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Goodbye analog (and some digital) STLs, hello IP
By now you know I'm a big fan of new technology. However, I also know there is still an important role in a radio station's technical facility for legacy equipment. Like many, I find it difficult to retire a piece of equipment that has served its purpose reliably over a 10- or 20-year (or maybe even longer) period of time. Some equipment is truly classic. It seemed very expensive when new, but easily proved its worth over the years. However, time and technology move on, and those classics' original design specs — good as they were then — are in many cases simply inadequate today.
Analog STL systems fit into this category well. While we've moved on to the current state-of-the-art in our STL systems here in New York, we still hold on to our old reliable STL systems as backups.
Perhaps you find the time has come to relegate your old analog STLs to backup status as well. There are too many advantages to ignore in a digital STL, such as constant audio performance over a varying receive level, or the ability to extend a local area network from the studio to the transmitter site. There are various radio systems available for any need in a broadcast facility.
Basic change from analog to digital
The digital STL systems for our 950MHz band will of course work on a channel that you currently have licensed, but because they use more bandwidth, you'll have to essentially relicense the channel to reflect the change. For example, a Moseley PCL 6010 has an FCC emission designator of 300KF8E (which essentially means it has analog modulation with 300kHz of bandwidth) while a Moseley Starlink (using 64 QAM) has an emission designator of 500KD7W (digital modulation with 500kHz of bandwidth). You will have to recoordinate with local users in the same band. The good news is that a digital STL transmitter is less likely to interfere with co-channel or first adjacent channel analog receivers (since whatever happens to be heard by the other receiver pretty much sounds like incoherent noise). Conversely, a digital receiver is more likely to be interfered with by an analog transmitter, because any analog modulation heard by the digital receiver has the potential to degrade its BER since it is essentially incoherent noise.
My experience is that digital STL systems are very good at ignoring other co-channel digital systems; some experiments showed that co-channel systems would ignore each other as long as the desired-to-undesired ratio exceeded 15dB. Your mileage may vary, but don't forget to consider fade margins if you put more than one system locally on the same channel.
The well-known STL manufacturers of course have mature digital STL products available.
TFT makes the Model 460 digital STL. This radio system will carry up to six channels of uncompressed digital audio, with the modulation scheme (and bandwidth) depending upon the number of channels actually used. For example: six channels of audio (at a 32kHz sample rate) calls for the 256 QAM modulation scheme. In that case the receiver (according to their published specs) requires, at minimum, a -84dBm signal to operate at a BER of 10
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