If I hadn’t been watching, I wouldn’t have noticed it.
I had been editing jungle field recordings from the northern mountains of Thailand. They sounded fine. I heard nothing unusual about the dense night insect drone I was mastering. But if I hadn’t been keeping an eye on my scopes and meters, this rare field recording flaw would have slipped by: DC offset.
I had encountered this error only a few times before while editing sound effects. Curious, I searched to see if any other field recordists commonly had this issue. What I found is that there is little information that explains what it is or how to fix DC offset in field recordings. New field recordists may have no idea what this problem is, how to detect it, or more importantly, how to repair DC offset in their sound effect recordings.
So, today’s post is a quick look at the issue. What is DC offset? How do you fix it? And how does it affect recording sound effects?
Let’s learn more.
What is DC Offset?
What is DC offset? Let’s understand the term, first.
“DC” of course is short for direct current, an electrical charge that flows in a single direction. This is contrasted by AC, or alternating current, which is electrical charge flow that can change direction.
DC offset (also known as DC bias) is when problems with the analog audio signal cause the waveform to be shifted or “offset” from its expected place.
What is this “expected place”? Well, you’re likely familiar with the typical waveform image: amplitude on the vertical axis and time on the horizontal axis:
Typically, waveforms always span the “zero crossing” of the amplitude line. However, DC offset introduces an error: the waveform no longer crosses at zero. Instead, it has a shifted crossing line either above or below zero, like this:
What Causes DC Offset?
So, what causes DC offset to happen?
An offset occurs when the fixed voltage is shifted somewhere in the audio chain. It happens in the analog side of the signal chain, before it is converted to the digital signal.
What causes this shifting? It can come from any number of issues, from a bad analog-to-digital (A/D) converter, broken hardware, or powering issues.
Why is DC Offset a Problem?
The most obvious problem is that playing back a sound effect with DC offset will produce a click at the beginning or end of the file. This can also happen when portions of the file are copied and pasted and overlap. Why? Well, since the waveform of a problematic DC offset file is shifted, the audio no longer intersects with the zero crossing line. When the waveform is not zeroed, it produces a pop or click.
Of course, clicks and pops happen with normal field recordings as well. So, it may not be apparent that a field recording is suffering from DC offset. This highlights another issue: if an editor assumes a click is occurring from another source, they may not be aware there are greater problems to fix. After all, DC offset in itself doesn’t introduce a significantly different sonic aspect to sound effects.
There are more subtle issues, too. Since the waveform is offset from its typical position, DC offset may produce less headroom. Think of it like this: if the recording is offset by 5 dB because of a DC offset error, that means you have less headroom to play with. Normalizing the same file will be 5 dB quieter than what it could be without the offset. In short, the recording will not be at its loudest when amplified.
DC offset adds noise floor and causes low level distortion. This is typically inaudible, however it may become apparent when the sound is altered in some way, such converting it to an MP3, or altering its frequency, and so on.
How to Detect DC Offset
OK, now you’re convinced you don’t want DC offset in your field recordings. How can you find DC offset?
As mentioned, other than the clicks and pops introduced into a file, it’s difficult to detect DC offset just by listening. So, instead of your ears, keep your eyes open for two issues.
The first is by watching the sound effect’s waveform. Field recordings plagued by DC offset will look strange. They’ll be shifted off-centre along the waveform amplitude. Here’s an example of the jungle field recording with DC offset:
In most cases, just watching the waveform will be enough to detect the problem, as long as you know what you’re looking for.
Sometimes, though, the shift is subtle. In this case, it’s helpful to keep an eye on your scope. I use iZotope’s Insight 2 when mastering all of my sound effects for Airborne Sound and all my other clients. If you have your Vectorscope set correctly, you can spot the symptoms of DC offset.
Here is what the same jungle sound file showed me:
Notice that the audio data is concentrated in the bottom portion of the scope, which is unusual. Also note that the audio is in phase, as indicated with the “+1” to the right of the image above, indicating further strangeness.
What’s interesting is that the error wasn’t apparent in Insight’s polar sample or polar level scopes. It was only apparent on the Lissajous scope. Here’s a gif switching between the three scopes:
Some of iZotope’s other tools can help with this as well. RX will display DC offset in the Waveform Statistics panel, expressed as a percentage (hover over it and you can see the decibel level).
iZotope’s Ozone is helpful too. Click the Dither button on the bottom right, then click I/O button beside it. This will show a meter you can use to track the extent of the DC offset in your field recordings. (NOTE: the Maximizer module must be enabled to see this.)
How to Repair DC Offset
So, how do you remove DC offset once it is there?
Thankfully, fixing DC offset is pretty simple. To repair the file, it’s just a matter of eliminating the DC component of the audio signal.
The easiest way is to use a high pass filter (also known as a low cut filter). Set your EQ at 5 Hz, and it should fix things. Some EQs don’t filter that low, and if so, setting it at 20 Hz or 30 Hz will do the trick.
Pro Tools has its own AudioSuite DC Offset Removal plug in. Just select the clip and process. Similarly, Reaper has its own tool, the JS: DC Filter.
For RX’s Ozone, enable the Maximizer module, click the Dither button on the bottom right, then click the I/O button, and then click the Filter button under the DC Offset meter. This will apply a 1 Hz filter to the audio. Ensure this is applied last in the audio chain.
Want more control of the outcome? In RX, note the DC offset percentage in the Waveform Statistics window as mentioned above. Then, open the Signal Generator module, select the “Silence” tab, set the slider to the inverse of the percentage seen in the Waveform Statistics window, click on the “Mix” setting on the right, and apply.
Any of these will fix DC offset in sound effects.
DC Offset Sound Effect Samples
Here are some samples of the DC offset jungle file.
Here’s the first, with the offset:
And the second with the offset repaired:
You’ll notice even the low-res Soundcloud waveform shows a difference. Also, you will hear the aforementioned “pop issue” on the first file, one of the signs of a field recording affected by DC offset.
You can download them both to experiment with DC offset yourself, just click the arrow in the upper right of the players above. They’re free to use, just please do not share them, even free of charge. (Please note they are raw, unfinished files, and are not improved or mastered in any way.)
My thanks to Eric T for his assistance with this article.
Want to know more about mastering sound effects? There are plenty of articles in the Creative Field Recording membership area.
You can also see more field recording mastering articles here.