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[This article is based on the Fries's new book, Digital Audio Essentials.]

Virtually all audio recordings, no matter how they were created, will contain some amount of noise. Noise is simply any unwanted sound, from hiss, clicks, and static to a door slamming or someone coughing. Noise levels are typically higher in analog recordings, but some noise exists in all digital recordings as well.

The good news is that noise is only a problem if its level is above the threshold of hearing. If a small amount of noise is present, but at such a low level that no one can hear it, who cares? Well, some people might, but if you can’t hear it, it’s not really doing any harm.

Even noise above the threshold of hearing is not necessarily a problem if the noise is masked by louder material. For example, hiss, clicks, and pops that would be very annoying in a classical piano recording might be completely unnoticeable in a heavy metal song.

Distortion is similar to noise, but there is a key difference: Noise is something that’s introduced to the signal from outside, such as hum, whereas distortion is a modification of the signal itself. Hit the Pause button and the distortion goes away, but the noise will remain. Another type of noise called thermal noise is generated by the electronic circuits themselves and can never be eliminated, though it can be minimized by the use of the higher quality components typically found in high-end sound cards and audio interfaces.

Types of Noise

Following are descriptions of several common types of noise. Different tools are needed to remove each type.

Broadband noise, also called continuous noise, includes sounds such as hiss and static that span a wide range of frequencies. There's a certain amount of broadband noise in all recorded audio—even in recordings made by professional engineers with top-of-the-line equipment. The important measure here is the ratio of the average level of the program material to the average level of noise. This is called the signal-to-noise ratio (SNR) and is specified by the relative difference between those levels in Decibels (dB).

Figure 1 shows the typical signal-to-noise ratios for several types of recording media in good condition. Figure 2 shows how broadband noise appears in the “silence” at the beginning of a track and how a click appears at different zoom levels.

Fig. 1: SNR Figure 1. This illustration shows the relative signal-to-noise ratios of typical recording media in good condition. The average level of noise is called the noise floor.

Fig. 2: Types of Noise Figure 2. In the image on the left, broadband noise appears as a fuzzy horizontal line in the “silence” at the beginning of the track. The first beat of the song contains a vertical line that indicates a click. In the image on the right, a zoomed-in view of the click reveals several smaller clicks.

Narrowband noise is limited to a narrow range of frequencies—typically a fundamental frequency and its harmonics, or whole-number multiples. Examples include 60-cycle hum and the whine of an electric motor. A recording containing 60-cycle hum will likely also contain hum at 120Hz and 180Hz (two and three times the fundamental frequency, respectively). Hum is often caused by incorrect grounding and poorly shielded cables.

Impulse noises include brief, sharp sounds such as clicks and pops. Clicks are usually caused by small scratches or specks of dust on a record. They also crop up during digital recording when digital devices aren't synchronized. Pops are caused by more severe scratches. When you view a recording in a waveform editor, clicks and pops show up as steep spikes or dips (see Figure 3). Clicks are narrow spikes and may span just a few samples. (Each sample or data point at standard CD resolution is 1/44100 of a second.) Pops last longer than clicks, and can affect the signal for several dozen samples.

Figure 3. This illustration shows examples of common “impulse” noises. From left to right: a large pop, a typical click, a smaller “tick,” and a digital click.

Irregular noise includes sounds such as background conversation, traffic, and rain. These types of sounds are very difficult to remove because they are made up of many random sounds that vary in frequency and loudness.

Noise Sources

There are many places where noise can get into an audio signal. Most are when the signal is in analog format. Following are some common sources of noise.

Original recording environment. When an original recording is made, noise will be picked up by any microphones and from the cables and circuits that handle the signal while it is in analog form. Once the signal is captured or converted to a digital format, it will be fairly immune from noise unless you edit it or re-encode it.

Defects and wear of analog media. When you make a digital recording from analog media, such as a record or tape, noise will be picked up from the media when it’s played, even if it is in good condition. Even more noise will be introduced if the media is worn, dirty, or damaged.

Analog equipment and cables. When you play an analog recording to capture it in digital format, noise will be picked up as the signal passes through the analog circuits of the tape deck or turntable, the preamp, and any interconnect cables.

Analog circuits in your sound card. A poorly shielded sound card will pick up noise as the signal passes through its analog circuits on the way to the digital-to-analog converter (DAC or A/D). The converter adds more noise from quantization errors as the voltage of the analog signal is sampled and rounded to whole numbers.

Errors during digital signal processing. Even when audio is in digital format, it’s not totally immune to noise. When you edit digital audio you can easily introduce noise in the form of digital clicks and noise from quantization errors that are an unavoidable result of digital signal processing.

Noise Removal Strategies

Tip: Listen with Your Heart (Shape)

If you plan to record voice or live music, you’ll pick up less background noise if you use a cardioid microphone, because it has a directional pickup pattern that cancels out noise from the sides or rear. Professional quality microphones usually have 1/4-inch phone or XLR connectors, which are not compatible with most consumer sound cards. You can get an adapter, but an even better choice would be to use an external audio interface like the Roland UA-3FX, which has a 1/4-inch microphone input built-in.

Noise removal always involves trade-offs, and often it can end up doing more harm than good. The type and severity of noise, the nature of the recorded material, and the noise removal software you use will all affect how much noise can be effectively removed. Your goal should be to reduce noise to an acceptable level, without excessively altering the desired material.

If you're too aggressive in removing noise, you'll also remove some of the desired program material. Even worse than losing good sounds, you may get sonic artifacts that are produced when noise removal tools are pushed to the limit. If you overdo it, you’ll hear sounds that resemble singing birds, robotic voices, and breathing. You can hear examples of both proper and excessive noise reduction at the end of this article. Following are seven strategies for effective noise removal.

1. Minimize Noise Before You Record

The first rule of recording is to minimize noise before you record. Properly cleaning your media and using decent playback equipment and cabling will eliminate many noises that are difficult, if not impossible, to remove later on.

When recording from a turntable, clean the record and make sure your stylus and cartridge are in good shape. When recording from a tape deck, make sure the heads are clean and demagnetized. In either case, use good quality shielded cables to reduce noise from electrical interference.

The components inside a computer generate a tremendous amount of electrical noise. This noise can be introduced into the audio signal whenever it is in an analog format. Poor shielding allows noise to leak into the signal, and low quality D/A converters add even more noise.

If you have a poorly shielded sound card, it’s fairly straightforward to replace it with a higher quality card. The Creative Labs Sound Blaster Live! and the Turtle Beach Santa Cruz cards are good choices for PC users. If you have a Mac with an open PCI slot, you can upgrade with an M-Audio Audiophile sound card. For notebook users, a PC Card (PCMCIA) audio interface such as the Echo Indigo offers a compact upgrade.

The best way to avoid picking up electrical noise from inside the computer is to use an external audio interface, which typically connects to your computer via a USB or FireWire port. If you have an iMac or other computer with a built-in sound card and no free expansion slots, an external audio interface might be your only option for cleaner audio. These devices often offer additional features.

The Edirol UA-3FX shown in Figure 4 is a USB audio interface that also functions as a simple mixer. It uses 24-bit A/D converters and supports both Macs and PCs. The UA-3FX has optical and S/PDIF (coax) digital input and output jacks, and stereo RCA jacks for analog connections. It also includes a headphone jack and a switchable phone jack for plugging in a guitar or microphone.

Fig. 4: Edirol Interface Figure 4. A USB audio interface, such as the Edirol UA-3FX, moves the digital-to-analog conversion outside of the computer, so it's less susceptible to noise than an internal sound card.

2. Set Proper Recording Levels

Tip: Play Your Cards Smart

Place your sound card in the slot farthest away from the computer’s power supply and processor and place your video card as far away from the sound card as possible. That can help reduce the introduction of electrical noise from other components inside the computer.

Any time you record audio, it’s important to set the recording level as high as possible to minimize noise. However, you don’t want to set levels so high the signal “clips,” because that will cause ugly distortion.

Waveform editing programs like Sound Forge and Peak have level meters that are more precise than the simple meters in the volume control panels of Macs and PCs. Level meters are usually labeled in dB (Decibels) with 0dB equal to the maximum level. Levels below the maximum are shown in negative dB, and the lowest possible level is referred to as infinity, which is indicated by a character that looks like the number eight laying on its side.

When using dB meters to set recording levels, make sure the peaks average around –6 dB and don’t exceed –3 dB. That will normally provide enough headroom to avoid clipping, while maintaining a good signal-to-noise ratio. If your recording level is set too low, any noise picked up by the analog circuits in your sound card will be more apparent, because it will be proportionally louder. Figure 5 shows examples of recording levels set too low, too high, and just right.

Fig. 5: Levels Figure 5. The level on the left is too low, with the peaks around –6 dB and the average around –12 dB. The level in the center is too high, with the peaks at 0 dB, and clipping in one channel. The meter on the right shows optimum levels with the average around –6 dB and peaks around –3 dB.

3. Use Different Approaches for Different Types of Noises

Tip: Hum a Bar and I'll Static

A good test, before recording from source like a turntable or tape deck, is to record a few seconds of silence from the gaps between tracks and then play it back. Listen for hum, hiss, and static. Hum may indicate a faulty cable or improperly grounded equipment. Hiss is unavoidable on tapes. Static or hiss when the source is paused indicates electrical noise from the external audio equipment, or from inside the computer.

You’ll find a variety of noise removal tools in good audio restoration programs and in many waveform editors. These range from general purpose “notch” and broadband noise filters to specialized click and pop removal tools and rumble filters. Most of these tools are highly configurable, so you can fine tune them to remove noises with different characteristics.

For example, the clicks, pops, and surface noise on a 1930s shellac record will have different characteristics from those on a vinyl LP, so the parameters that worked well for your Led Zeppelin platter are probably inappropriate for those Benny Goodman 78s. But if you have a 20-disc album of Beethoven’s sonatas, the noise reduction settings that worked well for the first disc should work well for all the others.

Some audio restoration programs, such as Diamond Cut DC6, have a wealth of noise reduction presets for common types of vintage recordings, from wax cylinders and acetate discs to shellac records and vinyl LPs. If you find other settings that work particularly well for certain types of recordings, you can create your own presets.

4. Be Conservative; Work in Stages

You can never remove all the noise without noticeably damaging the source material. Be conservative, work in stages, and keep backup files.

Noise removal tools can be effective when applied correctly, but if you process an entire file with settings that are aggressive enough to get rid of all the noise, chances are you’ll also get rid of a lot of music, especially “spiky” sounds, like cymbals and snare drum hits. You also risk adding audio artifacts such as the dreaded singing birds and robotic voices.

When working with a recording that contains a variety of clicks and pops it’s best to be conservative and shoot for the major clicks in one pass, the smaller clicks in a another pass, and then manually remove any stubborn clicks that are left. It’s always a good idea to work on a copy of the original file and to experiment with different settings until you're satisfied with the results.

5. Preview the Result; Listen to Just the Noise

Better audio restoration programs and plug-ins have preview, bypass, and noise-only options. The preview option lets you listen to the result and fine tune the settings before applying the change. The bypass option lets you switch back and forth between the processed and unprocessed signal to compare the results. The noise only option (sometimes called difference or keep residue) lets you hear exactly what is going to be removed. If you hear too much music mixed in with the noise, you can back off the settings a bit and try again.

If your program doesn't have a preview option, run the noise removal tool and then listen to the result. If you don't like what you hear, use the Undo command, and then repeat the process with different settings.

6. Record at a Higher Resolution

Any type of digital signal processing (including noise reduction) introduces quantization errors that accumulate and add noise to the signal. If you apply different types (or the same type with different settings) of digital signal processing, these errors can quickly add up to the point where they are objectionable. To retain the best possible fidelity, record at 24-bit resolution, remove the noise, and then convert back to 16 bits before you save the file. This takes up more disk space, but that’s only temporary. Quantization errors are not an issue if you are removing a few clicks individually or just trimming off silence.

Another option if you need to remove several different types of noise is to use a program like Diamond Cut DC6 or a plug-in like Sound Soap Pro that allows you to chain, or apply, multiple noise-removal filters in one process. All signal processing is done at a high resolution and the result is converted back to 16-bit resolution when the processing is complete.

Fig. 6: Sound Soap Figure 6. SoundSoap Pro from BIAS is available in both Mac and Windows versions.

7. Use the Right Software

Although you could drop thousands of dollars on high-end audio restoration hardware and software, such as CEDAR Audio or the Sonic Studio HD system, you can get decent results from dedicated programs and plug-ins without breaking the bank. The following table lists several capable audio restoration programs and plug-ins. Roxio's CD Spin Doctor is fairly limited compared to the rest, but it’s worth trying if you already have it as part of the Toast Titanium bundle.

Program

Manufacturer

Platforms

Price

CD Spin Doctor (Part of Toast Titanium)

Roxio

Mac

$79.95

Diamond Cut Millennium

Diamond Cut

Windows

$59

Diamond Cut DC6

Diamond Cut

Windows

$199

Noise Reduction 2.0 (plug-in only)

Sony Pictures Media

Windows

$279.97

SoundSoap 2.0 (no click and pop removal)

BIAS

Mac, Windows

$99

SoundSoap Pro

BIAS

Mac, Windows

$599

If you're digitizing LPs that are in good shape except for some occasional clicks, you may be able to get by with the vinyl restoration tools included in “lite” versions of professional programs such as Peak and Sound Forge. If your budget is really tight, you could even turn to the basic restoration tools found in CD burning programs such as Easy Media Creator, Toast, and Nero.

Fig. 7: DC Pro Figure 7. The continuous noise filter in DC6 includes presets for many different types of recordings. It lets you sample noise from recordings, fine-tune the settings, and create your own presets.

But if you're dealing with vintage records or any recording that has excessive noise, consider purchasing a full-blown audio restoration program or plug-in. If your time or your recordings are valuable, the investment will pay off many times over. These programs use more sophisticated noise removal algorithms, let you save and reapply settings, and offer such essentials as a “noise print” feature that determines the frequency distribution of the noise to remove. Just as important, you can preview the noise removal before actually processing the audio.

Mac users with Peak and PC users with Sound Forge can turn to the $99 Sound Soap noise removal plug-in, but it does not have tools for click and pop removal. SoundSoap Pro adds click and pop removal, and other advanced features, but it will set you back $599. PC users with DirectX-compatible sound editors such as Sound Forge can turn to Sony’s $279.97 Noise Reduction suite of plug-ins (which comes bundled with a click and crackle removal tool), or the $199 Diamond Cut DC6, one of the best audio restoration programs on the market.

Music Examples

Following are clips of Scott Joplin's “Solace” recorded from a vintage record: first with no noise removed, then with noise reduction applied correctly, and finally with noise reduction applied using settings that are too aggressive.


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