posted 08 May 2001 04:14            

picketfence.kym

This one has both comb and picket fence filters built in. The first delay is a feedback delay that simulates a tube with one end closed. The second delay is a feed forward delay that simulates a tube opened at both ends.

When the !Fdbk is exactly the same as !Fdfwd the filter is absolutely flat in amplitude and phase. When !Fdbk is greater than !Fdfwd then you get a picket fence filter whose bandwidth is governed by how close you make !Fdbk near unity. The opposite holds when you increase !Fdfwd.

So how do you turn a quarter wave resonant tube inside out? It's easy, just use a half-wave resonator. In tandem they exactly cancel. Where the quarter wave feedback delay plants poles, the halfwave feedforward delay plants zeros.

Just for fun... (try driving !Freq from an envelope or LFO)

[Huh! Curious that a closed tube feeding an open tube cancels out and leaves only air... Why wouldn't the pair be equivalent to another closed tube, only longer? ...I know why... do you?

Also, could a 200 Hz resonator filter be used to precompensate for room resonances in a loudspeaker driven environment?]

- DM