TheEC: High Pass, Low Pass (audio filtering)

Oct 11, 2017

Today (Oct.11) I modified the settings on our Omnia 9 audio processors on 102.7FM (WRNI-FM) and 89.3FM (WXNI) to add both a high-pass filter and a low-pass filter.

...I'm sorry, whenever I say that I always picture this scene from Austin Powers in my head.  Moving on...

Low-pass and high-pass filters are a bit counterintuitive, but they mean literally what they say: a low-pass filter means you pick a given frequency...in this case, 15,000Hz (or 15kHz)...and allow all frequencies below that frequency to be passed.   All frequencies above it get "rolled off" (reduced, or "turned down") so many dB (decibels) per octave, so there's a gradual curve; the higher the frequencies, the greater the rolloff is.

High-pass is the reverse: pick a freq (82Hz) and everything above is passed.  Freqs below are rolled off.

Why are we only passing frequencies between 82Hz and 15,000Hz?  Because of the limitations of FM radio, especially in regards to spoken-word audio like RIPR's news/talk format.  

The typical human ear can hear sounds as low as 20Hz, and as high as 20,000Hz, but even a very good ear cannot hear sounds as well at those extreme frequencies as it can more in the middle (roughly 200 to 8000Hz).  This is why "white noise", which is just noise at equal volume across all frequencies, sounds very "bright" or high-frequencied to the human ear; we hear certain frequencies better than other.  That's why there's also "pink noise", which has the volume levels at certain frequencies enhanced/reduced so that it "sounds the same" to our ears.

But the upshot is that it doesn't help listeners much to push out frequencies below 100Hz, nor above 15,000Hz.  Even if there's audio information there, it's information our ears are not attuned to make much use of, and thus it doesn't help with how good/bad RIPR will "sound", nor how easy/hard it is to understand what's being said (aka "intelligibility").

But what pushing those frequencies CAN do is cause distortion within the FM transmitter (specifically the "exciter"...the specific device that takes audio sound and turns it into an RF broadcast).  And that distortion can be audible across many frequencies.  So it helps make things sound "cleaner" to roll off the low- and high-end of frequencies a little bit.

To make a long story short, the audio on 89.3 and 102.7FM should be a little "cleaner" to your ears now.

A similar effect has been in place on 88.1 for a couple of days, after I tweaked the HPF/LPF and turned down the bass-boost at 120Hz in the parametric EQ of the Wheatstone Vorsis AP2000 audio processor I recently installed there.  

By the way, keep an eye on the real-time status update page (or just subscribe to @riprengineer on Twitter) about technical problems, repairs and improvements.