[sdiy] TR-808 Kick Drum modelling explanation

[Richie Burnett]

> Paragraph 8.2 is the most important ingredient :)

Yes. I agree.

Although their comments about leakage through R161 causing the pitch "sigh" 
are a bit misleading. It's really the current out of the collector of Q43 
due to reverse transistor action when Vcomm swings negative that contributes 
most to the higher pitch and brighter tone at the beginning of each 808 kick 
sound. They do talk about this effect too though.

The first few cycles near the start of the bass-drum sinewave essentially 
slew downwards slightly faster than they slew upwards.  This is what causes 
the pitch fall effect and introduces the harmonics that the 808 kick is 
known for.

It's ironic that Roland's own Tr-8 still doesn't really model this properly.

-Richie,