What we have discovered so far about noise is far from encouraging. Noise shows up in pretty much all work circumstances, but it is a nuisance we must absolutely get rid of, or at least cut down as much as possible. However, the situation isn't as bad as we think if we consider that noise, although it is present, often has quite a limited amplitude which is easily overpowered by the audio signal. For each kind of noise a more or less effective countermeasure has been found. With reference to the previous paragraph, let's now look at the noise reduction techniques for each of the different mentioned noises.
For HVAC-type noises or those resulting from vibrations, not much can actually be done except to try to attenuate the frequency in question. However, this isn't a particularly "healthy" solution and should only be used in extreme cases, seeing that this way not only is the noise attenuated, but also the frequency band of the audio signal we are manipulating. For the buzzing resulting from the irregularity of the power supply system, the best remedy consists in two independent power supply panels, one for HVAC appliances and one for the audio equipment.
As the name implies, these are emissions that carry an electric or magnetic field (for a more detailed description of electromagnetic waves refer to any physics textbook). Both generate an interference on the sound signal being transported on a cable whose frequencies are the same as those being transported by the electromagnetic waves. These interferences mainly affect the microphone cables seeing that very low intensity signals travel through them. Two different expedients are employed to protect these cables from the interferences. To stunt the electric field a Faraday's cage is created, in other words the signal-conductors are coated with metal. This kind of coating eliminates the electric field inside it (once again, a physics textbook would supply more specific information on the processes involved).
To halt the magnetic field (even though the following technique works for all kinds of interferences) two conductors wrapped in a spiral, each carrying the signal, are present inside the microphone cable. The sound signal passes through the first of the two cables, whereas through the second one we have the same signal inverted in phase. The two conductors are wrapped in a spiral so that both have more or less the same amount of exposure to the magnetic field. When both the signals reach the mixer, the second signal is once again inverted in phase and the two signals are added together. This leads to a doubling of the original signal's amplitude and the elimination of the noise which at this point is inverted in phase between the two conductors.
The following diagram illustrates the various steps of this operation:
Let's take a more detailed look at the single steps of this manipulation:
(a) to make things easier let's presume that the input signal is a sinusoid
(b) the signal is divided into two and one of the two copies gets inverted in phase
(c) the signals travel through the cable and are subjected to the same electromagnetic interference, therefore having the same distortion
(d) the signal that had previously been inverted in phase gets inverted once again and now both the sinusoids are in phase again, whereas the noise is inverted in phase between the two signals
(e) the two signals get added together, which results in obtaining the original sinusoid, its amplitude now having doubled and the noise having finally been eliminated
The same trick is used on humbucking pickups on electric guitars (if we take a close look at a humbucking we'd see that it consists in two single-coil pickups. That's why the sound of the humbucking is the most powerful sound of all the pickups).
This type of connection is called a balanced connection [Balanced electric connections ] and is widely employed in professional work contexts. When on the other hand, a connection includes just a conductor carrying the signal plus the ground (the metal sheath that envelops the conductor), the connection is called unbalanced [Unbalanced electric connections ] . In this case protection from the electric field still remains, whilst we are no longer screened from the magnetic field.
In this case noise covers the entire audible frequency spectrum, therefore we need to act on the whole sound signal. The classic example is the hiss noise which is almost second-nature in analogue recordings using magnetic support. The process for reducing this specific noise entails three successive phases: compression, expansion and equalizing.
In the previous diagram the noise reduction process is illustrated. In the example the signal we will record has a 90 dB dynamic and the noise is above the dynamic's minimum value, in other words it would cover the original signal's lower sounds. Let's then apply a 2:1 compression to the entire audio signal and amplify it before going on to record it (encoded signal). This way, thanks to the compression, we have managed to amplify the signal without saturating it and at the same time the whole of our signal's dynamic is now above the noise (signal on magnetic tape). When we recover the signal from the tape we apply a 1:2 expansion, thus restoring the original signal. The good news is that now the noise is far below the minimum value of the dynamic range. We've reached our goal: the lowest sound on our signal now covers the noise: noise is no more!
In the following diagram the process is illustrated from a different point of view. In this case we can see how during the encoding phase the compression curve (in this case a straight line) squashes the input signal's dynamic. In the central phase we can see how by amplifying the compressed signal before recording it, we lift it above the tape's background noise. The decoding phase illustrates the expansion curve, and how it gives us a background noise that is below the original signal's minimum value.
This is the fundamental principle for broadband noise reduction. From this starting point we can go on to performing other manipulations to further refine the effects of our noise reduction. One of these consists in applying an equalizing operation called pre-emphasis. Seeing that the hiss noise present in magnetic tapes is perceived more prominently at high frequencies, we can improve the compression/expansion process by amplifying the original signal's high frequencies. This process is illustrated in the following diagram. The first phase shows pre-emphasis, namely, where amplification of the signal's high frequencies takes place. The second phase shows the recording of the signal onto the magnetic tape. In the third phase the high frequencies get attenuated in order to restore the original signal; this way the high frequencies present in the noise are also attenuated.