Effects divide themselves up into those that alter the volume and those that alter the time-base of a signal.
Reverberation, flanging, phasing, pitch-shifting and echo are all types of time-based effects. Compressors, limiters, eq and
wah-pedals are examples of devices that alter the volume.
The situation is somewhat befuddled because most of the new generation of multi-effects do all
types. That is because they digitise the signal and then can perform just about any time or volume based alterations as a
result of set routines burnt into ASICs (application specific integrated circuits) as a set of software commands. So a machine
like the Eventide Ultra Harmonizer can be a multiband compressor for use in mastering in one programme, or a set of reverberation
halls in another.
But let’s pretend, for the time being at least, that these machines do not exist and look
at the various effects as if they still exist as individual boxes (most plugins for Apple and PC based systems still do).
(It may be a bit of a red-herring at this stage, but it is worth remembering that if you were to buy every
effect that one of these multi boxes has on board as a plugin, your plugin collection would cost more than anything else in
your studio! So if you do go down the PC/Apple based DAW path, think about plumbing in a good multi effect via its digital
IOs as an alternative, or an augmentation to plugins.)
Time based effects
All time based effects use the principle of storing a bit of the signal, altering it if necessary
and then replaying it. There are three ways of storing audio:
1. Mechanically (plates, reverb springs, or the natural reverb in a room or reverb chamber),
2. Electronically using a series of tiny condensers that each store the signal for a few milliseconds
before handing it over to the next in the line. Because this worked rather like men in a bucket chain trying to douse a fire
in the days before fire engines, this was called a bucket-brigade-device or BBD.
3. Digitally, in that the signal is converted to digital, stored and possibly processed for
a while and then converted back to analogue. Today, nearly all effects are digital.
Before the days of digital effects, added reverb was achieved by placing a speaker and a microphone
or two in an empty, tiled room. This was not done very often and very few changes could be made. At best, the speaker or the
microphones could be moved or changed. In larger studios, a room in the cellar was usually set aside as a reverb chamber.
Soon after this, plates were introduced, often in the same room. The first plates were huge sheets of thin
steel, at least two meters long and one meter high, suspended in a welded metal frame, with a small speaker mounted on the
steel at one end and two microphone capsules mounted at the other end. The speaker made the sheet vibrate and the microphone
capsules picked up a very realistic reverberation. Different sizes of sheet steel were used to simulate different sizes of
room and hall.
The first digital reverb machines were cascaded, multiple echoes that did not sound very realistic.
Today's digital reverbs sample parts of the audio and play them back, interwoven with one another in very complex manners
that are often very closely guarded company secrets.
Echo is the simple repetition of a sound at regular intervals until it dies away. Some of the
earliest echoes were created by feeding the signal from the playback head back into the record path. Ultra long echoes could
be created by feeding the tape to another machine and the length of the echo was governed by the size of the gap and the speed
of the tape. In 19XXXX the WEM Copy Cat was born with several dedicated playback heads and a loop of tape and today these
machines are much sought after. Thee first electronic echo machines used so called BBDs (bucket brigade devices) that were
simple chains of capacitors that each delayed the signal by a few milliseconds. Each capacitor worked rather like a man handing
on a bucket of water to the next man in the row to douse a fire in the days before the invention of the fire engine - hence
the name. The only big problem was that each capacitor degraded the audio slightly, so longer delays that were repeated often,
sounded very woolly. With the first digital delays, the woolliness disappeared.
Flanging and Phasing
At college, my friend Trevor (who later was to become a research scientist - Clever Trevor!) rushed up to
me in the common room during break and told me that he had heard the latest record from the band The Small Faces on Radio
270 (I was a Radio Caroline man myself) and that it featured a strange sound effect that made the middle eight ("What did
you do there? I got high! What did you touch there? I touched the sky! It`s all too beautiful.") sound as if it was on a fading
shortwave station. The song was Itchicoo Park, so we called it the Itchicoo effect. We were fascinated and over the weekend,
Trevor experimented with his two tape recorders and discovered that if one played the same song from both recorders back through
the same system and delayed first the one and then the other by touching the outer flange of the feed reel on the tape recorders,
when the two signals coincided, they cancelled each other out in strange ways. This was the Itchicoo effect! But because it
was done by touching the reel flange, it became known as flanging. This effect is caused by the comb filter effect of the
audio cancelling itself out as the plus and minus phases of the waves pass over each other and milder forms tend to be known
ADT stands for automatic double tracking and was first achieved by delaying the signal by a
few milliseconds (typically between 10 and 30 ms) and then sweeping that delay time slightly, so that it really sounds like
Pitch shifting works by storing the audio in very small portions and replaying them at different
speeds to raise or lower the pitch. These sections carry a time stamp and overlap so that if they have to be played faster
to raise the pitch, there are no gaps in between. The first pitch shifter was 'The Harmonizer' built by Eventide and used
a BBD that made the human voice sound like a robot. Although today there are many, many good pitch shifting effects sold both
as hardware and as software plugins, the name Eventide Harmonizer remains synonymous with altering pitch.
We often forget that filter effects like the eq on a desk or a wah-pedal are volumetric effects
as they alter the volume of a range of frequencies. But we usually mean dynamic effects, so let's have a look at the main
types of dynamic effects:
Compressors reduce the dynamic range of an audio signal and is possibly the most important dynamic or volumetric
effect in the box after equalisation. Compression can be used to tame a wildly varying vocal or beef
up the drums kit. With the attack set to 20ms and the ratio set high, we got that disco-cardboard-box effect
in the 80's on the bass drum.
The very first compressors were built using light bulbs and a light-variable resistor that pulled the volume
back as the bulb lit up. Later, variable gain valves were used and today, nearly all analogue compressors use voltage
controlled amplifiers (VCAs).
Some compressors are frequency-selective, compressing only a certain range of the frequency spectrum.
When several of these are combined to work on different parts of the signal, it is called a multi-band compressor and is often
used in mastering.
One signal can be used to compress another in a process known as side-chaining. This can be used to
tighten up a horn section or a choir, or to allow a DJ to automatically talk over music on the radio.
Noise Gates are a special type of expander that can be used to reduce or eliminate a signal
below a threshold level. It does this by heavily attenuating signals with levels that fall below the threshold. In the days
of tape, it was used to totally cut off the signal level on a single track during a musical pause so as not to pass background
noise. It is often used to give drum hits greater impact by making the signal appear to be even more sudden
An expander is the opposite of a compressor, i.e. it increases the dynamic range of a signal
Limiters are compressors with a very high compression ratio. In theory, this is infinity to one, but about
10:1 would be a normal setting. This has the effect of reducing or "limiting" input signals that exceed a specified threshold
level, so that the output does not increase in gain beyond that point. In other words, a limiter only allows the dynamic range
at its input to increase up to a certain point (determined by the threshold setting). Beyond this level, as the input continues
to increase in gain, the output level remains relatively constant and does not increase in volume.