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Signal Processors

Phil Walsh DIYversifies to take a look at effects pedals


These days the guitarist/keyboard player/vocalist/drummer can call upon a wealth of electronic gadgetry to modify the basic output of his instrument.

I suppose the interest in signal processing first really took off in the early to mid sixties with records such as the Stones' Satisfaction (with that gorgeous fuzz sound — incidentally, legend has it that it was produced by using an amp with one of its output valves blown!) and the Small Faces' Itchicoo Park (the very first flanging sound?). Live performers started demanding effects that were previously the exclusive domain of the studio. There arose a folklore concerning effects with all sorts of ideas such as — "If you use the same effect as your guitar hero you'll sound just like him", "Effects will make a great player out of an inferior one", and "Effects are simple to use — just plug in and play". We now appreciate what a load of old rubbish these ideas were but they launched a whole industry manufacturing foot operated, battery powered, portable effects units. These days the electronic musician is confronted with a bewildering array of effects which can vary the sound of their instrument virtually beyond recognition. Over the next few months I will be meandering through the various effects available today with a look at how they produce their varying sounds.

Signal Processing



The first point to get over is exactly what we mean by signal processing. Basically it is the altering or modification of the 'clean' input signal which is either then outputted in its modified form or as a mix of modified plus clean signal. The modification of the signal usually takes place in one of four forms:

a) Modification of the signal level — these alter the gain or volume of the signal. Examples of this type of processor are:

1. Preamps
2. Limiters
3. Compressors
4. Noise Gates
5. Volume Pedals
6. Distortion (Fuzz)
7. Attack-Delay
8. Tremolos

b) Modification of Signal Frequency. Examples of this type are:

1. Frequency doublers
2. Octave dividers
3. Harmony Synthesizers
4. Vibratos

c) Modification of Frequency Response — these alter the range of frequencies that are outputted and the relative volume of those frequencies. Examples of this type are:

1. Graphic Equalisers
2. Parametric Equalisers
3. Wah-Wah pedals
4. Phasers
5. Vocoders
6. Envelope Shapers

d) Time Shifting devices. Examples of this type are:

1. Tape Echo
2. Electronic analogue echo
3. Digital echo
4. Chorus
5. Flanger
6. Reverb

Now if I dealt with each one of these units in turn, from scratch, I'd still be going at Christmas, but luckily many of them use the same basic building blocks (electronic, not Lego!) so it won't be quite as bad as that.

FIG 1. VOLTAGE AMPLIFICATION


Units which modify the Signal level.



FIG 2. PREAMP VOLUME CONTROL CONFIGURATIONS

A preamp simply takes a weak signal and makes it stronger (see Figure One). At a first glance it would seem that that is all it does but first impressions can be deceptive. Firstly it can amplify a weak signal, and secondly it can create distortion effects by sending such a strong signal to the main amplifier in put that it overloads its input stage allowing an overdriven sound to be produced at relatively low volume levels. Thirdly it can alter the tone quality of the sound by acting as a high impedance buffer. This presents a guitar pickup (say) with a high impedance load (typically about 500Kohm which is much higher than a normal amp input impedance) which drains the pick up to a lesser extent than a standard guitar amp input allowing a much richer sound.

The only control normally found on a preamp is a volume control and the way this is connected to the preamp has a dramatic effect on the signal to noise ratio. Figure two shows the three ways a single pot can be wired to a preamp module.

Circuit a) shows a pot in the input line. This works by altering the input signal. Unfortunately every preamp produces some residual noise and this is unaffected by the volume control. Hence reducing the input signal gives a lower output signal but the noise level stays the same — at low output volumes the noise can be relatively very loud. (See Figure three)

Circuit b) overcomes this problem because, with the pot on the output, it reduces the noise level at the same rate as the signal.

FIG 3. S/N LEVELS FOR A PREAMP WITH PRE-VOLUME CONTROL


Circuit c) is usually only used if an extremely clean signal is required and is usually more expensive as a more complex design is required. The input and output lines are left alone and the amplifying power of the preamp is altered directly by the pot.

FIG 4. FLEXIBLE GAIN CONTROL FOR PREAMPS


One way of getting the best of both worlds is to have two volume pots, one in front of and one behind the preamplifier. (See Figure four). This allows you to set both the input sensitivity and the master volume. The sensitivity control is commonly set for minimum distortion (although distortion is available by cranking it up) allowing the master volume to control the overall input level.

Limiters and Compressors



A limiter is a preamp that has an automatic volume control. It allows quiet signals to pass through unimpeded until the strength of signal reaches an adjustable, preset limit — usually known as the threshold. At this point the gain (amplifying power) of the preamp is adjusted so that the maximum output of the preamp is fixed at that level, no matter how much stronger the input signal becomes. As soon as the input level reduces below the threshold level the limiter reverts back to normal preamp operation. It works by feeding some of the output signal into a comparator circuit in which the threshold level is set. When the output level rises above the threshold level a rising voltage is sent out from the comparator. If this signal were applied directly to another, special preamp called a Voltage Controlled Amplifier (VCA), we would have a rather unfortunate effect. The gain of the VCA increases as the voltage signal applied to it increases. Thus as the signal from the comparator increased the output from the VCA would increase. This obviously gives the reverse effect to what is needed so the control signal from the comparator is first reversed (ie a rising voltage is changed into a falling voltage) before it is fed into the VCA. This reversing or inverting function is performed by another basic building block called an inverter. The final circuit looks something like Figure Five. The rectifying diode simply converts the alternating output signal into a varying DC level which gives more reliable triggering of the comparator.

FIG 5. LIMITER CIRCUIT BLOCK DIAGRAM


This gives a useful device which prevents overloading of, say, a tape recorder by preventing over strong signals from reaching its input.

FIG 6. COMPRESSOR CIRCUIT BLOCK DIAGRAM


Compressors work in a similar way but with one vital difference. A compressor increases the gain of low volume signals as well as limiting the volume of high volume signals, therefore giving a smooth, averaged-out sound having approximately the same volume no matter what the input signal strength. Unfortunately this tends to make compressors a bit noisy as, with no input signal, the circuit tries to amplify like mad and only succeeds in pushing out a high noise level. Most compressors feature a threshold control (often labelled sensitivity) allowing you to adjust the lowest level at which the circuit will trigger and therefore to cut out a lot of the noise, though you dare not play too quietly or the compressor won't 'recognise' the signal.

FIG 7. 'INCREASING' SUSTAIN USING A COMPRESSOR


Many guitarists use a compressor to extend the sustain from their guitars by setting it to level out the output from high to medium input levels. As the guitar string vibration decays the compressor continues to give a constant, though lower level, signal (see Figure seven). A limiter can be used in a similar way but with the added advantage of less residual noise. As can be seen from the graph there are a couple of disadvantages. Firstly the compressor (and limiter) take a small amount of time to start working once an input signal is detected and this leads to an overshoot spike. Secondly, once the input signal falls below the threshold level the unit switches off rapidly and the sustain is stopped dead.

Next month Noise Gates, Distortion Pedals etc — the rest of the signal processors.



Previous Article in this issue

Vocal Points

Next article in this issue

The Beat Goes On


International Musician & Recording World - Copyright: Cover Publications Ltd, Northern & Shell Ltd.

 

International Musician - Feb 1985

Feature by Phil Walsh

Previous article in this issue:

> Vocal Points

Next article in this issue:

> The Beat Goes On


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