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Article from International Musician & Recording World, June 1985

The basics of processors propounded by Phil 'DIYners card' Walsh


Last month we looked at equalisers and explained their action by considering the bandpass and bandcut filters they use. You will remember that a simple parametric equaliser usually has only two filters that you can manually tune up and down the frequency range until you find the frequency you want to cut or boost. If you happened to do this whilst playing an instrument through the parametric you would get a peculiar sounding effect as a particular band of frequencies (centred around the resonant frequency, which you are shifting around) is either cut or boosted.

FIG 1. WAH WAH FILTER RANGE


Wah Wah Pedals



What I have just described is the basis of a wah wah pedal. The wah wah pedal contains a bandpass (boost) filter with a resonant frequency that can be shifted along the frequency scale (see figure one). It is not particularly advantageous to be able to sweep the entire frequency range as the sweeping is usually done via a footpedal. With a large range there is little control over the sound as the slightest twitch of your foot would shift the frequency a great deal, giving a very choppy and unpleasant sound. Usually a range of between one and two octaves is used, centred around the 1kHz mark. As with a parametric equaliser, many wah wahs have a resonance (or Q) control to adjust the sharpness of the filter response peak. A high Q gives a biting, incisive sound whereas a lower Q gives a rounder, smoother sound.

In most wah wahs that's all you get, but in a few the broad range of input frequencies is allowed to pass unimpeded through the circuitry and the boosted frequency notch is superimposed on top of this (see Figure two). It's all a matter of personal choice — this rarer type gives a much more subtle effect but I must confess I prefer the 'raunch' of the common type.

FIG 2. WAH WAH SIGNAL SUPERIMPOSED ON NORMAL RESPONSE CURVE


From the outside a wah wah pedal looks very similar to a volume pedal and uses the same type of mechanism(s) to turn the pot that sweeps the filter along the frequency spectrum. (Many manufacturers use the same case — it saves on tooling costs). All the pros and cons of mechanical versus optical pedals still apply, however in the more recent wah wah units a new type of control has began to rear its little head. As, unlike a mechanical volume pedal, the wah wah already contains a battery or some form of low voltage supply, a Hall effect sensor can also be used. This is really very simple and consists of a slab of the same type of material from which transistors are made. (For the technically minded, doped silicon). If a voltage is put across opposite edges of this slab and a magnet is then brought close to one of the slab's faces, a voltage is developed across the other two edges. The nearer the magnet, the bigger the voltage. This changing voltage is used to alter the resonant frequency of the filter. As the only moving part is a magnet attached to the pedal top, it eliminates all the clunks and scratchiness associated with mechanical pots. As magnets don't burn out or consume any power, the Hall effect switch also gets rid of the two major disadvantages of the optical system. A Hall effect wah wah block diagram might look something like figure three.

FIG 3. A HALL EFFECT WAH-WAH


Figure three introduces another basic building block used in signal processing — the VCF (Voltage Controlled Filter). As the control voltage rises, (ie the magnet comes closer to the Hall effect sensor) the resonant frequency of the VCF is raised and vice versa. Hence with the footpedal pressed forward the high frequencies are emphasised and, as the pedal is angled back, the resonant frequency of the filter shifts steadily to the lower frequency end of its range.

Envelope Followers



FIG 4. AN AUTOMATIC WAH-WAH

T'would seem a pity to let the opportunities offered by the VCF to be ignored, so the next stage in the development of the wah wah was an automatic version. If an envelope follower is used to supply the control voltage to a VCF we have a device which will shift its resonant frequency according to the amplitude of the input signal. The envelope follower essentially converts the AC input signal into a DC 'envelope' (see figure four). As the note (say from a guitar string) is hit a high control voltage is produced so the VCF has a high resonant frequency. As the note decays the control voltage and hence the resonant frequency drop. Here we have an automatic wah wah which can react note for note to what you play; the harder you hit the string the higher the filter's resonant frequency. There are, however, some drawbacks and the attack and decay problems I mentioned earlier when looking at limiters, compressors and attack/decay devices all hold good for the automatic wah wah. The bottom line is they're great for lead lines but if you want to play chords you get a cleaner, less cluttered sound from a manual wah wah.

As a final note, even though they are often called envelope followers, an automatic wah wah is not an envelope follower, the envelope follower is just one module in the circuitry — it is also used in many other effects.

Phasers



FIG 5. PHASE COMPARISONS

And now... Uncle Phil's little homily for the month — to understand phasers you've got to understand what we mean by phase.

Figure five shows a lot of squiggly lines but far from being drawn at random they actually represent something. Let's start by looking at the top waveform which represents an audio signal. I've labelled this one 0° and a look at the one labelled 180° shows that it is the same waveform half a cycle later. After a further 180° (ie 360° after the 0° position) the waveform will be back in an identical position to 0°and is said to be 'in phase' — the 0° and 180° waveforms are said to be 'out of phase'. In between these positions there are a whole range of phase relationships which are out of phase by varying amounts. If a 0° and a 360° signal (ie two in-phase signals) are added the result is a signal having twice the amplitude (volume). If two signals which are 180° out of phase are added (mixed together) the result is silence (cancellation). Adding signals having phases other than these gives some in between volume. The effect is known as interference. So far I've only considered static phase relationships. The familiar swirling, ethereal sound that a phaser gives is due to continually varying the phase of an incoming signal and then mixing this back with the original signal causing the interference effects. Figure six shows the general layout.

FIG 6. A SIMPLE PHASER


In practice the phase shifting element only shifts between 0°and 180° and a single element produces a fairly feeble effect. The way round this is to feed the output of one phase shift element into another one (or more). As each element shifts the phase by up to 180° a five element phaser offers 0° to 900° (5x 180°) of phase shift. Feeding some of the phase shifted output back into the output sends it th rough again and even greater effects are possible. The amount of phase shifted signal fed back is controlled by a pot usually labelled resonance, regeneration or feedback. A more practical phaser is shown in figure seven. The depth control in this design is a dual gang pot — as the pot is turned the mix between the straight through signal and the out of phase signal is altered between all straight through and all phase shifted.

FIG 7. A PRACTICAL 3 ELEMENT PHASER


Vocoders



FIG 8. A SINGLE VOCODER CHANNEL

By using a vocoder the human voice (or any other input signal you choose) can be used to modulate the sound of another instrument. Though seeming to be very complicated the basic idea of a vocoder is pretty simple. Figure eight shows a single channel from a vocoder. The filters shown are very similar to those found in a graphic equaliser and the instrument and microphone filters are both centred on the same resonant frequency. If a note within the range of the filter is sung the microphone filter will allow through a signal which acts as a control voltage to the VCA thus allowing it to amplify any instrument sounds that fall in the same frequency range, but with the dynamics of the human voice. This in itself would not produce a particularly pleasing or interesting sound but gang up, say, 24 filter channels to cover the audio range and you have a device which will track human speech and open the appropriate filters so that the musical instrument sound follows the dynamics of the speech.

All that remains to be done is to mix the instrument VCA outputs together to produce a final output sound. As only the VCAs corresponding to the frequencies present in a particular vocal sound will open at that particular moment it follows that the effect will work at its best if the instrument sound has a wide harmonic content — chorusing the instrument prior to feeding it into the vocoder helps to really thicken up the sound.

Next month: Echo units — tape, analogue and digital.


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Absolute Beginners

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"I Murdered The Drummer!"


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

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International Musician - Jun 1985

Donated & scanned by: Mike Gorman

Feature by Phil Walsh

Previous article in this issue:

> Absolute Beginners

Next article in this issue:

> "I Murdered The Drummer!"


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