Signal Processor Buying Guide

Like if this guide is helpful
Signal Processor Buying Guide

Although signal processors can have many industrial and non-audio uses in electronics, for musicians, audio engineers, and audio enthusiasts, they are powerful devices capable of transforming and enhancing music and other audio in a huge variety of ways. Some audio signal processors focus only on producing a specific type of effect but offer a wide range of adjustments. Others may offer a wide range of different effects with varying levels of adjustment. Signal processors can be built into equipment such as synthesisers but can also be purchased as separate units. Many are rack-mounted, but they can also be found in other forms, including effects pedals, table-top units, and in software form.

Signal Processing

In digital audio systems, signal processing involves taking any audio signal, such as that produced by, for example, a singer singing into a microphone, converting it to digital form, and then altering the data to transform specific properties of the audio waveform. This can include changing frequencies, intensities (amplitude), or phase of the audio signal. Different processes result in different families of effects. The following are what to look for when choosing a signal processor.


Everyone knows what reverb sounds like. It's considered an essential rather than an extra in most music production. There are many different reverb effects, however, and a good reverb unit will allow a range of adjustments to be made. Reverb is a series of diminishing echoes. Normally they are so close together that they can't be heard as individual sounds but just as a single warm, sound with depth. How rapidly they repeat can be decreased until they are heard as individual repetitions. How long the echoes take to die down can also be controlled. The two in combination provide a wide range of reverb, echo, and variable digital delay effects.


Audio signals in live music contain a wide range of frequencies. From deep and rich bass tones to the highest treble pitches, any portion of that wide spectrum can be boosted or reduced (cut) by means of filters. Some filters can operate on very precise frequency ranges, others may be more limited. Low-pass filters, for example, block or reduce signals above a certain frequency, and high-pass filters do the opposite. Graphic equalisers and parametric equalisers, on the other hand, can, in their own slightly different ways, make very fine adjustments to all parts of the signal's frequency band. In addition to enhancing the natural tone of the audio, filters can also be used for other important purposes, such as noise reduction.


Modulation processes are concerned more with effects that produce interesting sounds rather than enhancements of the audio signal. They do this by reproducing the audio signal in a slightly altered form and then mixing it with the original in specific ways. These processes result in the effects known as flanging, phasing, chorus and other modulation based effects, such as ring modulation. It's easy to produce strange and alien sounds with these effects; producing tasteful and musically pleasing effects, however, takes a bit of practice and restraint.


While distortion in the wrong place can completely ruin any piece of music, distortion in the form of electric guitar rock solos is virtually indispensable to that genre. The expressive capability of controlled distortion is well-known and a staple of all types of rock music, and the harder the type of rock, the more intense the distortion. Manufacturers typically label their distortion-type signal processes as overdrive, gain distortion, and fuzz. Technically, overdrive and gain produce relatively mild distortion at high volume and none at low volume, while those labelled distortion or fuzz distort at all volume levels, with fuzz being the stronger of the two. Don't expect all manufacturers to conform to that usage, however. There's always a fair amount of overlap between different manufacturers' distortion effects. As distortion is mostly used by electric guitarists in live situations, distortion units are often sold as effects pedals.

Compression and Expansion

In addition to having a wide band of frequencies, live music also has a wide range of sound intensities from very soft to very loud. If the range is too great, any recording of the performance may not be able to capture all the sounds. If the recording levels are set to capture the loudest sounds, the softest sounds may not be picked up. If they are set to capture the softest sounds, the loudest ones may be too powerful and result in unwanted distortion. Compression is the answer. Compressors reduce the loudest sounds and boost the softest ones. Sometimes, the opposite effect may be required; the loudest sounds are boosted while the softer ones are reduced. It can add a fullness and richness to the sound. This process is called expansion, and signal processors dedicated to this effect are called expanders.

Pitch Control

Pitch controllers change the pitch of the audio signal by a chosen fixed amount. These processors enable transposing and harmonising effects to be obtained. Depending on the quality of the signal processor and the actual process employed, however, some pitch shifting can result in a loss of quality. Autotune can change the pitch of notes automatically by comparing the frequency of a given musical tone with the nearest correct frequency according to the musical scale.

Required Effects

As signal processing in audio applications covers a large range of effects, it's essential to ensure that any signal processor being bought has the effect or effects that are required. A good signal processor should also allow a lot of adjustments to be made that will subtly change the effects as required. The following table shows which effects to consider for specific uses.

Required Effect

Signal processor

Tone enhancement, noise reduction

Graphic equalisers, parametric equalisers, low-pass, band-pass, and high-pass filters

Sound transformation

flangers, phasers, ring modulaters, chorus

Controlled lead guitar distortion

Distortion, overdrive, gain, fuzz

Echo effects

Reverb units, digital delay

Dynamics control

Compressors and expanders

Pitch control

Harmonisers, transposers, octavers, auto-tune

Digital and Analogue Signal Processing

Modern signal processing is done digitally, but many musicians still remember, and miss, the quality of effects that analogue signal processors provided. Analogue signal processors achieved their effects using electronic circuitry designed to process signals in a variety of ways. They are mostly obsolete nowadays as digital signal processing is a simpler way to achieve the same types of effect and offers greater flexibility and stability. However, some musicians and engineers feel that certain analogue signal processors were superior in terms of quality. In response, signal processor manufacturers, many of whom are also major music keyboard and synthesiser manufacturers, have produced processors that model or emulate the characteristics of their earlier analogue signal processors, amplifiers, synthesisers, and more. The exact circuitry of analogue signal processors can be modelled digitally, or even have some analogue hardware included in order to reproduce the exact sound of those earlier classic models. If those are the effects required, look for them being clearly advertised as analogue modelled processors or some similar term. Keep in mind also that there are still original fully-analogue signal processors being sold as used models. Some classics have collector value too.

Finding Signal Processors on eBay

To find signal processors on eBay, enter the search term ‘signal processor’ from eBay's homepage, and then browse within the most relevant category that is displayed on the left hand side. The most relevant category is Pro Audio Equipment. Click that category and subcategories within Pro Audio Equipment will be displayed underneath. The most relevant of these is Signal Processors & Effects. Drilling down to this level avoids being presented with an unsorted list of signal processors, some of which will be non-audio industrial-use signal processors. Also keep in mind that many signal processors are listed by their owners according to the effects they are designed to produce. A reverb unit is more likely to be called that than signal processor. Searching for specific effects by type within the Pro Audio Equipment category can produce more relevant results. To further tailor the search, click any of the check-boxes that are listed below the category names. This will narrow the results according to the various types of signal processor as well as other features, such as Condition (new or used) and Buying Format (Auction or Buy it now).


While it's very tempting for inexperienced musicians to use signal processors to their full capacity and revel in the myriad sound effects produced, most experienced musicians and sound engineers use them according to the less is more philosophy. Always use them tastefully and with subtlety. Used properly, signal processors can make an enormous difference to any music in terms of sophistication and quality.

Have something to share, create your own guide... Write a guide
Explore more guides