Audio Global Viewpoint – September 2018

Killing Hiss in Audio

It pops up randomly — ruining otherwise excellent audio takes and often leaving the sound operator perplexed as to the cause. Hiss is an enemy we all fight at various times, especially on video shoots. How can hiss in audio be prevented in the first place?

Essentially, hiss is a broadband noise that spans the entire audible spectrum with greater intensity in the high frequencies. It is similar to the sound of blue noise. Hiss comes from the electronic components themselves, and is usually called self noise. The level of a circuit’s inherent noise is called a noise floor, expressed in decibels (dB).

All audio circuits generate some level of self noise. It is the inevitable result of the heat energy created by moving electrons. The level of noise depends on the quality of the components and the design of the circuitry. This is why more expensive audio gear — which typically use higher quality components and better designs — has a lower noise floor.

Since lower quality audio components have the greatest potential to create noise, it makes sense to look for the weakest link in the audio signal chain when trying to prevent hiss. Unfortunately for videographers, the weakest link is often the camera’s mic preamp.

In most video cameras, the money you pay is for better video, not audio. This is particularly true with low-end DSLR and mirrorless cameras that have a 3.5 mm microphone input jack. Sometimes, when working with lower-end video gear, it is best to disconnect from the video camera completely and do double-system sound.

Azden, a Japanese audio manufacturer, recently published a white paper on audio hiss and offers several solutions that videographers can use to prevent or reduce it.

The best way to combat a noisy microphone preamp, said Azden, is to feed it a audio signal with plenty of gain. The idea is to avoid adding gain using the camera’s preamp (by leaving the volume setting low), which would add noise. The microphone signal should be boosted by another, less noisy means.

When it comes to external microphones for a video camera, use a microphone that has a gain control switch that allows the addition 20 dB of clean gain to the microphone signal before it is sent to the camera. In addition to the 20 dB gain boost, these mics have settings for nominal gain (no boost) and a 10 dB reduction.

AGC control

AGC control

Auto gain control (AGC) is commonly found on many video cameras but many users are not aware of it. AGC is an audio compressor that acts to regulate the perceived volume of recorded audio by turning up the gain when the sounds are quiet and turning it down when things get loud.

The problem is when the AGC raises the audio level during a quiet section of video, it increases everything else too. This includes the level of the preamp’s inherent noise, making it even more audible. The resulting effect is often heard as a whooshing sound as the noise becomes amplified in quiet audio segments. This doesn’t just affect the inherent noise in a low quality preamp, but also the ambient noise from the surrounding environment.

Turn off the AGC on any video camera being used to avoid unwanted noise and hiss in the audio. Unfortunately, for some camera models, turning off AGC is not an option. You might need to check the camera’s manual to know if your camera has AGC and how to defeat it. Otherwise, avoid the problem altogether and go to double system audio recording.

XLR to 3.5mm cable

XLR to 3.5mm cable

When using an XLR microphone, be careful how it connected to a camera with a 3.5mm mic connector. Azden notes there are a few important things to consider when searching for a solution to adapting XLR to 3.5mm connectors.

One, the cable needs to be wired correctly since a balanced audio connection is being adapted to an unbalanced one. Two, the length of the unbalanced section of cable should be relatively short as the set-up is losing the noise rejection capabilities of a balanced connection. And three, the ideal adapter will also use an impedance matching transformer to help compensate for any lost gain as a result of the change over.

Even with the proper adapter cable, the camera’s volume might have to be increased significantly. This can increase the noise floor. That’s because XLR microphones in general are designed with the expectation that they’re going to be plugged into a microphone input that is of a higher quality than what most DSLR or mirrorless cameras can provide. It is best to avoid and XLR to 3.5mm connection altogether.

Azden-FMX-42u mixer.

Azden-FMX-42u mixer.

A better solution is to use a portable audio mixer with XLR inputs. This will guarantee higher quality XLR inputs, each with a variable gain stage. It also allows the addition of the proper amount of gain for the XLR microphone without adding more noise to the signal. This is why most professional crews always use a portable professional sound mixer on location.

What about hiss found after the fact on a video recording? Fortunately, Azden said, there are many software solutions available to remove noise from audio. Video editing software such as Adobe Premiere and Apple’s Final Cut Pro have and hiss and other noise reduction options built in. Also, Apple’s free iMovie app has a very easy-to-use noise reduction adjustment that works reasonably well. It’s in the audio equalizer setting.

Audacity interface. Click to enlarge.

Audacity interface. Click to enlarge.

When using a video editor without noise reduction, consider adding one of many free audio editing software packages. One tried and true free app with noise reduction is Audacity. Using it would require exporting the audio track as a separate audio file and then importing the adjusted audio file back into the video editor. This is a little more work, but it could prevent hiss from ruining the sound on your big project.

Sonex De-Buzzer Plug-in. Click to enlarge.

Sonex De-Buzzer Plug-in. Click to enlarge.

Be warned that the more aggressive the use of noise removal, the more artifacts are added to the sound. Overuse can distort the audio. When applying noise reduction, strike a balance between improving the audio and making it worse.

There is an old saying about all post-production, which came along with the old cliché, “we’ll fix it in post.” Always get the best sound in the field when recording. Listen for hiss and other noise on location. Don’t wait to discover it later. Post-production solutions are never as good as a clean recording to start with.

And while we are on the subject of old cliches, here’s one more. Buy the best gear you can afford. Hiss is indicative of cheaper gear, not the best. This a key reason certain well-built gear costs more than less expensive options. In the end, we all get what we pay for.

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