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 Input Level Output Level No effect Limited 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 Output Level No effect Limiting Threshold 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 Summary Input CompressionLimiting OverdriveDistortion

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 InOut

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. Impulse Response 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 InOut Time 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

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)