Break­ing it down

How spec­tral syn­the­sis works

Computer Music - - Make Music Now / Spectral Synthesis -

So what ex­actly is spec­tral syn­the­sis, and how does it work? Well, we men­tioned spec­troscopy on the pre­ced­ing page, and it rep­re­sents the sim­plest way to de­scribe the tech­niques we’ll be ex­plor­ing in our tu­to­ri­als. The most fa­mil­iar form of spec­troscopy will be fa­mil­iar to any­one who has ever shone a light through a prism or, for that mat­ter, any­one who’s ever seen the cover of Pink Floyd’s The Dark Side Of The Moon!

As any school kid knows, a prism breaks up in­com­ing light sources and splits out a rain­bow of coloured light. In­deed, a rain­bow is na­ture’s own spec­tro­graphic dis­play, with at­mo­spheric mois­ture act­ing as a prism. It takes in the light of the sun and sep­a­rates it ac­cord­ing to the length of the waves that make up that light. Wave­lengths are also re­ferred to as ‘fre­quen­cies’.

Sounds can sim­i­larly be bro­ken down into their con­stituent fre­quen­cies by us­ing one of two tech­niques: band-pass fil­ters or Fast Fourier Trans­form (FFT).

The for­mer is what drives the most fa­mil­iar in­car­na­tion of spec­tral syn­the­sis: the vocoder. Here, a com­plex in­com­ing sig­nal (the mod­u­la­tor – say, a voice) is plumbed through mul­ti­ple band-pass fil­ters, each al­low­ing a spe­cific fre­quency – or range of fre­quen­cies – to pass through. The am­pli­tude of the sig­nals pro­duced at the out­put of each fil­ter is used to drive the am­pli­tude of the car­rier sig­nal – usu­ally a synth tone.

The con­cept be­hind Fast Fourier Trans­form is a lit­tle bit more dif­fi­cult to un­der­stand. It’s based on the idea that any sound may be bro­ken down into a col­lec­tion of in­di­vid­ual sine waves, each with its own am­pli­tude and phase. There are a num­ber of al­go­rithms that can be em­ployed, but the im­por­tant thing for you to re­mem­ber is that we are left with a visual rep­re­sen­ta­tion of that sound as it oc­curs over time. This will be our spec­tro­gram.

The re­sult­ing spec­tro­gram may take any one of a num­ber of forms, but it will al­ways tell us what fre­quen­cies are in play, how loud they are, and when they oc­cur in time. Com­monly, the ver­ti­cal axis of the image will rep­re­sent the fre­quency, while the hor­i­zon­tal axis will in­di­cate the pas­sage of time. The am­pli­tude of the fre­quen­cies in play is rep­re­sented by the colour dis­played and/or its in­ten­sity.

Analysing the spec­trum of a sound al­lows for a deeper un­der­stand­ing – and deeper tweak­ing – of your au­dio

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