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T1 STL Digital Studio Transmitter Link for HD Radio
Digital landline makes an excellent studio to transmitter link for digital radio broadcasting.

By: John Shepler

Over the air broadcasting is in the process of making the transition from analog to digital transmissions. In radio broadcasting the solution is HD Radio using a hybrid digital technology developed and licensed by iBiquity Digital Corporation. HD preserves the station's existing AM or FM transmissions while adding an additional digital signal to the same channel. The FM version can have more than one digital program channel available.

Higher Audio Quality Brings Challenges
The upgrade to digital transmission gives AM stations the ability to sound like FM stations and FM stations the ability to sound like CD quality. This can be a two-edged sword because the upgrade in signal quality puts demands on the studio and transmission systems to handle the improved frequency response, lowered noise and distortion, and improved stereo channel separation.

In many cases, the radio station's studios are in a different location than the transmitter facilities. Many broadcast companies now own multiple radio properties and feed them all from a common studio complex. So the question becomes: how to maintain the high quality of audio signals through the STL or studio to transmitter link?

Conventional STL Limitations
Equalized analog telephone lines were once the standard for sending studio audioto transmitter plants. But these are expensive and hard to come by any more. They are also subject to noise pickup, frequency limitations and phase shift from different length stereo pairs. Microwave transmission can carry the signal in either digital or analog format. But the STL frequencies are crowded and the signals are easily blocked by tall buildings or terrain. Stations that have been using microwave STL may find themselves out of luck when they want to add more programming channels, move a studio or transmitter site, or acquire new properties that need links never before installed.

Why T1 Lines Make Sense for Audio Transport
An excellent option for digital radio is to maintain the audio signal in digital format from source material through digital transmission and only convert it to analog for the analog transmitter input. A T1 telecommunications line offers the bandwidth needed to do this and is readily available in both urban and rural areas. Since this is a digital landline system no FCC license is needed, unlike microwave STLs. That also means no interference issues with other stations since the line is dedicated to your exclusive use. No line of sight is needed. It makes no difference where the studio and transmitter are located in relationship to each other. They can be a few miles apart or hundreds, even thousands of miles distant.

T1 lines were once completely dedicated to telephone company operations. More recently, as prices have come down, they've been adopted by even small and medium size companies to send data files between locations and to connect to the Internet. An STL application uses what is called a point to point private line. That's just what it says. The T1 line is a two way conduit between two locations. It operates at 1.5 Mbps in each direction and is highly reliable.

What Equipment is Involved
To implement a T1 STL, you need specialized equipment designed for this purpose. For HD Radio, Harris offers the Intraplex STL HD specifically tailored to HD radio needs. It consists of two T1 identical multiplexers, one located at the studio and the other at the transmitter. These boxes provide any necessary analog/digital conversions and interfaceto the T1 line through a circuit called a CSU or Channel Service Unit. The STL HD samples at 44.1 KHz and will transport uncompressed stereo audio with a frequency response of 1 Hz to 20.5 KHz, with distortion of less than 0.003% at 1 KHz, crosstalk of greater than -80dB and dynamic range greater than 91 dB.

Two-Way Bandwidth
Since the T1 line is bi-directional, there is an equally high quality stereo channel available in the backhaul direction from transmitter to studio or TSL. This can be used to carry an over the air monitor signal, satellite downlink, RPU audio or telemetry.

A T1 line has only enough bandwidth to carry one full quality uncompressed stereo program audio channel. You do have the option to transport multiple audio channels on the same line by using MPEG Layer II, Layer, III and apt-x100 compressed audio. The Harris Intraplex STL Plus system has the flexibility to do this with plug-in modules.

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