Field Report: Harris Intraplex HD Link

October 6, 2011


Harris Intraplex HD Link

The Intraplex HD Link is a 950MHz radio system that passes not only the typical program audio, but also HD Radio traffic between a studio location and a transmitter site. If integrated with an IP return path from the transmitter site back to the studio, the system will pass TCP traffic between the two ends, and it will manage packet routing based on the user configuration and path accessibility. The HD Link has all the features we've come to expect from an STL system, and more.

The first thing I do with a radio system (or any for that matter) that has as much capability as the HD Link is put it on the bench and back-to-back the units. I did this by just putting a dummy load on the transmitter, and a small whip on the back of the receiver. After powering the units, they came right up with all their default settings in place. Read the Quick Start Guide (pages three and four in the manual) so that you can configure the system to meet your requirements. Specifically, you'll need to go to the RF profile and to put the system on the correct frequency; and you'll need to set the RF bandwidth so that it matches the emission designator you used in the license application (for example, 500KD7W for 500kHz of bandwidth).

You may want to change the parameters of the two audio pathways from their defaults. One of the default RF profile settings is "any modulation," which allows the unit to change from 32, to 64, to 128 or even 256 QAM depending upon what you ask the system to do. There is only so much data-handling capability available for a given system bandwidth and it's clear that as you ask for more, the QAM number goes higher. For example: With both audio pathways set for 48kHz sampling/stereo/linear, and with the user-IP bandwidth set for 256kb/s, the system modulation scheme goes up to 256 QAM. Changing audio path two to mono (as one example) immediately changes the modulation scheme to 128 QAM. These parameters have a finite number of choices obviously and the system will prevent you from trying to make changes that don't work. As another example: If I leave audio pathway one at 48kHz sampling/stereo/linear, and change audio pathway two to match that, I find that (at 256 QAM) I can get 320kb/s through on the user-IP link; trying to get more than that results in an invalid error message back from the radio.

Performance at a glance
Analog and digital audio I/O
Two G.722 audio paths available
Two stereo program channels
Linear or Enhanced apt-X audio
Multiple Ethernet links

By the way, all the configuration changes I've suggested so far can be done via the front panel. You can also make all those changes and many others by using the Ethernet interface. That allows me to segue nicely into the next important feature of this radio system: its ability to be integrated with a full-duplex IP network between the transmitter site and the studio site.

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All about the data

The system supports two separate LANs, the idea primarily being that the LAN known as HD carries E2X (exporter to exciter) UDP traffic. (You would make use of this if you had both the importer and exporter back at the studio.) The other LAN will also carry traffic in one direction (i.e., UDP). It's important to note that the HD traffic is given priority over any traffic you try to send to the far end via the (lower priority) LAN port. In this way you can ensure that the E2X traffic makes it.

So you may wonder then how you would access the receiver via IP if the traffic only flows in one direction? Of course the answer is that you can't, unless you integrate a full-duplex IP connection into the system. The HD Link is built to do that, and it's a wonderful feature.

To test this feature on my bench, I had to build up the system as best I could, simulating the E2X traffic. The goal of the test then was to pass the UDP traffic along the HD LAN port, and to pass TCP traffic on the lower priority LAN at the same time. It's important to note however that the system can be configured to totally re-route all the LAN traffic, plus all of the audio links, to the duplex connection, should the RF link fail.

">Figure 1. Intraplex HD Link Test Configuration. Click to enlarge.

Figure 1. Intraplex HD Link Test Configuration. Click to enlarge.


See Figure 1. To generate traffic for the E2X link, I set up a Barix pair: the Instreamer was connected to the HD LAN port on the transmit side. Conversely, the Exstreamer was connected to the HD LAN port on the receiver end. The other LAN configuration simulates a typical LAN connection in the rack room of the radio station. The LAN port of the HD Link transmitter connects to a switch-port on a Layer 2 switch. Another port on that same switch (same VLAN) connects to a gateway that gives you access to some type of duplex IP connection to the transmitter site. For my test I used a couple of Cisco routers connected via a T1. On the far end, the other router becomes the gateway for the receiver side. It connects directly to the LAN port of the HD Link receiver.

Now there is some simple configuration necessary to make this arrangement work. You need to configure proxy ARP (Address Resolution Protocol) on the transmitter side with the address of the target on the far end.In my test configuration I considered the HD LAN to have only the ability to pass traffic in on direction; so for this reason, proxy ARP needs to be used at the transmit end on the HD LAN. When the traffic generator (in my case the Barix) sends an ARP request, the HD Link transmitter actually responds with its own MAC address. That way, the Barix can actually build all the frames correctly. The transmitter then sends that data to the far end.In the path redundancy table on the transmit side, tell the system to use both the forward and return IP path, and configure the peer address (which is the address of the LAN link port on the far end receiver). On the transmit side, configure the Ethernet service to fail over from RF to IP. In the path redundancy table on the receive side, tell the system to use forward and return IP path, and configure the peer address (which is the address of the HD Link LAN port on the transmit side).

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So now with the RF and IP links up and running, the system is accomplishing all the following:
• Passing two separate audio paths from transmitter to receiver
• Passing the E2X traffic along the HD LAN
• Passing TCP traffic over the other LAN

In the event that the RF path fails, traffic gets moved completely onto the IP link. When the RF link comes back up, traffic flow reverts.

So sitting in your office you can browse into both the receiver and transmitter, which is especially handy if you want to make other changes in the configuration, or perhaps more importantly, look at the real-time performance of the system.

Observations

A couple of items that I want to mention about this system that you should know about: The transmitter is a little noisy, because of fan noise - so you won't be able to put it in a spot that must be quiet. The system takes a large fraction of a minute to boot up - so I would recommend having a UPS on both ends of the link.

Harris
800-231-9673
www.broadcast.harris.com
broadcast@harris.com

For my evaluation, I used a beta version of the software that was be released for the product in September 2011. In this final release you'll find a new feature that allows the user to configure a backup profile with reduce audio coding rate as well disabling of non-essential services. This allows the user to use a lower speed IP path for a backup or allows the RF link to automatically adjust (increasing channel coding or reducing modulation) when receiver experiences degradation.

I've barely scratched the surface on the features that this radio system has; but I can tell you that in the testing I carried out, the system performed very well, doing everything advertised. I'd also like to thank Keyur Parikh and Jeff Merrow from Harris for their help in this evaluation.


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



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