Multimedia
Blogs
Forum
Affordable HD
Whitepapers
Newsletters
Events
Advertisers
DCP Directory
MillimeterSalvaging Bad Audio
Sep 26, 2001 12:00 PM, Steve Cunningham
As a professional audio guy, I'm very aware of the need to capture clean and accurate audio along with clean and accurate video. When on a shoot, I follow accepted practices by using a separate directional microphone (or two), keeping the mic as close to the sound source as possible, and keeping the record levels as high as possible without clipping. But even with those precautions, noises occasionally creep into my recordings and must be reduced or eliminated.
I'm also asked to edit and compress other people's footage, and often there's no telling what distracting noises or just plain bad sound exists on their audio tracks. Recently, I embarked on a project to clean up and transfer to CD several hours of live Cuban music recorded in the early 1950's on ¼-inch analog tape. When it¹s done, I will encode the music for streaming on the owner¹s website.
All of these jobs call for some audio surgery.
Fortunately, there are several hardware and software tools that can greatly improve bad audio tracks and deliver sound that is much improved over the original recording. The trick, though, is finding the right audio tool for the specific noise problem. To do that, you must first break the audio problem into separate parts and then deal with each individually.
Undesirable noises can be broken down into two categories: fixed frequency noise such as that caused by AC hum, air conditioning units, cameras, or generators; and dynamic noise, which changes in spectral content and level over time. Classic examples of dynamic noise include car and airplane pass-bys and wind and surf noise. Fixed frequency noise is by far easier to repair than is dynamic noise, but both can be improved.
Unintelligible Speech
You first need to determine if you're a victim of Bad Recording Techniques. Poor microphone choice or bad mic placement can leave you with an audio track that's muffled or indistinct. The use of a camera-mounted microphone can result in the same symptoms. Happily, this is the easiest problem to fix, perhaps requiring only the judicious use of equalization and volume control to reduce the level of noise. Boosting frequencies around 2.5 KHz for men and 3.5 KHz for women with EQ will make the spoken word louder and will help it to cut through. Boosting frequencies around 6 KHz will increase sibilance (the "ssss" sounds in speech) and intelligibility, but it may also increase background noise and hiss, so a light touch is required.
By cutting frequencies below 250 Hz you may be able to turn up the track's overall volume without distortion. The telephone company discovered long ago that most of the audio information in human speech exists between 300 Hz and 3 KHz, so there's little harm in cutting below 250 Hz for a talking head. But remember that EQ will affect all the sounds in the recording; so a little bit goes a long way.
Background Noise
If the original recording level was low, then you may find that boosting the volume during editing also boosts background noises. These noises can include camera noise or handling noise, or environmental noises like air conditioning.
If the background noise is only audible in-between sentences, your best solution is to use a noise gate. A noise gate allows sounds at or above a certain volume level, called the threshold, to pass through it unaffected. But when the volume drops below the threshold level, then the noise gate turns the sound off altogether. When the volume again exceeds the threshold, the noise gate opens up and allows sound to pass again.
A close cousin of the noise gate is the expander. Whereas a noise gate clamps the volume to a minimum level when it falls below the threshold, an expander just reduces the volume to a user-specified level. It is a kinder, gentler noise gate that is used to increase the dynamic range of a track, and can significantly reduce background noise. Often a noise gate and expander are combined into one effect. Both can be quite effective at reducing or eliminating low-level background noise.
If the noise has a fixed frequency, as with AC hum or air conditioning noise, you can often use filters to reduce or eliminate it. However, basic bass and treble filters are too broad for this work, and will audibly affect your program material. Look for parametric filters that will let you control the width of frequencies affected (known as the filter's "Q"). Parametric filters can be set to cut the level of a very narrow range of frequencies, leaving the rest of your track untouched.
Some software packages offer specialized filters for eliminating hum. Apple's Final Cut Pro, for example, now offers a dedicated hum filter. In addition to specifying the frequency of the hum, you can also choose to eliminate up to five harmonics of the hum. Gentle experimentation is always best, and be prepared to hit the Undo button.
Distortion and Clicks
Audio distortion is the worst of the problems, and the most intractable. Distortion occurs when an audio input is too loud for the input of the next stage to reproduce cleanly. Consequently the tops of the audio waveforms are clipped off, hence the term "clipping". Distortion generally imparts a "raspy" quality to the sound, and in extreme cases can make your sound unintelligible. While analog distortion is the most common, digital devices are also susceptible to distortion. Analog devices distort in a gradual fashion as gain is increased, but when digital devices distort they do so at one particular threshold level, and the result is instant unpleasantness.
The best way to eliminate distortion is to prevent it before it happens. There are few remedies for distortion once the recording is done, especially in extreme cases. If the amount of distortion is slight or the distorted segments are short, then you can use your software audio editor's Pencil Tool to literally draw smooth tops on the waveforms.
But if the distortion is severe or prolonged and there's no chance of re-recording the sound, you may be best served by taking your distorted audio files to a facility that has Sonic Solutions' NoNoise, which is part of their Sonic Studio HD product http://www.sonic.com/sshd_home.html.
NoNoise is a venerable computer program specifically designed to eliminate unwanted noises from audio recordings, and it is particularly effective when dealing with distortion. It can even generate resynthesized audio to replace damaged sound. NoNoise is also your best choice for dealing with dynamic noise that does not respond to filtering. It is the industry standard in noise elimination, but a NoNoise system comes with a hefty price tag, so it's best to farm this work out. Depending on where you live, expect to pay anywhere from $60/hour to $150/hr for NoNoise work. Your budget may not like it but sometimes there is no other option.
Clicks and pops can result from bad audio cables, or channel breakup with wireless microphones. Also, if you choose to use an old phonograph recording of music, you’ll likely find that it is full of clicks and pops. These can be handled in much the same way as distortion. Many software audio editors include a function that will automatically find and repair clicks. I don¹t recommend allowing the program to automatically remove all clicks, as this can destroy transients that you want to keep, like ³t² and ³p² sounds in speech. You¹ll do better to find them one at a time manually.
Noise Removal Software
There are several specialized software tools designed to remove various kinds of background noise. They can perform many of the same functions as NoNoise, but at a much lower price. For example, DART Pro from Digital Audio Restoration Technology http://www.dartpro.com is a program for Microsoft Windows that includes several filters, click and hum reducers, and gain changers that can be chained together to process your audio tracks in one pass.
Ray Gun from Arboretum Software http://www.arboretum.com is a Macintosh-based program that takes a much simpler approach to reducing noise, by providing presets for attenuating noise, removing clicks and pops, and reducing common hum. This approach may be easier for non-audio professionals to use, but its effectiveness is somewhat limited on anything but minor noise problems.
While these systems are convenient, care must be taken in their use. Post-production pro Mark Berger makes the point that, "piranhas are better than sharks.” This is just an aphoristic way of saying that many small bites are more effective than one large gulp. The idea is to use a little filtering, then use a little gain change, and then use a little compression. In this way each device is doing a little bit, which it does best, and the sum adds up to a more pleasing, effective sound.
According to movie mixer Randy Thom, "One thing that most people don't know about the process of trying to get rid of noise, especially environmental noise (as opposed to electronically generated noise) is that manual gain riding and EQ almost always play a part in what turns out to be the most successful approach.”
The best technique is often to use a digital algorithm to reduce the noise a few decibels. More than 6 or 8 dB will usually generate unwanted digital artifacts unless the noise is very narrow band. And then use some gain riding and dynamic EQ to reduce the noise still further. Some choose to start with the manual part of the process, and then apply the algorithm.
The best way to eliminate background noises is to make sure they are not recorded in the first place. But if you find yourself having to salvage an audio track, there are plenty of tools out there that can help.
Steve Cunningham is a voiceover actor and producer and principle of Acme Voiceworx.
