In December 2002, the decision was made to upgrade WQSX-FM to IBOC. That decision brought many questions, not only about how to properly install the system at the transmitter site, but also how to best get the audio to the transmitter site.
We were already using a digital STL system on a T-1 circuit to send discrete left and right channels to the site, but there was a compression algorithm applied that, although acceptable to us in our analog setup, was of concern to us with the new technology.
Everything that I had read up to this point directed us to keep everything uncompressed wherever we could. We also knew that we were going to potentially have other functions that we would want to accommodate with the evolution of the new IBOC technology.
After some research, we knew we wanted the Intraplex STL Plus system. We were already using the STL Plus on two of our other stations with great reliability and flexibility to accommodate our needs. We found that working with the Intraplex sales engineer made the process painless.
The STL Plus is a versatile STL system because of its frame design. A three rack-unit frame with slots accommodates the interface cards for the given application. We knew that for our setup, we would have the main audio going to the transmitter site. We also knew that the STL Plus would allow us enough bandwidth to backhaul left- and right-channel off-air audio for monitoring the radio station. We also have two RPU receivers at the site that we needed to backhaul to the studio on their own cards. Last, but not least, we could use the STL Plus to carry the data for our transmitter remote control.
For WQSX on-air audio, we chose the PT-353 card to deliver the audio to the transmitter site. A fairly new card made by Intraplex, it has the flexibility that any station would want when considering IBOC.
Audio response of this card is up to 22.5kHz depending on the sample rate chosen. It will work with 48-, 44.1- or 32ks/s sampling rates. The card has the option of left and right discrete inputs or AES/EBU inputs, but also allows the user to feed the input with left and right discrete audio and take the output in AES/EBU format at the transmitter site. The last feature of the card is a 9.6kb/s data channel, which we are using to transmit RBDS data.
PT-350C cards are used for the off-air audio backhaul from the transmitter site to our studios, which are analog-only cards delivering linear audio quality of 15kHz. For the audio channels carrying our RPU audio, we used the PT-150C cards, which use Apt-x compression at a rate of 4:1. For our application, it was an acceptable compression rate that allowed the RPU audio to still sound quite good while reducing the bandwidth that it occupies within the T-1 circuit.
For our transmitter remote control data, we use the VF-25 card, which is a four-wire data application. Converting our remote control system from two-wire to four-wire was necessary, and in the end saved us the cost of a data line from the telephone company by allowing us to use the Intraplex system.
The last item we purchased for the system was a redundant power supply; basically low-cost insurance.
Our future plans have us adding another Intraplex STL Plus system later this month, but instead of using a T-1 circuit, we will use a Harris Aurora 5800 wireless 5.8GHz spread spectrum link.
When it came to the installation, it was plug and play. Because all of the cards were set up and tested at the factory, we were up and running with our Intraplex STL system within a couple of hours. Our initial reaction was one of marvel at how much better the on-air audio sounded with the uncompressed PT-353 card from our old system, which, as I mentioned, we thought sounded quite good.
This system has been on for almost a year, now. Our experience with not only this Intraplex STL system, but our other two has been great. When we've had questions, we have always found the Intraplex tech support to be responsive and knowledgable. And, most importantly, I can say that we've had no failures, ever, with our Intraplex equipment … and, knock on wood, that will stay that way for a long time.
Kennedy is director of engineering, Entercom Boston.
Performance at a glance
Multiple I/O card options available
Work with wired or wireless T-1 links
PT-353 card good for IBOC applications
3RU chassis has slots for many cards
Redundant power supply option available
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