We don't live in a perfect world - many things conspire to introduce unwanted sound into your treasured recordings in the form of noise. Noise is a term which covers many different things - there are many different types of noise. Arguably, some people go out of their way to buy what, to other people, appears to be noise (of a sort). Here are some of the most common forms of noise:
- Electrical Noise - introduced by mains power supplies in your equipment. This takes the form of a low hum based on the frequency of your AC (alternating current) power supply (50Hz in Europe, 60Hz in America). In severe cases, the signal may also involve high-frequency "echoes" of the signal known as harmonics. These will appear at multiples of two up the frequency scale (i.e. 100,200 in Europe 120,240 in America).
- DC Bias - 0Hz audio introduced by DC (direct current) power supplies. This is usually inaudible, but can cause a sharp pop at the beginning or end of a recording. It also limits the dynamic range of a recording.
- Tape Hiss - caused by dirt on tapes or just low grade tape. All analogue tape has some degree of this and it sounds like hiss or, more accurately, white noise - a random collection of different frequencies at different amplitudes.
- Rumble - caused by the mechanics of your tape deck or record player. This is a low frequency sound.
- Warping - caused by old records, particularly those left in the sun, which have become slightly bent. The needle will move at slightly different speeds as it goes up and down the twisted surface and change the frequency of the audio up and down as it goes.
- Clicks and Pops - caused by dirty or scratched records, dirty styli etc - these take the form of sharp popping sounds during recording.
Interesting Note: the use of the word white in conjunction with noise belies the fact that noise doesn't come in just the one 'colour'. Audio engineers also recognise brown and pink noise, which are white noise filtered to exaggerate particular frequency ranges. These forms of noise don't generally appear as unwelcome noise on storage media but are deliberately generated in order to check sound setups for problems such as feedback.
Many types of noise are caused by wear and tear on LP's and cassettes - one of the reasons for the popularity of CD being that it doesn't suffer from them. Early CD's often bore the legend "Compact Discs can show up limitations of the source master" as the old recordings had been made on tape and would therefore be subject to some wear and tear. Digital Remastering is the process of taking old tape masters and cleaning up the sound, producing a cleaner digital recording to remove many of these limitations.
Most noise reduction methods for broad sprectrum noise (i.e. noise at a wide range of frequencies such as hiss) work by looking at a portion of audio which contains only noise to work out what to remove from the rest of the material.