Audio - Digital noise reduction

The definitions for sound artefacts are generally onomatopoeic - they sound like they are described and termed. Common artefacts include hiss, which is just that - a hissing type of sound in the background of a recording. In contrast, you may have recorded from your source (a vinyl record, for example) perfectly but the .wav file on your computer has clicks, pops and crackling.

PREVENTION IS BETTER THAN CURE: To achieve a great recording, it is best to attempt to reduce noise from the beginning. One of the best ways to prevent artefacting when recording is to use a good quality sound connector with your sound card - digital is preferable, as are gold analog connectors. This is where it pays to have a good quality sound card. Newer and more expensive sound cards have a better signal to noise ratio (SNR or S/N), an indication of your desired audio signal's strength in relation to background noise. Measured in decibels (dB), the higher SNR your sound card has, the better.

Breakout boxes (an external connector box) that move the sound card's audio connectors to the outside of your PC are also useful in reducing the chance of unwanted sound impurities.

When recording, remember to keep sound levels moderate and don't forget to record through the line-in if you have one - not the microphone jack. If you are using the line-in - make sure you've got your microphone muted. Under Windows, double click the yellow speaker next to the system clock in the bottom right of your Windows taskbar. The Volume Controls will appear.

REDUCING BACKGROUND NOISE: Cool Edit 2000 has a variety of benefits for the inexperienced user. It boasts a clean and easy-to-navigate interface combined with great tools that can be accessed even in its demo version.

After installing Cool Edit 2000 from this month's cover CD-ROM, open it, selecting the Save and Noise Reduction features.

Next, open the file that you wish to try to improve. Using the mouse, highlight a portion of the waveform that is basically silent, with no musical information; about one second should be enough. You'll often find that the best place to look is at the start and end of a recording. Press the spacebar to hear what you've selected and you should only hear the noise or hiss you are about to try to remove. Next, select Transform-Noise Reduction-Noise Reduction. Click Get Profile from Selection. You are able to save and load profiles also. Click Close.

This procedure allows Cool Edit to distinguish between the background hiss in your sound selection, and what it needs to remove. This is called setting a noise floor. To remove it from the entire file, press +A to highlight the whole waveform and then go back to Transform-Noise Reduction-Noise Reduction. This time, click OK.

Cool Edit will now display a dialogue box with a noise reduction progress metre. Listen to your results. You should be able to hear less background noise - if any - but if you hear an underwater type of effect, this may mean that you selected some of the musical information when setting the noise floor. This is not a problem: simply go back and try again, it's all about experimenting.

The FFT (Fast Fourier Transform) filter setting can cause the biggest change in quality. It determines the number of individual frequency bands that are analysed. In layman's terms, the noise in each frequency band is treated on its own, so the more bands you have, the more precision you have in reducing noise. If you make the setting too high, you will hear an echoey sound.

In the Noise Reduction window you'll also notice the second graph window - the reduction graph. This is just like a graphic equaliser and will let you draw a curve for the frequency spectrum. If you need noise reduction only in higher-ended frequencies, just use the nodes to adjust the chart accordingly. Click on the graph at the location where you want a node to be placed. The "reduce by __ dB" field will help reduce any bubbling background effects. Experiment with settings between 5 and 100.

In summary: less is more - be subtle with effects and noise reduction, and experiment until you like what you hear.

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Danny Allen

PC World
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