Field Report: WorldCast Systems APT WorldNet Oslo

May 1, 2010

Over the last 20 years I've used the whole gamut of wireline STLs, including those from Graham-Patten, QEI, Intraplex and now the APT Oslo from WorldCast Systems. We use the Oslo for the main STL system for all of Clear Channel's FM stations here in New York City — and my hunch is that Oslo is well on its way to becoming one of those classic pieces of equipment that engineers will talk about for years to come. Oslo is a frame-based system (3RU) that accepts plug-in modules performing various functions. An entire system is made up of two frames.

Putting the system together

As with any piece of equipment with a tremendous amount of capability, configuration of the Oslo involves many choices. First, the user decides the number of audio channels to be transported along with the type (if any) of audio encoding (including the audio bandwidth and word length). These are the factors that determine the number of bits that need to be assigned to the payload audio. The user may also decide to add auxiliary data services, which will use part of the overall available bandwidth. By using two of the T1 transport modules, up to four T1 circuits can be configured, allowing for up to 96 timeslots (6.4Mb/s of available bandwidth). Here in New York we use the Ethernet transport module, and we use about 11.5Mb/s of data over a 100baseT connection.

Performance at a glance
Interfaces with Ethernet (IP) or T1/E1 circuits
Four audio channels per module
Six modules per frame
Analog and/or AES3 input/output
Audio bandwidth from 10Hz to 22.5kHz
Enhanced apt-X and PCM encoding
RS-232 or Ethernet control
Supports auxiliary data services

Another configuration option for Oslo is to use one T1 interface card (connects up to two T1s) in addition to an IP/MUX card (Ethernet transport). This allows the capability to drop and insert timeslots from T1 to Ethernet and vice versa.

Once the system requirements are determined, the factory will put the system together, configure and test it prior to shipping.

The bench test

When the units arrived, I connected them via an Ethernet crossover and finished the final configuration myself. We hadn't decided on the IP addresses when the unit was tested by the factory. I used this opportunity to learn how to use the NMS GUI as well. I set up a hub so the computer was on the same network as both the MCU cards (these are used to communicate with and configure the system).

Once the GUI was running, I viewed a page called the tree, which shows only the computer. From there I added icons to the tree view to correspond to each individual frame. Double-clicking on the frame icon opens all the unit-specific configuration tabs.

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While the system came with a basic configuration, I added optional alarms. The frame has seven form-C relays that correspond to the alarms. I assigned one relay to silence on a particular audio module (we use five), one relay to report critical alarms such as a power supply failure or loss of IP on the IP transport card, and another alarm that reports loss of AES on any of the five inputs to the frame. On the far end we program silence sensors. The audio threshold levels and the delay time are adjustable parameters.

IP-based audio transmission

As I mentioned earlier, we use Oslo for our main STL systems between our HQ in the Tribeca neighborhood of NY and our main transmitter sites at the Empire State Building. We also have a complete backup facility for each of the five FMs at 4 Times Square, and we have another complete Oslo system, identical to our Empire system, for our backup site. In each case, the Oslo frames talk via IP. Our sites are connected via T3 data circuits. We use Adtran MX2800s to mux 28 T1s together into the T3 data format. 16 of the 28 T1s are assigned to connect two Adtran 5305 routers — one in our master control and one at the far end. In the configuration of the Adtran routers I have made two networks: one that we use for a remote LAN (7 T1s worth of data) and the other for Oslo (9 T1s worth of data). In that way, Oslo has its own, non-contentious network. We connect Oslo to the Adtran directly via an Ethernet crossover on both ends.

I will note that Oslo is designed to work just fine over a shared network, and has parameters that can be reconfigured by the user (if necessary) to compensate for the particulars of the IP network.

We are fortunate though to have the network completely under our control, end to end. And that control pays dividends: The performance of Oslo over this network is identical to its performance with an Ethernet crossover cable connecting the two frames together. Via the GUI, you can drill down into the performance monitoring to see how well the system is doing. As I write this, I'm looking at our own performance monitoring, and there are four streams (linear, 48kHz sample rate, 24-bit word length) that have each sent more than 750 million packets, with zero loss. The fifth stream has lost a grand total of two packets.

Worldcast Systems

It's been my experience that the audio quality of devices built by APT is unsurpassed, and Oslo is no different. It's a real pleasure to listen to. The unit is built extremely well, and I have the utmost confidence in it. I have had questions for factory service, and APT is very attentive to its customer base.

Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at

Editor's note: Field Reports are an exclusive Radio magazine feature for radio broadcasters. Each report is prepared by well-qualified staff at a radio station, production facility or consulting company.

These reports are performed by the industry, for the industry. Manufacturer support is limited to providing loan equipment and to aiding the author if requested.

It is the responsibility of Radio magazine to publish the results of any device tested, positive or negative. No report should be considered an endorsement or disapproval by Radio magazine.

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