The Signal Sucks: FM Side Band Hiss

Does your FM station sound like crap these days?

Is the sonic gravel getting you down?

If so, I bet you are listening to the modern sound of side band garbage through a vintage FM tuner.

The FM frequency spectrum is increasingly being crammed with signals that were never envisioned by the designers of medium decades ago. Here in Austin, the classical FM station KMFA has broadcast a benchmark quality signal since its inception, but these days you are wasting our ears trying to listen to it with a vintage tuner. Recently KMFA sold off the edges of its frequency range to another signal, and you can hear it as hiss behind the music.

The FM signal broadcast is not just a single licensed frequency but includes 130 kilohertz of bandwidth above and below the designated frequency. According to KMFA's consultant engineer the FM broadcast system originally used the entire licensed spectrum to minimize distortion, and our vintage tuners reflect that philosophy. They tend to grab the full width available with broad filters. However, the modern broadcast signal now lives in a much narrower portion of the licensed band (15 khz) , and the edges of that band are a business opportunity.

What is that business opportunity? There are a number of possibilities, but in all likelihood the offender is HD radio. I highly recommend this Wikipedia article. Note the FM spectrum images down the page on the right. The second image is of the signal that gives your vintage tuner a hard time. According to the article only 1% of the station's power is devoted to the HD signal. A move is afoot to increase this to 10% in order to improve the quality of the HD signal at the expense of the analog signal. (Shudder.)

So what prompted this little rant? I hadn't listened to the wonderful KGSR in a week or two on my Advent 300 receiver, but tonight the hiss was just depressing. I recently restored this little Henry Kloss/Tom Holman gem using Elna Silmic II caps with a few 1 mfd films and the tuner is sounding great. I guess I need to learn something about FM filters...