Digital wireline STLs

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The link from the studio to the transmitter is a criticial, but often overlooked part of the transmission system. There are generally two options: wired or wireless. There are some common reasons behind building a wireline STL system as opposed to using a radio system:

  • A move to a new studio location where there is no line-of-sight to the transmitter.
  • A move where there aren't any channels in the 950MHz band that can be coordinated.
  • The radio station already has a radio link established, but wants a wireline STL as an alternate or backup.
  • The station has data or program audio (such as a satellite receiver) that needs to be backhauled from the transmitter site.

    There could easily be additional reasons, but these offer a good foundation. I'll look at what is available in the equipment marketplace for wireline STLs in the categories of what we used to call audio loops; equipment that makes use of T1; and finally, I'll expand the capability greatly and see how to use T3.

    Audio lines

    Ordering 15kHz loops from the telephone company used to be standard operating procedure for STL purposes (and remote broadcasts of course). The results were hard to predict ahead of time: it depended very much upon the quality of the telco techs you just happened to get to align the system from end to end. If you ordered a stereo pair, you were really willing to test the limits of your own patience.

    Pulsecom PCAU

    Pulsecom PCAU

    Here in New York City, our local phone company (Verizon) still offers 15kHz loops but fortunately for us, the modern version. These circuits are built around the Pulsecom PCAU. The PCAU card looks very much like the old Tellabs 4008 cards, and it accepts analog in, and puts analog out on the far end. That's pretty much where the resemblance ends though. The reality is that the PCAU is an A/D converter and communicates with the far end via a digital path through the phone company. By making use of Apt-x coding, the bandwidth requirement is lowered (making telco happy). According to Pulsecom, the units automatically align themselves with one another, for flat frequency response and zero loss on the far end. Two units can be made into a stereo pair by means of a short interconnect cable on both the near and far ends.

    Pulsecom also makes the HD PCAU, which is suitable for HD Radio purposes. This card has 20kHz of audio bandwidth (once again relying upon Apt-x coding); accepts analog or AES; has provision to accept the sample rate reference clock; and finally, it has built in provisioning to transport PAD and SIS data to the far end as well.

    Moving on to T1

    The explosion in data requirements for both the cellular telephone system and other types of wireless data have caused local phone companies to greatly expand their infrastructure into remote mountain tops and other tower sites in order to accommodate these customers. Fortunately broadcasters have been able to take advantage of this. Several manufacturers offer equipment designed to use the now-ubiquitous T1 for transport.

    There are several compelling reasons to use T1 for transport of an STL system:

  • Buying in quantity reduces the unit price. Obviously I don't know every tariff in every state, but my experience in California, Washington and New York is that the cost of a stereo pair is usually at least as much if not more than an entire T1.
  • Not only do you buy the A to Z direction with a T1, but you get the Z to A direction as well. This makes it easy to configure a TSL system should you need it.
  • The TDM nature of T1 makes it easy to combine multiple types of service in to one link: audio, telephone, serial data, and ethernet can be combined into one system.

    Let's take a look at some of the equipment out there.

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