Forums > MaxMSP

4-pole filter with [biquad~]?

January 3, 2011 | 9:40 am

Based on the documentation it seems that biquad is a 2-pole filter? How can I make a moog style filter (without externals?) For a 4-pole, should I just patch 2 two-poles in serial?


January 3, 2011 | 11:11 am

Not sure whether it is exactly the same or kind of the same. But if a 4 pole filter is capable of filtering at 24db/oct compared to 12 db/oct for a 2 pole filter, then that steepness can be achieved indeed by putting two biquads in series. The cascade~ object is more flexible in this regard. Also take a look at the cascade message to the filtergraph object.

_
johan


January 3, 2011 | 5:20 pm

yup, 2 in series, and halven the Q factor.


January 3, 2011 | 10:56 pm

Thanks. Using cascade. Why do you need to halve the Q?


January 4, 2011 | 12:46 am

do you halve both Q’s or just the second?


January 4, 2011 | 4:47 am

both.


January 4, 2011 | 7:14 am

but why? because the two summed is twice the gain in the Q’s bandwidth?


January 4, 2011 | 12:02 pm

cant give you a scientific correwct answer why. it is just what i found out by trying.
if you put 2 biquad in series with both Q 0.99 it is simply getting too loud.

with both at 0.5 the gain of the output signal is exactly as high as with one filter at 1.0

it might be that the curving of the Q parameter is no longer linear, but that does not
really matter in practice.

-110

p.s. and always use phase display mode in filterpgraph, it can be very interesting, yet not
explaining, when fiddling with biquads.


Viewing 8 posts - 1 through 8 (of 8 total)