Editor’s note: Welcome to “Ask the Engineer,” an occasional series of technical advice columns from Rob Byers, sound engineer, loudness advocate, and technical coordinator for American Public Media. Thanks to Erik Stromstad at American Public Media for help with this column.
By Rob Byers
Phone tape can be a real pain when it comes to audio quality, but it’s so useful that it’s a necessary evil. So here’s the quick and simple way to improve the sound of your phone recordings: filters.
But before we get into how to use these handy tools (ones you already have!) there are a few things you need to know about why phone tape needs special handling.
Old-school analog phone lines only include the frequencies between about 300Hz and 3400 Hz (there are multiple reports out there of what these frequencies are, but we don’t need to get too specific).
This means there is no useful information below 300 and above 3400Hz in phone audio. Unfortunately, by the time the phone call makes its way through the telephone system to your recorder, a bunch of noise and distortion is added above and below those frequencies.
It’s possible to end up with a pretty bad phone recording depending on the quality of the infrastructure, weather conditions, and other influences. It all gets worse and more unpredictable if you connect to a cell phone. (Always ask your guest to use a landline!)
Unprocessed audio on the left, processed audio on the right. The filters remove a good deal of that phone junk. (Image courtesy Rob Byers)
To eliminate this unnecessary “junk,” you will use two of my favorite tools: a high-pass and a low-pass filter. The high-pass filter will remove all of the junk below 300Hz (it allows the highs to pass) and the low-pass filter will remove all of the distortion and hiss above 3400Hz (it allows the lows to pass).
So, what can filters do? Take a listen:
Using these filters was one of the first steps I learned as an engineer. When I started in public radio at NPR, it was my responsibility to fix audio delivered to me by reporters in the field, which included quite a bit of phone tape.
Early on, I encountered a piece of phone tape with really bad distortion. I didn’t know what to do, so I just let it go through my hands untreated, and it made it to air in a newscast.
Minutes after the distorted tape aired, the engineer who drove the newscast came into my suite with a stern and somewhat disappointed look on his face. He showed me that a low-pass filter around 3400Hz (sometimes a little lower) will almost always remove distortion — and then told me to never let distorted phone tape air on his newscast ever again. You’d better believe I never did!
Here’s what you can do to avoid disappointed looks from your engineer colleagues (and improve your phone tape):
In your audio editor of choice, load an EQ plugin on your phone track that gives access to both a high-pass and a low-pass filter.
Turn both filters on (likely by clicking on “ON” button or a “HP” or “LP” button).
Set the high-pass filter (the lower one) to 300Hz.
Set the low-pass filter (the higher one) to 3400Hz.
Change the slope for each filter to 24dB/octave.
Play back the audio and see if you like the results.
It should look something like this:
(Hindenburg doesn’t have a true low-pass filter, so in this example I used some plugins that come with most Mac installs.)
Now that you’ve filtered out the “junk” in the phone line, things should sound much cleaner. You may need to adjust the filters slightly. Some hints:
• Phone tape distortion typically lives in the high frequencies, so if you still hear distortion after following the steps above, move the filter down to 3200Hz or even 3000Hz to remove more of the high end.
• If the phone tape sounds a little muffled, move the filter up to 3500Hz or above to put some higher frequencies back in.
• If the tape sounds like it’s a little thin or lacking warmth, you can move the high-pass filter (the lower one) down to 200Hz to add some low end back.
• You might be wondering about Skype, FaceTime Audio, and other voice-over-IP apps. While those tools might use a cellphone, they don’t use phone lines and instead they try to provide higher-fidelity audio and capture the full range of the human voice. They attempt to represent a wider range of frequencies than a phone line.
If you try the above trick on Skype audio, it won’t really work — the low-pass filter will make it sound muffled. That said, you can always try using the high-pass filter to remove low-end rumble and p-pops, which can be a problem in Skype setups.
• You may want to compress the phone tape to control some of the peaks. Put your compressor after these filters for best results. I may cover compressing phoners in another post if there is interest — let me know by emailing firstname.lastname@example.org, or tell me on Twitter at @robbyers1.
• To make the process even quicker for next time, save a preset in your EQ plugin.
• Use this quick trick to clean up all of your phone tape.
I start with these filters every time I work with phoners. Your ears — and your audience — will thank you.
• Rob Byers (@RobByers1 on Twitter) is a technical coordinator at American Public Media, an NPR “alum,” a classically trained percussionist, a jazz enthusiast, and a member of AIR’s network. Over the years, he has been lucky enough to engineer recordings with Yo-Yo Ma, Rufus Wainwright, The Knights, Béla Fleck, Joshua Redman, Josh Ritter, Yo La Tengo, Cyrus Chestnut, and many others. He gave a recent, terrific webinar about audio levels, and he thinks everyone needs to know about #loudness.
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