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Digital wireline STLs
For example, the APT Oslo, mentioned earlier, can also be built for IP transport by the addition of the dual IP MUX card (the dual part referring to its separate Ethernet ports, which can live on separate networks).
Harris/Intraplex offers the Net Express, which is very similar to the system I wrote of earlier, but T1 transport is replaced by IP transport.
Another possibility for the transmission of audio over a network such as this is the STL-IP from Audio TX. This is a 1RU box with analog or AES inputs and outputs, wordclock in, and of course the network interface (RJ-45 100baseT). Sample rate up to 96kHz and word length up to 24 bits; linear PCM, AAC, layer 2, layer 3 and G-722 are the most well-known of its available coding schemes. Management is done via a Web browser or a telnet session using command line.
Likewise, many of the codecs commonly used for remotes, such as Tieline, are capable of communicating via an IP link and can be used to interface via a network.
And finally, you can take the approach that your transmitter site isn't really far away after all, and that it's just another node on your network. This would be your approach should you chose to go with Axia and one of its AES nodes. This device has eight AES inputs, and eight AES outs, and makes use of an Ethernet network for transport of audio from point A to point Z. (An entire full-bandwidth 100baseT connection is required between the two nodes.) If you have that much bandwidth available to your transmitter site, this is an approach you may want to consider. Alternatively, if the site is in the general proximity of the studio, you may want to consider a fiber optic run between the two points. You could extend your network by adding two Ethernet switches (one at point A and the other at point Z) that are trunked via a fiber interface.
Wireline STLs have come a long way in the last 10 years or so. As the proliferation of networking has changed the way radio stations are put together, so has it expanded the possibilities for STL systems. I hope I've given you some ideas about how powerful and useful the current technologies are, along with some ideas to consider in the future.
Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at firstname.lastname@example.org.
When studying the specs of T1 equipment you need to keep a couple of things in mind:
The various services will use up some number of timeslots in the T1, each of which is 64kb/s. At minimum, any one service will use one 64kb/s timeslot (or ‘ds0') and at maximum, 24 timeslots.
The grand total of all timeslots used cannot exceed 24 since that is the most data that can be carried by a T1.
For example, if you wanted to use 18 timeslots for your audio path, and six for the Ethernet bridge, there would be nothing left over for any other services. Six timeslots would give your LAN extension a maximum bandwidth of 256kb/s, which is rather slow considering all that goes on at transmitter sites nowadays. Read on in the main article to see how to get around that problem.
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