tech stuff – Page 14 – Engineering Radio

MPX over IP

In the progression from Circuit Switched Data to Packet Switched Data, I can think of many different applications for something like this:

The FMC01 MPX to IP encoder can be used for multi-point distribution (multi-frequency or same-frequency network) of FM Composite audio, or as a backup solution over a LAN bridge, LAN extension, or public network. I can think of several advantages of using this for a backup when composite analog STLs are in use. There are many compelling reasons to extend the LAN to the transmitter site these days; Transmitter control and monitoring, security cameras, office phone system extensions, internet access, backup audio, etc. I would think, any type of critical infrastructure (e.g. STL) over a wireless IP LAN extension should be over a licensed system. In the United States, the 3.6 GHz WLAN (802.11y) requires coordination and licensing, however, the way the rules are set up, the licensing process is greatly simplified over FCC Part 74 or 101 applications.

Another similar CODEC is the Sigmacom Broadcast EtherMPX.

Features include:
• Transparent Analog or Digital MPX (MPX over AES), or two discrete L/R channels (analog or AES).
• Built-in MPX SFN support with PTP sync (up to 6.000km in the basic version). No GPS receivers!
• Unicast or Multicast operation to feed an unlimited number of FM transmitters with MPX from one encoder.
• Linear uncompressed PCM 24-bit audio.
• Very low audio latency: 2,5mS in MPX mode.
• Perfect match with Sigmacom DDS-30 Exciter with Digital MPX input.
• Can be used with high-quality 802.11a/n Ethernet links.
• DC coupled, balanced Analog inputs & outputs with -130dBc noise floor.
• No modulation overshoots due to compression or AC capacitor coupling.
• Decoder provides simultaneous Analog & Digital output for transmitter redundancy.
• Aux RS232 serial transparent link, Studio to Transmitter.
• Auto switchover to Analog input when Digital signal is lost.
• Centralized remote control & management software

One last thought; separating the CODEC from the radio seems to be a good idea. It allows for greater flexibility and redundancy. Using an MPX-type STL allows sensitive air chain processing equipment to be installed at the studio instead of the transmitter site.

Transmitter Haiku

I wrote a little Haiku about Thanksgiving dinner:

Telephone rings
Old transmitter beckons
Dishes get cold

Not exactly the 5/7/5 of a traditional haiku, but close enough. This year, it was the nearly 30-year-old Broadcast Electronics FM35A at WEBE. A set of readings from the remote control reveal; zero forward power, zero plate current, and 12.8 KV plate voltage. My first assumption was some sort of drive issue; a failed exciter or IPA driver. After starting the backup transmitter and making sure that it was running stably, I spoke with the program director and told him we would be out the next morning.

Upon arriving at the transmitter site, I found the BE transmitter had no filament voltage. An obvious clue, I began working backward from the tube socket until I found this:

Broadcast Electronics FM35A filament voltage regulating transformer

This is the auto-transformer that regulates the filament voltage. Schematically, it is noted as T204 and it is in series with one side of the filament transformer. This one is burned open. The bad news; Broadcast Electronics does not stock this part, it is a special order item, the replacement part costs $2,800 dollars and it will take a few weeks to get here. The good news, after digging through our stock of old transmitter parts, I found an exact replacement:

Replacement part name plate, T204, BE FM35A — Replacement part nameplate, T204, BE FM35A

We will be installing it on Monday morning.

No food or drink in the studio

This is a rule that I always find difficult to enforce. Since switching into contracting mode, I am often at any particular studio once per week or less. It seems to me, no matter what signs are posted or what words are spoken, the DJ seems to hear; “It is okay to eat and drink in the studio.”

Of course, with that attitude, the inevitable is bound to happen:

RS-18 Millenium console on/off button membrane

To make things worse, this was spilled on the main mic on/off buttons. These button membranes come in groups of six and are not inexpensive. The complaint was “The main mic will not turn off.” Ah well, I am paid to fix things after all. The DJs are only inconveniencing themselves at this point.

Whatever happened to the CFA?

Remember way back when, perhaps in high school or college, you met this really cool person who seemed to be wonderful in every way? Yeah, then you got to know them a little better, and, well, those first impressions changed a little bit.

Crossed Field Antenna, Courtesy of Wikipedia

The Crossed Field Antenna (CFA) sort of reminds me of my first prom date. There was a lot of promise there, but plans fell through.

From a 1999 Radio World article:

Imagine an AM antenna one–fiftieth of a wavelength long, that needs no radial ground system, occupies a small parcel of land, produces little or no RFI (Radio Frequency Interference), has great bandwidth and performs better than a full–sized vertical radiator.

This potential new antenna was all the rage during the early 00s or whatever you call that decade. I remember thinking to myself; I will believe it when I see the test results. At one point, there was a battery of tests run in the installation in Egypt and China. The test results are spotty at best, however, none of these installations performed up to expectations. While it looks like a cool idea, and it would have been great to see it succeed, it seems that sheer willpower alone will not make a particular system work outside of the laws of physics. There are a few of these still in operation out in the wild, mostly in Egypt.