STL and TSL

The Studio-to-Transmitter Link (STL) is what allows the tower to be in one place and the radio studio in another. In the early days of radio, the music, the DJ, the transmitter, antenna and tower all shared the same real estate. This was a big step forward as it allowed two things.
1. Ideal rather than compromised tower placement
2. Clusters with shared towers and studios.

So you gotta take the good with the bad. The original form of an STL was a wire connection. This could be laid as a dedicated cable at great expense or more commonly supplied by the phone company, which in that era would have been American Telephone & Telegraph Company's Bell system.

Both would be an analog wired circuit, typically a dry pair of straight wire with no intermediate devices. They had no no transformers, compressors, or any kind of audio processing. The upside was that the lines could be used for AC or DC current. The downside was that frequency response was inconsistent especially over long distances.

Studio-to-transmitter links, and to some degree inter-city relays are familiar elements of most radio stations. They allow stations the flexibility to build their studios in locations that may be miles from the transmitter, allowing certain creature comforts or marketing opportunities. To all but those who must maintain them, they are a silent and sometimes forgotten step in the transmission chain

The legacy form of an STL is a wired path. Traditionally supplied by the local telephone company, an analog wired circuit was once the most common form of STL. The most basic type of circuit is a dry pair. This is a straight-wire path between two points. There are no active or passive devices between the source and destination. This type of circuit is also sometimes called a burglar-alarm circuit because it is commonly used for signaling by monitoring companies. Because there are no transformers or equalizers, these lines can pass ac and dc voltages. Their frequency response can be unpredictable and will suffer at long distances.

Modern STL is wildly more advanced using digital data, point-to-point microwave connections, dedicated T1 lines, or a broadcast on another licensed radio frequency ouside the AM & FM bands usually in the 950 Mhz band. Most stations using an STL have a transmitter-to-studio link (TSL) that returns transmitter data. Both the STL and TSL are considered broadcast auxiliary services (BAS) by the FCC and require separate licensing from the CP under their facility ID.

What's in the Signal Pt 1

There is more to radio than what you hear. I dont mean all the goings on behind the mike. I'm referring to the signals in the broadcast that you can't hear. The audio you hear is data on a carrier wave. There is more to it than audible sound.

There are 100 possible FM channels between 88.1 and 107.9 MHz. These are allocated in 200 kHz increments. Because of limited bandwidth a radio station can only transmit audio which is up to 100 kHz. But hey, that's still much wider than the range of human hearing. In practice, FM audio is usually restricted to 15 kHz. This leaves a lot of spectrum for other uses. FM Stations are permitted to place subcarriers in this unused portion of the spectrum.

Here's what's on that carrier wave that you can hear:
Normal baseband audio: consists of the right and left audio mixed together (R+L). This is so that when listening on a mono receiver, you hear both channels of sound.

Difference signal: This is harder to imagine, it's the difference between the right and left channels. When a songs in stereo both ears are getting very similar but not exactly the same audio. This is just the parts that are different or (L-R). This is transmitted on a 38 kHz subcarrier using FM modulation.

Mind blowing isnt' it? FM radio does not broadcast the Left and Right Channels. It's not a damn think like the channels on a cassete tape. Here is how how stereo audio is decoded. Of course, as much of a genius as Edwin Armstrong was, it was nto his system that gave us stereo audio. The Armstrong system was rejected by the FCC because it did not allow sub-carrier services. The the Zenith system has gone on to become the standard method in most countries.
The Armstrong system was more noise resistant I understand. Instead today's stereo FM signals are far more susceptible to noise and multipath distortion than mono FM signals. This is due to several factors, including the following:

1. the addition of the two sidebands of the difference subcarrier to the baseband signal increases the noise bandwidth of the signal by a factor of three (9.5 dB) as compared with a mono signal.
2. The pre-emphasis is applied to the audio signals results in the pre-emphasis acting in the wrong direction on the lower sideband of the difference subcarrier, i.e. decreasing the level as the frequency rises, which will have a further deleterious effect on the S/N of the difference signal. yadda yadda , I know thats too technical. Btu I cant think of a way to simplify it.

So heres what we got and how it works:
If you fed the baseband audio into a receiver's antenna input it will demodulate the signal. This demodulated signal would be the R+ L audio. If you combine this with the Difference (R-L) signal you get a mess, but that's kind of how it works. What it does is add the R+L and R-L signals, leaving just the right (R) channel. the remainder being the LEft channel left (L) channel. ta-dah!

The upside of this complexity and reduced reception qualityis that mono and stereo stations can all operate with this system as can all FM tuners. Also side bands can exist and aren't those nifty. More tomorrow!