonta, NY

The main studio had existing studio furniture that was in reasonable shape so we decided to reuse it.  While we had the studio ripped apart, the paint and carpet where updated.  The main microphone is an Electrovoice RE-20, the guest mics are Heil PR-20UT which are inexpensive and have excellent characteristics for a dynamic microphone.  Since this faces a fairly busy street, I put in some very basic DBX 286S mic processors with a little bit of downward expansion.  Adobe Audition is used for production.  I have also used Audacity which is available in both Windows and Linux flavors.  Acoustical wall treatments are coming soon.

Main studio, Oneonta, NY

Main studio, Oneonta, NY

The counter tops in the smaller studios were traded out with a local kitchen supply company.  We used Middle Atlantic BRK-12-22 racks with castors on them to install a limited amount of rack equipment.  Each one of these studios is nearly identical; a AudioArts Air4 console with JBL powered monitors.  The microphones in these studios are Heil PR-20UT with console supplied mic preamps.  These studios are used for WSRK, WDOS, WBKT and WKXZ.  All studios are off line when in automation, which means each can be used for production and other purposes.

Studios B-E, Oneonta NY

Studios B, Oneonta NY

Studio C, (WDOS) Oneonta, NY

Studio C, (WDOS) Oneonta, NY

Studio D, (WSRK) Oneonta, NY

Studio D, (WSRK) Oneonta, NY.

We started the TOC from scratch. This area was occupied by a bunch of empty file cabinets previously. The original equipment racks where in Studio A.

A riser was installed from the racks straight up to the roof for the STL, monitor antenna and satellite dish transmission lines.  Everything is grounded with a star grounding system connected to the main building ground which consists of driven ground rods and the water main.  The STLs have Polyphaser IS-PT50HN lightning protection devices installed.

900 MHz lightning protectors on STL transmission lines

900 MHz lightning protectors on STL transmission lines

The racks are Middle Atlantic MRK 4031.  Since this building was built sometime in the mid 1800’s, the floors are a bit uneven (along with almost everything else), so a fair amount of shimming and leveling was needed to get these units bolted together.

Racks and equipment

Racks and equipment

Each rack has its own UPS in the bottom and they are all on separate breakers.

A manual transfer switch controls a dedicated electrical sub panel.  All of the racks and studios are powered from this sub panel.  Below the transfer switch is a NEMA L14-30 twist lock male receptacle for generator connections.

Studio/TOC sub panel and transfer switch

Studio/TOC sub panel and transfer switch

The total load about 18 amps.  The station is looking to trade out some generators for various transmitter sites.  I suggested that they get a couple of the portable Honda inverter generators, which are very good have excellent power regulation, frequency stabilization and fuel economy.

The existing Scott’s Studio 32 system was updated with new computers.  This is an interim step until a new automation system can be installed next year.  Each station has it’s own BT 8.2ss switcher which can select any studio to go on the air with.  That flexibility makes moving from studio to studio easy.  It also allows for all the stations to be simulcast, which is handy in the event of an emergency.

Punch blocks are mounted on plywood attached to the back wall.  We left extra space for a new phone system.

The EAS monitor assignments are met with roof top yagi antennas.  I like drawings and diagrams, as the saying goes, a picture is worth a thousand words.  This is an image I created on Google Maps using the transmitter site coordinates for each of the EAS monitoring assignments.  That gives me good local aiming points for the various antennas needed.

EAS monitor assignment headings

EAS monitor assignment headings

Other drawings include a floor plan and block diagrams for each station.  I have a Viso template that I use for these.  I find that having these diagrams on hand in a book is very helpful in the event that somebody else needs to go to this station to work on things.

Block diagram for WDOS, Oneonta, NY

Block diagram for WDOS, Oneonta, NY

Finally, the wiring documentation which shows where each wire originates and terminates. Again, if I am not available and somebody else needs to do work here, this is very helpful. All the studios are laid out the same, so figure out one and then the rest falls into place.

Screen shot of wire run spreadsheet

Screen shot of wire run spreadsheet

There is still a little bit of clean up left and some old equipment to get rid of.  Otherwise, it’s a wrap.

Gone and apparently forgotten

Can a 50,000 watt AM station disappear from the airwaves and no one notice?

The answer is yes, if you live in the Albany, NY area.  WDCD, 1540 KHz, (formerly WPTR) has surrendered its license to the FCC last Friday, September 28, 2018.  Seventy years on the air and quite the legacy as a Top-40 station in the 60’s and 70’s.

Unfortunately, the station had fallen on hard times the last few years, being silent twice for long stretches of time.  In the end, I suppose it was simply time to pull the plug.

This was my first CE gig in the early 1990’s.  What I remember was, I had a lot of fun working there.

So long and thanks for the memories.

 

Axiom


A pessimist sees the glass as half empty. An optimist sees the glass as half full. The engineer sees the glass as twice the size it needs to be.

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~1st amendment to the United States Constitution

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~Benjamin Franklin

The individual has always had to struggle to keep from being overwhelmed by the tribe. To be your own man is hard business. If you try it, you will be lonely often, and sometimes frightened. But no price is too high to pay for the privilege of owning yourself.
~Rudyard Kipling

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~Universal Declaration Of Human Rights, Article 19

...radio was discovered, and not invented, and that these frequencies and principles were always in existence long before man was aware of them. Therefore, no one owns them. They are there as free as sunlight, which is a higher frequency form of the same energy.
~Alan Weiner

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