Fixing Audio in Post

We’ve lectured for years on the benefits of proper audio gathering and getting it right at the shoot. Obviously, this makes things easier, cleaner and more professional sounding when you begin to edit the footage. But for whatever reason, it seems that something always happens to mar your perfect audio recording. It may be a simple little thing or some unforeseen catastrophe, but someday, it’s inevitable, you’re going to have to fix something in post. You can relax because today’s audio editing tools – and even video tools – offer virtually everything you need to repair those unfortunate audio accidents, and these tools can help you enhance the audio even more.

Options Galore

Fixing audio in post used to be a nightmare. Just imagine all the equipment it took back in the golden age of television. Video was recorded and edited on tape, so all the audio processing took place in real time with real hardware processors. Need some equalization? Hook up a box. Need some dynamics control? Hook up another box. And don’t forget, many of those boxes were expensive. Of course, today all our audio processing and cleanup takes place inside the computer. Adding a virtual processor can be as simple as dragging something onto the timeline. If you don’t get what you want the first time, just back up and try again with different settings. If you need a processor that’s not included in your editor, just download one from the Internet – many times they’re even free.

The majority of audio repair tools fall into just two categories. First are the filters. These include High-Pass, Low-Pass, Band pass and general purpose equalization. Filters affect the tonal quality of your recording and usually serve to add or subtract some portion or portions of the audio spectrum from your audio track. Next on the list are dynamics processors. Compressors, limiters and expanders all fit this category. Dynamics processors generally alter the difference between loud and soft portions of the soundtrack. It’s most common to reduce the dynamic range of a recording in post to gain clarity and even out volume levels.

Another popular audio tool is digital noise reduction. By sampling a section of only noise, you tell the noise reduction processor what to remove from your recording. This is a powerful tool that has the potential to rescue noisy recordings and significantly reduce hiss without affecting the quality of sound. Other tools such as de-essers and enhancers operate using a combination of filtering, dynamics control and other techniques. Regardless of the tool or tools you’re using, the goal is the same: clean up that audio track.

Working Inside the Editing Program

The most effective audio repair tool you have available isn’t a signal processor at all; it’s the cut tool. With clean cuts you can eliminate many of the noises that pop up during the shoot. For interviews, a tight cut gets rid of all the shuffling, bumping, umms and ahhs that drove you crazy in the first place. In an action scene, you may be able to cut before the noisy motorcycle drove by or just before someone behind the camera tripped over a light stand. In addition, you have the power to decide which take requires the least repairs after the fact. The cut tool is your first line of defense.

Of course, cuts alone won’t fix everything, but it’s a good place to start. Once you have an edit you’re happy with, it’s time to clean up the leftovers. Another good non-processor cleanup tool is the use of volume envelopes. Envelopes are drawn on the audio track which creates automated mutes for the tracks. This works especially well on dialog when you plan to cover the gaps with room tone, music or sound effects. Otherwise, you may have to resort to other techniques. When working on a dialog track, start with a high pass filter. By setting the cutoff frequency around 100 Hz or so, you effectively eliminate any audio below that point. That includes rumbles from wind and mechanical noise. Removing this portion of the audio spectrum makes a cleaner track for any additional processing you do.

If you have volume level issues, consider using an audio compressor. Start with a 2:1 ratio and listen closely to the effect. You should hear a leveling of audio levels – soft sounds become a little louder and loud sounds don’t get so loud. It’s easy to over-do this effect, so listen to the bare audio track when you make changes to minimize over processing of the sound. Another volume trick is placing a hard limiter on the master output. Sometimes called a brick-wall limiter, this processor puts a lid on the audio volume and keeps any audio spikes under control. This ensures a distortion-free final audio track that won’t overload playback systems downstream.

Export and Process

If your editing program offers little in the way of audio cleaning tools, consider exporting the finished audio track and fixing it in a separate audio-only program. This option provides all the audio processing power you’ll need. Inside a Digital Audio Workstation or DAW, you can clean and polish audio tracks with a variety of tools. In addition to the standard filters, your DAW offers graphic and/or parametric equalizers to eliminate harshness, get rid of audio mud and brighten tracks. You will likely have access to a spectrum analyzer, too, which provides visual animation of your audio and helps pinpoint areas that need further attention.

In addition to the digital noise reduction we mentioned earlier, most DAWs offer a plug-in architecture, allowing you to include any compatible audio processor you like. Whether paid or free, there are hundreds – maybe thousands – of general purpose and specialty audio processors available today. Using one or more of these, you can widen or narrow the stereo width of your audio, add audio effects like echo and reverb or make your audio sound like it’s coming from an over-driven bullhorn. The only limits are your imagination, patience and ability to find useful plug-ins on the Internet.


Mr. Fix-It

In a perfect world, we’d all record flawless audio in-camera the first time and editing would be a breeze. Keep dreaming. It’s a cruel world out there for the video producer and it would be easy to let a lot of audio errors go. Nobody will notice, right? Maybe not on YouTube, but if you’re c reating video for paying clients or for your business, it’s worth the time and effort to fix the audio. The nice thing is, whether you do it in your editing program or your DAW, the tools are there if you’re willing to invest the time.

Sidebar: Change in the Wind

The Audio for Video landscape is changing. HDSLRs have had a large impact on audio gathering techniques and many producers record their audio on separate recorders. In post, you may have to combine a shotgun mic track recorded in the camera with one or more tracks from pocket recorders on talent. The editors’ challenge is unifying these sound sources to create a seamless end product. It’s certainly more complicated that it used to be, but this is where things are going. Get friendly with the workflow and your audio processors – you’re going to need them. Watch for our “Audio Tips for HDSLRs” coming soon.

Contributing Editor Hal Robertson is a digital media producer and technology consultant.

3 COMMENTS

  1. It’s okay Julie, Aaron is one of those “know it all” kinda guys. He’s just used to one way of doing things. Too bad he’s not open minded about things.

LEAVE A REPLY

Please enter your comment!
Please enter your name here