Technical notes index

Principles of audio noise reduction
Principles of audio noise reduction
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            CONTENTS

1. INTRODUCTION
      Distortion
      Frequency response
      Pitch errors
      Noise

2. FIXED PITCH NOISE

3. IMPULSIVE NOISE

4. RANDOM NOISE (HISS)

5. HISS REDUCTION
      Examples
      Companders

6. NOISE REDUCTION EXAMPLES

Impulsive noise (clicks and crackle)


As long as they are clean and quite sharp in their sound, clicks are relatively easy to remove. Before electronic methods were available it was common practise to physically cut clicks out of tape recordings: of course you remove a small section of the sound in doing this, but as long as you use a high tape speed such as 15 inches per second the effect is minimal.

Early electronic methods simply detected the sharp edge of the click and reduced the level for a fraction of a second, either holding it down to the level of the surrounding audio, or muting it all together. Neither method is entirely satisfactory, though the results are usually acceptable: modern digital systems can remove the click and interpolate the audio by looking at the audio either side of where the click was and filling in the gap. Results can be very good, though there is always a danger of instruments with sharp transients such as piano or percussion triggering the click removal and producing distortion: so as with any noise reduction methods it has to be applied with discretion.

This example shows both the advantages and the limitations of click reduction. This LP has picked up some clicks, one of them being quite loud and another being unusually so:


[click1.mp3]


Click reduction using Click Repair has successfully removed the first click, but the second was loud enough to vibrate the pickup arm, and this has caused a 'thunk' which the program cannot recognize as a click. This may not be particularly audible on small speakers, but it can be clearly seen in these expanded waveforms:

Before treatment: and after treatment:
Here is the treated audio:


[click2.mp3]


In this particular instance I could have removed it by simply cutting it, and making the joint a rapid crossfade: but of course if the music had been rhythmic this wouldn't be possible. Cutting the bass just for the duration of the thunk would help.

 This 78 has a typical repeating click caused by a very fine split running across the opening section of the recording:



In this case it can be successfully removed with only the faintest trace left, again caused by the click vibrating the pickup arm.



[click4.mp3]

It's also possible to remove crackle, which is in effect  a very high number of clicks close together. This record isn't worn, but suffers from granulation - bacterial attack has caused a large number of very small hard lumps in the shellac, causing a level of crackle which makes the record quite difficult to listen to. This effect is very common on inter-war HMVs (which this record is), much less so on Columbias, or indeed on HMVs pressed in India or Australia.



[crackle1.mp3]

Removing the crackle has to be carefully done, because in the case of musical instruments with sharp transients, such as piano or percussion, the transients can be mistaken for crackle and distorted; this can also happen with trumpets. Speech is always easier for any form of noise reduction, and in this case almost all the crackle can be successfully removed.



[crackle2.mp3]

This still leaves a considerable amount of hiss, but at least it's a continuous sound which can be reduced by the use of a different technique, which will be discussed on the next page
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© Roger Wilmut.