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.
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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