EE2F2 - Music Technology

4. Effects

Effects (FX)

Effects are applied to modify sounds in many ways – we will look at some of the more common

Effects processes can be broadly categorised as:

Filtering/equalisation effects Altering the frequency content of a sound

Dynamic effects Altering the amplitude of a sound

Delay effects Modifying a sound using time delays or phase shifts

Equalisation Effects Equalisation is probably the most widely used

effect, so much so that it is usually provided as standard on most mixing desks

We looked at equalisation in some detail during lecture 2 on mixers. As a reminder, however, it is used for many purposes including:

Correcting a non-uniform microphone response Suppressing resonant modes Enhancing vocal clarity Suppressing high-frequency noise (hiss) Suppressing low-frequency rumble (e.g. traffic) Modifying wide-band sounds (e.g. cymbals) to avoid

masking other parts

Dynamic Effects The ‘dynamics’ of a musical signal refer to how

loud or soft it sounds Dynamic effects can be thought of as automatic

volume controls They mostly work by turning the volume down

for loud signals and back up again for soft ones Differences between dynamic effects are:

How quickly they respond Length of the window over which the input volume is

estimated How much the gain is altered in response to volume

changes

Limiting

If the sound from the TV is below a threshold, everyone’s happy

If it goes above that threshold, the volume needs turning down.

Limiting Example

InputLevel

OutputLevel No effect

LimitedThreshold

Threshold

Time

Input Signal

Threshold

Time

Output Signal

Compression

Compression is a less severe form of limiting.

Note that ‘compression’ in this context is not the same as data compression

Input Level

OutputLevel No effect

LimitingThreshold

Compression

Application Compression and limiting are used to reduce the

dynamic range of a signal They “smooth-off” the peaks Compressed sounds can be made louder on average

without overpowering a mix. Compression is very commonly used for vocals and

bass guitars Usually, compression is a subtle effect – you shouldn’t

really notice it

Overdrive & Distortion Compression and limiting work by monitoring the

average level, or the envelope, of the input If the input voltage is monitored directly, with no

averaging, a different effect is produced A non-averaged version of compression is known

as overdrive, and the equivalent of limiting is known as distortion

Overdrive and distortion don’t just affect the signal level, they also change the shape of the waveform and, thereby, alter its timbre (how it sounds)

Very popular effects with electric guitarists and on electric organs

Dynamic Effects SummaryInput

Compression

Limiting

Overdrive

Distortion

Delay Effects

This group of effects all work by combining two or more time-delayed versions of the input signal

Delay effects are particularly useful as they model many ‘real-world’ environments

The differences between them are mostly concerned with the length of the delay:

Very short delays: Chorus, flanger, phaser Medium delays (>100 ms): Echo Long delays (several seconds): Reverberation

Echo

Simplest possible delay effect models a single, fixed echo

The input signal is attenuated and time delayed The output is the sum of the delayed signal and

the original This creates a very crude echo

Delay

+

+

Attenuation

In Out

Chorus, Flanger and Phaser

Using much the same single delay structure are: Chorus

Very short modulated time delay Sounds like more than one instrument

Flanger Like chorus but slower modulation Creates a ‘swirling’ effect

Phaser Like flanger but uses phase shift rather than time

delay

Reverberation Real echoes are the result of multiple reflections

from several surfaces This is reverberation

Modelling Reverberation Most realistic way to model a reverberant environment

is to: Go there Measure the impulse response of the room Convolve that with the input Example: The basilica at Foligno, Italy.

Impuls

e R

esp

onse

Time(1.5 seconds)

Impulse response

Synthetic Organ

Organ * reverb

Comb Filter Processing-wise, a more economical method is to

simulate the multiple reflections using comb filters A comb filter can simulate the multiple back-and-

forth reflections between a pair of parallel surfaces

Delay

++

Attenuation

In OutTime

In

Time

Out

An Economical Reverberation Model To model a typical room, several comb filters are

used in parallel to simulate different pairs of surfaces The delay and feedback attenuation of each filter is

different in order to mix up the reflections

Comb Filter

+

+

Attenuation

In

Out

Comb Filter

Comb Filter

Comb Filter

Summary

Effects are applied for many reasons, e.g. EQ

Corrective treatment Creative control of tonal colour

Dynamic effects Aid to mixing vocals (compression) Modifying sounds (overdrive and distortion)

Delay effects Special effects (chorus, flanging etc.) Adding realism to synthetic sounds (reverberation)