Continuing our look at processors which modify the signal level, we start with the noise gate.

Noise Gates

The trouble with effects, particularly distortion pedals, is that they often produce unwanted noise and hiss. This is not always a problem as the noise is usually masked by a stronger audio signal. We run into trouble during pauses in the music when the noise becomes audible. Now, if you're really nimble footed you could connect a volume pedal at the end of the effects chain and take the volume down in the gaps. This is okay if the stuff you're playing is relatively slow but you could end up with a hernia trying to keep pace with Mark Knopfler! The obvious answer is to replace the relatively slow reacting mechanical system with a fast acting electronic one — enter the noise gate.

Once again our old friend the voltage controlled amplifier (VCA) comes into its own. It is linked up to a level sensing circuit in such a way that if the input signal is above an adjustable threshold the VCA allows the signal through, whereas a signal below the threshold causes the VCA to switch off. (See Figure One).

In practice the threshold level is set just above the noise level so that signals louder than the noise level will pass through the VCA but as soon as there is only noise present the VCA shuts down (Figure Two). This sounds like a wonderful cure-all to eliminate noise, but you don't get something for nothing and noise gates present problems of their own. For example, if you hit a guitar chord the noise becomes more significant as the chord decays. Strings tend to decay erratically at the end of the decay causing the input signal to wobble back and forth across the threshold level, which in turn causes the gate to switch rapidly on and off chopping up the sound. To avoid this many noise gates have a built in delay so that the gate does not switch off abruptly but does so over a short period of time (usually about a tenth of a second) — in better units this delay (or decay) time is adjustable. This explains why, in Figure Two the output signal was not chopped to ribbons every time the waveform dropped below the threshold. There again if the noise level is quite high the threshold must be set so high that it chops out a considerable portion of the audio signal. This leaves us with the strange conclusion that noise gates are only okay if you have a small amount of noise. With large amounts, or a weak audio signal, they tend to do more harm than good.

Attack-Delay Pedals

An attack-delay pedal works in a very similar way to a noise gate with the major difference that whereas the noise gate switches on immediately the input signal rises above the threshold, the attack-delay fades up the signal over a period of time which is adjustable. This enables a guitarist to alter the attack time from short, giving a sound like a violin, to long, like an organ swell. The more expensive units have two threshold controls, one for the switch-on threshold (attack) and one for the switch-off threshold (decay).

Volume Pedals

We're all familiar with the common or garden volume pedal. They come in two basic varieties, the mechanical and the photoresistive types. In the mechanical type the movement of the pedal simply turns a potentiometer shaft (Figure Three). Each manufacturer has their own pet method of doing this with differing results as far as the range of volume control is concerned. The one shown in Figure Three is like a car rack and pinion steering system in reverse. The differences in linking systems affect the volume range available as a full sweep of the pot requires a turn of about 270° from full onto full off. Most volume pedals don't manage this and so you can adjust the pot position to get differing sweeps, say from off to three quarter volume, or from quarter volume to full volume — I haven't yet found a mechanical pedal that gives full onto full off!

The photoresistive pedal essentially does the same job but electronically. A photoresistor (often known as a Light Dependent Resistor or LDR) has an electrical resistance which varies according to the amount of light falling on it. The gearing and potentiometer of the mechanical system are replaced by a lamp, an LDR and a tapered shutter, moved by the foot pedal which runs between them (see Figure Four). Often the LDR is linked to a VCA so that the light falling on it varies the VCA's gain.

Each type has its advantages and disadvantages. The mechanical system is cheaper and needs no power supply but both gears and potentiometer tracks can wear leading to noisy operation, both mechanical and electrical. Photoresistive pedals are usually smoother in operation, are not so prone to noise and usually offer a wider range of volume control but are more expensive, require a power supply to light the lamp, and if the lamp blows in the middle of a number you're stuck! Still, you pays your money and you takes your choice.

Whichever type you go for the volume pedal is without doubt one of the most versatile pedals you can buy, offering, in addition to normal volume adjustment, control of attack-decay charcteristics, a mechanical noise gate, a (slow) tremolo unit and a way of controlling sustain and feedback (particularly if you place it after a compressor-fuzz chain). Normally a volume pedal is placed at the far end of your effects chain but it's well worth experimenting with it in other places too (no, not there Rodney!) as some glorious (and appalling) effects are possible.

Distortion (Fuzz) Pedals

The area of distortion is, possibly, the one that generates the most heat amongst guitarists. The tube vs transistor argument has raged for so long that anything I might add would be superfluous. Suffice it to say that all distortion pedals work by overdriving an amplifier of some sort. The result is that the signal is said to be 'clipped'.

Hard Clipping. Figure Five shows an undistorted input signal of one volt being fed into a preamplifier with a gain of X10. This amplifier is powered by your standard PP3 nine volt battery. The amplifier dutifully tries to amplify the signal to one volt X 10 — 10 volts but runs out of headroom. Any part of the signal beyond nine volts is chopped off; this is known as hard clipping. The output in this case is too high for a guitar amplifier to handle without itself overloading so the signal is usually reduced (whilst still retaining the clipped shape) before feeding into the main amp. In this example a signal of less than 0.9 volts will be amplified but not clipped, so in practice the gain of the amplifier is increased to a couple of hundred times and the clipping point is reduced to, say, three volts. This gives distortion for a wide range of input levels.

Soft Clipping. With circuitry that is a little more subtle, soft clipping can be achieved. Rather than violently distorting any signal above the amplifier's headroom level and leaving anything below this undistorted, as in hard clipping, soft clipping causes the output to become steadily more distorted as the input level increases. This gives a less harsh, more creamy sounding fuzz (see Figure Six) which seems to be preferred by the majority of guitarists. (For example the IM&RW 'Squarer' is a soft clipping device).

Tremolos

A bit old fashioned and out of favour these, so just a quick word or two. Tremolos work by varying the output signal volume up and down at a presettable speed. (Incidentally, the so-called tremolo arm on a guitar isn't! Tremolo is an adjustment of volume, vibrato is an adjustment of frequency, so strictly speaking they are vibrato arms — not a lot of people know that!) The tremolo effect is achieved by hooking up a Low Frequency Oscillator (LFO) (no it's not the London Fymphony Orchestra) to a VCA, in such a way that a signal from the LFO reduces the gain of the VCA. The frequency of the LFO is adjustable (speed) as is the amount of gain reduction it produces in the VCA (depth).

So there we are — a quick run down of level altering pedals. Next month I'll be looking at the frequency altering devices.