Using High Pass Filters for Video

Let’s admit it: It happens a lot. Even when you pay careful attention to the audio you record for a video, and you used a good mic (you did use a good mic, right?), you can still end up with a bunch of background rumble and noise in your recording. It happened just the other day to someone at the video production company I work for. They were shooting in a grocery store, capturing some interviews. They used a good shotgun mic, with good directivity to cut down on the ambient noise. However, there were those dreaded coolers all over the store. When you're in a grocery store, you don't notice the noise because your brain tunes it out. Back in the studio, it sounds like a truck going by the entire interview.

Because it’s a complex noise source, trying to run a noise reduction program on it probably won’t work well (and even when the noise goes away, it is often replaced by unwanted digital artifacts of the FFT process used to perform the noise reduction—but that’s another post). However, we do have one tool in our utility belt that can help (actually two, I’ll get to the second, which should actually be the first, in a second): Enter the high pass filter.

A high pass filter is just what it sounds like—it lets high frequencies pass, while blocking low frequencies. Super-basic HPFs are a simple on and off switch with a threshold frequency (the frequency at which it “passes” signal) and slope (how quickly it drops off the signal below the “pass” frequency) set at the factory. Better HPFs that come with higher level editors like Premier Pro and Final Cut Pro allow you to select the threshold frequency and adjust the slope

A sample high pass filter with a threshold frequency of 120 Hz and a 12 dB per octave slope.

You can see how the frequencies above the threshold frequency pass by unaffected, while the ones below get rolled off pretty quickly.

This is when it actually gets useful. For the male voice, the fundamental frequency of the lowest notes one speaks is between 85-155 Hz. For a female, it’s a little higher, perhaps 165-225 Hz. This means that there is no real information that we need below 85 Hz for males and 165 Hz for females. And in reality, because of the way we hear and the way the voice is produced, there are plenty of harmonic frequencies that our brain will interpret clearly to make up for missing fundamentals.

So let’s say we have a compressor running in the background of a female interview. We can safely dial up a HPF with a pivot frequency of 165 and not loose any of her voice. We can take it up even higher to eliminate more of the noise, and the clarity will improve markedly. In fact, the voice will “sound” louder once the low frequency stuff is removed because we can hear it better.

So this is exactly what we did for grocery store woman. We dialed up an HPF with a pivot at around 150 Hz, and it totally transformed the audio. There was still some higher frequency noise, and it was obvious she was standing in the store and not a studio, but the clarity of here voice was improved substantially.

Using High Pass Filters in Final Cut Pro (or Express)

So let's look at how to apply and use a High Pass Filter using a few poplar editing tools. I like Final Cut Pro (and Express), so I'll show you that. Then we'll take a quick look at Soundtrack.

To apply a high pass filter, first select the clip you wish to affect. I recommend you do this prior to editing the clip. The reason is simple: If you apply it before you start slicing up, all of the slices will retain the effect. If you wait until you finish editing, you'll have to copy and paste the effect.

Step one is to double click the clip to bring it into the viewer. Then choose "Audio Filters" from the Effects menu.

You could choose the AUHiPass from the Apple filter selection, though controls are a little different (we'll see them in the Soundtrack portion). The Filter is really easy to use, it only has 2 controls. To use the filter, hit play and adjust the "Frequency" control until you hear the rumble go away. Just don't turn it up so far that your voices sound thin and tinny.

You'll likely want to leave the "Q" setting low, as this gives you a rapid slope downward. You can play with the setting, but I bet you'll end up at .71 (the minimum).

So what does this sound like? I've made up a small sample to help demonstrate what we're talking about here. Play the movie below and you'll hear the effects of a HPF. You may not hear it if you're listening through small computer or laptop speakers (they are, in effect a high pass filter), but it should be pretty obvious if you have a decent set of monitors, or with headphones.

Applying a High Pass Filter in Soundtrack

It's much the same as in Final Cut Pro. Select the Clip, open your Effects Tab, and look under the Apple category. You'll see the High Pass Filter.

Click on the "Advanced" button and you'll see the following controls:

All of the frequencies below the "Cutoff Frequency" are cut (cut off is a bit harsh, more appropriately, they are attunuated). The "Resonance" is similar to "Q" in the Final Cut Pro version. It's how much and how fast those unwanted frequencies go away. In this case, you'll probably want to pull that down (into negative numbers) to achieve the effect you want.

Most other editors (Vegas, Premier Pro, Edius) have similar controls. For some reason, different companies call the parameters different things, but if you play with them a bit, you'll get the same results. Hey, it's digital editing—if you don't like it, there's always undo!

Earlier I mentioned we actually have 2 tools in our tool belt. The other one may be on the mic itself. Many professional shotgun mics (and some interview mics, and the occasional lapel mic) have a HPF built in. For example, my beloved Audio Technica 835B has a switchable roll off at 180 Hz at 12 dB per octave. That means at the lowest fundamental of a male voice the mic will be 12 dB down, which is generally not a big deal unless you’re interviewing James Earle Jones. Normally, I like to leave this switched on because it eliminates a lot of room rumble, AC noise and other nasties right at the source. It’s just a good idea. If you use this when you shoot, you will require less processing in the edit suite.

Of course, you’ll want to listen to it through some good headphones first to make sure you’re happy with the sound. You do have good headphones, right?