Au­dio-rate fil­ter mod­u­la­tion

Future Music - - FEATURE -

Most plug­ins have two dis­tinct pro­cess­ing sam­ple rates

Imag­ine a sine LFO smoothly sweep­ing a fil­ter’s cut­off up and down. When the LFO is cy­cling around at a speed be­low 20Hz, we per­ceive this low-fre­quency os­cil­la­tion as an ob­vi­ous wob­ble of the fil­ter’s fre­quency. Speed the LFO rate up into au­dio-rate ter­ri­to­ries, how­ever, and the wob­ble in­creas­ingly blurs, in­tro­duc­ing com­plex har­monic over­tones and noise-like dis­so­nance into the fil­tered sig­nal. This is akin to FM syn­the­sis, whereby the fre­quency (pitch) of one os­cil­la­tor is used to mod­u­late an­other’s for a sim­i­lar sort of har­monic com­plex­ity. As it’s such a dis­so­nant ef­fect, au­dio-rate fil­ter mod­u­la­tion is usu­ally best used for trashy drum-man­gling, sci-fi FX de­sign and lo-fi degra­da­tion.

Cy­tomic head hon­cho An­drew Sim­per goes deeper into the com­plex­i­ties of coding this type of rapid fil­ter move­ment. “Most plug­ins have two dis­tinct pro­cess­ing sam­ple rates: one for the au­dio, and a slower one for the mod­u­la­tion. This slower rate is usu­ally called ‘con­trol rate’, and it is typ­i­cal that this is around 64 times slower than the sam­ple rate. This means that there is only one new con­trol value – for ex­am­ple, one new cut­off fre­quency for the fil­ter – for ev­ery 64 sam­ples of au­dio pro­cessed. If the au­dio sam­ple rate is 44.1 kHz, and mod­u­la­tion is pro­cessed at 64 sam­ples, then the max­i­mum fre­quency that can be rep­re­sented is around 700Hz. This is a rea­son­able rate for slow mod­u­la­tion, but for fast mod­u­la­tion the re­sul­tant sound is very harsh.

“Au­dio-rate mod­u­la­tion isn’t much use un­less all the mod­u­la­tion sources pro­duce their anti-aliased out­put at au­dio rate as well. Since there is no dif­fer­ence be­tween an au­dio sig­nal and a con­trol sig­nal, this is as close to the ana­logue world as pos­si­ble. Un­for­tu­nately, this speed of up­date can be dan­ger­ous for many com­mon dig­i­tal fil­ter al­go­rithms – it can crash and sound ter­ri­ble.

“In The Drop, all mod­u­la­tion sources and des­ti­na­tions are cal­cu­lated at au­dio rate, the mod­u­la­tors are anti-aliased, and the fil­ters all pre­serve the struc­ture of the cir­cuit and po­si­tion and shape of all ma­jor non­lin­ear­i­ties, so ev­ery­thing sounds ul­tra smooth and as ‘ana­logue’ as pos­si­ble. This does take more CPU, but you can push it much harder.”

Newspapers in English

Newspapers from Australia

© PressReader. All rights reserved.