There is a group of processors which are designed to alter frequency response and these fall neatly into two groups — those which trim the output to give a more 'realistic' (whatever that means!) sound and those which alter it for effect.

Equalisers

The first group can be generally termed the equalisers. This includes everything from a simple single pot tone control up to a full blown graphic equaliser.

Simple Tone Controls

The simplest tone control around is the single pot type, as found in most electric guitars. The two basic types are shown in Figure One. They are based on the use of a component called a capacitor. Capacitors allow high frequencies to pass through them much more easily than low frequencies. In musical terms they pass treble signals much more easily than bass ones.

A look at the Bass Tone Control circuit helps us to understand how it works. As the capacitor passes treble rather than the bass, the treble will pass unimpeded to the output jack whereas the bass signals have to battle their way through the pot. Thus the pot will progressively reduce the amount of bass arriving at the output jack as it is turned — it is a bass cut control.

A look at the Treble Tone Control tells a different story. In this circuit the bass signals get through uninterrupted but the treble signals are progressively bled away to earth as the pot is turned — it is a treble cut control.

The treble cut circuit is the most popular type used in guitars and as higher value capacitors produce a bassier tone (ie pass lower frequencies) this gives you a very easy (and cheap) way of customising the sound of your guitar. The capacitor(s) fitted can be easily replaced as a cost of about 20p so it's not going to cost you a fortune to experiment. As a rough guide try a capacitor in the range 0.01 uF to 0.1 uF — you can get a rough idea of where to start by either doubling or halving the value that is originally fitted.

It is worth noting that both of these circuits do not boost any frequencies — they only subtract from the original signal. As such they are generally referred to as passive tone controls. To be able to boost or cut frequencies at will we need to add amplifying stages and produce an active control.

Graphic Equalisers

One of the most useful of the active tone controls is the graphic equaliser. This consists of a number of separate active tone controls all in one box. These tone controls differ fundamentally from the previous ones in that they boost or cut a particular frequency. To achieve this two types of tone filter are used — the bandpass and the bandcut (notch) filter. Typical response curves for a bandpass filter are shown in Figure two. Three possible curves are shown, each one peaking at 500Hz — the resonant frequency. Response curve A shows a fairly sharp curve (though it could be a lot sharper) whereas B and C are more spread out. Response curve A has a reasonably high resonance or Q whereas the Qs for B and C are lower. A filter with a high Q affects only a sharp band of frequencies — hence curve A shows a filter which effectively operates in the range 400-600Hz whilst B operates around 350-650Hz and C from 200-800Hz, but they are all 500Hz filters!

In selecting the Q for his filters the manufacturer will need to look at the overall frequency range that he wants the graphic to cover and how many filters (or bands) he wants to cover the range. The idea is to overlap the frequency response curves from one filter to the next to cover the entire range in as smooth a way as possible. In practice this means that a six band equaliser is likely to have filters with a much lower Q (wider bandwidth) than, say, a 20 band equaliser.

So far we've only looked at the boost side of the coin. The bandcut filter (Figure three) works in the reverse way to the bandpass but all the comments about Q and resonance apply. A small five band equaliser (having five pairs of bandpass/bandcut filters) would therefore have a response curve looking something like Figure 4. The overall curve is pretty uneven and could be significantly improved by slipping another four filters, of similar Q, in the gaps.

The boost/cut is performed, for each band, by a slider potentiometer (hence the term 'graphic' — you can see at a glance which frequencies are cut or boost as the slider knobs form a graphic representation of the frequency response modification) which is usually calibrated in dB. Three dB (three decibels) represents a doubling of voltage (obviously -3dB is a halving) and boosts/cuts of around ±12dB are fairly common. With that amount of alteration possible it really pays to go for a graphic having long throw slider pots as these give you a lot more control and repeatability.

Parametric Equalizers

If a graphic is so good why are people raving about parametrics? Well it's all down to the problem of Q (or bandwidth — call it what you like). If you want to control a specific frequency (such as the particular frequency that induces feedback howl in a foldback system) you have a bit of a problem with a graphic. For a start Sod's Law states that the particular frequency you want to cut is in one of the troughs in the graphic response curve (such as the one marked with an asterisk on Figure four). At that particular frequency a -12dB graphic would not be capable of giving -12dB of cut (how much it would give would depend on the Q of the filters) and in cutting the troublesome frequency you would also have to lose a large chunk of your top end response. One answer is to get a graphic with more filters but this only reduces the problem, it doesn't eliminate it!

Enter the parametric. A parametric equaliser usually has only two or four filters performing a similar boost and cut function to those in a graphic. But the difference is each filter can be scanned along the frequency range (ie its resonant frequency can be altered) and the Q of the filter (ie its sharpness) can be adjusted. Hence using a parametric you choose the filter in whose frequency range your problem lies, adjust its resonant peak to conincide with your problem frequency and then increase theOso that frequencies either side of the troublesome frequency are unaffected.

It's a matter of different horses for different courses. For equalising a room's acoustics a graphic is the obvious choice but for boosting up a dead spot in a guitar's response the parametric wins.