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Noise, Dolby and all that stuff

Noise abatement expert Martin Sheehan takes the hiss out of recorded music.

NO NOISE reduction system is perfect. Errors in recording cause errors in the system, and vice versa. Anyway, if the recording medium was perfect in the first place, there wouldn't be any need for noise reduction. It's horses for courses, and the world would be a much noisier place without the Dolbys and dbxs of this world.

There are double ended noise reduction systems and there are single ended noise reduction systems. The first work on an encode/decode principle and have to be applied at the recording stage before they can be used on playback. The second can be used just on playback and to deal with noises at source.

The first technique incorporates "compansion" which is basically a combination of compression and expansion.

If, for example, the music we wish to record is 10in tall, and our compander ratio is 2:1, then the music will be compressed to 5in tall before it goes into the tape recorder. Once inside, there are two limits it needs to worry about which form a doorway, if you like. The first is the top of the doorway, the 'headroom', which is how loud the signal can be before it begins to overload and distort the tape (tape saturation). The other rests on the floor — it's all the low level, mucky noise which exists to spoil your quiet moments, because that's when you'll hear it. A dirty, long pile carpet of noise taking up the bottom inch of the doorway.

If the doorway is a foot high (and I think we can say it is) then a 5in tall signal can easily slip through without banging its head on the lintel or muddying its feet on the carpet. As the music exits from the tape recorded on playback, the other half of the compander will expand it back to its true 10in stature. However, it's not just the music that gets expanded. The whole doorway is stretched, having the useful side effect of forcing the carpet downwards, through the floorboards and out of sight (hearing?)

Now throw in some pre-emphasis and de-emphasis. This is delicate EQ-ing which helps to move those frequencies most likely to trip up in the carpet even further out of danger. The above is a loose description of the dbx noise reduction principle. A fine system, but if it's used badly you run into a problem known as breathing — the music 'wheezes' to itself as it's first squeezed then expanded.

The various types of Dolby noise reduction also make use of compansion, but tend to differ from dbx in that the bits of the music most likely to get soiled receive the most treatment, whereas the parts most likely to sneak through unaided are left to do just that.

The Dolby B system most often found on domestic equipment affects the most susceptible high frequencies of the music. The newer Dolby C system does a similar thing to the B system but it does it twice to give double the carpet clearance. The Dolby A system is the professional one which splits the music into four separate bands and caters to the needs of each band individually.

Apart from the double ended noise reduction systems, of which the types mentioned above are the most common, there are also the single ended variety which can be used to help deal with sources other than tape noise.

The simplest of these is the noise gate which simply turns off when the signal level drops below a certain point. This shut down point is variable and is known as the "Threshold". If, for example, you have a background buzz or hiss generated by your effects pedals, you can place a noise gate last in the chain and adjust the threshold to a point between your quietest note and the buzz. Now when you stop playing, the noise gate will take your pedals out of the circuit. The buzz will still be present during the playing but should be masked by the music.

A more advanced form of the noise gate is the expander. This is also a simple version of the back end of our compander system. If we imagine the doorway again with the expander's threshold set at a level corresponding with the surface of the dirty carpet, then any signal dropping onto the carpet will be expanded downwards, together with the carpet, through the floorboards and out of harm's way.

One of the most useful forms of single ended noise reduction is the dynamic noise filter. It is basically a dynamic treble cut control which works progressively downwards as the signal level drops. Consequently, when there is little high frequency information present in the signal, it will act as though the treble control has been turned down taking any high frequency noise down with it. Conversely, as the signal level increases so the filter opens up and when sufficient signal is present to mask any noise the filter will be fully open. Dynamic noise filters can be used in any application where the simple-open or shut principle of the noise gate is not sufficiently versatile and a gentler approach may be required.

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