Why We Use an Audio Equalizer on 146.76

First, Some Background

LEARA‘s 146.76 repeater is a complicated machine, and necessarily so. We want to provide a wide coverage area that’s easily accessible to mobile users. We accomplish this by having a powerful transmitter (180 watts). The transmitter is connected to a high-gain antenna that’s high up on a tall tower. The tower is located on a moderately-high hill. That’s why users can hear the repeater from mobile and fixed stations in several counties. But that’s only part of the picture.

Big Mouth, No Ears?

A repeater is not useful if far away users can hear it but can’t get in. The question becomes: How do we ensure the repeater isn’t an “alligator” – with a big mouth and small ears? We enhance the receive coverage by using several receivers spread out over the repeater’s coverage area. These receivers are basically cross-band repeaters: They receive signals on 146.16 and re-transmit them on individual UHF link frequencies. At the main site, we have receivers for each of these link frequencies. Those receivers feed into a voting comparator (“voter”). The voter selects the remote input with the best signal (more on how voters work in a future article). That audio is then passed to the equalizer. The equalizer is connected to the repeater controller, which processes it and then passes it to the transmitter.

Making it Sound Great

Our goal is to make the repeater sound as close to simplex as we possibly can.

So what does all this voting and remote input stuff have to do with the audio equalizer? It’s simple: The audio changes slightly and loses fidelity as it goes through each of the steps above (remote receiver, remote link transmitter, link receiver, voting comparator, controller). Each of those devices does some audio processing. The resulting audio tends to be lacking in the sub-1000Hz frequency range, which makes it sound “empty.” We compensate for this by amplifying the 250-1000Hz range (we do also amplify the higher range, but to a lesser extent). This tends to “bring back” the lower frequency range and makes the audio more satisfactory.

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