Tips & Tutorials | September 25, 2014
Learning to visually identify noise and other audio problems by sight will greatly help you—it means you’ll be able to use any software that includes spectrogram technology. In this blog post, we’ll consider how to use a spectrogram to identify audio problems like hum, buzz, clipping, and intermittent noises. Spectrogram technology is included in our award-winning audio repair toolkit, RX.
To learn more, read our blog post on the basics of how a spectrogram works.
Hum is usually the result of electrical noise somewhere in the recorded signal chain. It’s normally heard as a low-frequency tone based at either 50 Hz or 60 Hz depending on whether the recording was made in North America or Europe. If you zoom in to the low frequencies, you’ll be able to see hum as a series of horizontal lines, usually with a bright line at 50 Hz or 60 Hz and several less intense lines above it at harmonics. See the example below:
RX TIP: To zoom in on the spectrogram with RX, you can scroll the mouse wheel, or use the magnifying glass tool.
TIP: Hum Removal is ideal when frequencies of hum do not overlap with any transient useful signal.
In some cases, electrical noise will extend up to higher frequencies and manifest itself as a background buzz. See the example below:
TIP: Hum-removal tools usually focus on low-end hum, so when the harmonics extend to higher frequencies, a denoiser is more effective at removing the problem.
Unlike hum and buzz, broadband noise is spread throughout the frequency spectrum and isn’t concentrated at specific frequencies. Tape hiss and noise from fans and air conditioners are good examples of broadband noise. In a spectrogram display, broadband noise usually appears as speckles that surround the program material. See the example below:
TIP: Denoisers are very effective at dealing with this type of broadband noise.
Clicks and pops are common on recordings made from vinyl—but can also be introduced by digital errors, including recording into a DAW with improper buffer settings, or making a bad audio edit that missed a zero crossing. Even mouth noises such as tongue clicks and lip smacks fall into the clicks category. These short impulse noises appear in a spectrogram as vertical lines. The louder the click or pop, the brighter the line will appear. The example below shows clicks and pops appearing in an audio recording transferred from vinyl:
TIP: Declicking tools can recognize, isolate, then reduce and remove clicks such as these.
Clipping is an all-too-common problem. It can occur when a loud signal distorts on input to a sound card/converter, mixing console, field recorder, or other sound capture device. A spectrogram is not particularly useful for identifying clipped audio—for this you’ll want to work with a waveform display.
RX TIP: Move the slider below the spectrogram to the left to superimpose the waveform display.
As you’ll see in the image below, the clipping appears as "squared-off" sections of the waveform.
Many software programs allow you to zoom in on a waveform and see in detail where the waveform has been truncated.
TIP: Declipping tools can intelligently redraw the waveform to where it might naturally have been if the signal hadn’t clipped. Sometimes, heavily limited audio will also appear “squared off” when zoomed out, but this doesn’t necessarily mean it will sound as heavily distorted. You can zoom in to see if individual waveform tops are clipped.
Intermittent noises are different than hiss and hum—they may appear infrequently and may not be consistent in pitch or duration. Common examples include coughs, sneezes, footsteps, car horns, ringing cell phones, etc. The images below represent two different examples of these noises:
Sometimes a recording may have short sections of missing or corrupted audio. These are usually very obvious to both the eye and the ear! See the example below:
TIP: Noises and dropouts like the example above are often unpredictable, and usually need to be removed or patched manually using a visual/spectral editing tool.
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