Filter Design

[Bissell, Harry]

These are "switched capacitor" filters which are better suited to fixed
filter designs with very high roll-off slopes, most useful for
"anti-aliasing" filters in sampled data systems. You will need a filter
response from about 20Hz to about 20KHz. This is a 1000:1 range. If you take
the highest fundamental frequency (20Khz) times the clock rate (usually 50
or 100 times the highest frequency) you would need a clock frequency of 2MHz
at the high end, and 2KHz at the low end. So, you are very likely to hear
all sorts of clock breakthrough at the lower frequencies, as well as
aliasing from the higher harmonics of the input signal. You can't add an
input R/C pre-filter to prevent aliasing, and you can't add an output
post-filter that will work with the 2KHz clock rate and not be completly
overkill at the 2MHz clock rate. Bottom Line: Switched Capacitor filters are
not really musically useful.

Filters needed for Synthesis are: at least one Lowpass-Resonant filter with
a center frequency range of 1000:1 and resonance (Q) from less than 1 to
maybe several hundred. Slope is usually from 12db to 24db per octave.
Alternate choices would be a lowpass-bandpass-highpass or some combination
of the above. There are lots of filter designs to choose (copy) from. Try a
filter like the "Wasp" (a 12db state variable filter with high-low-bandpass
and notch responses) and tell your professor how interesting it is to use
logic ICs in a linear application), or the all time favorite "moog ladder
filter" a 24db lowpass-resonant filter, but add a twist like a second,
tapped output at the 12db point (I simulated this with a spice program and
it looks like it would be neat). But don't... don't use switched capacitor
filters for electronic music. Unless you want to demonstrate to your
professor that they SUCK !

You can find the schematics for lots of filters on the web, and the people
at DIY have almost all of them.
Happy Hunting: Harry

> -----Original Message-----
> From:	Allan [SMTP:allan.houston at lineone.net]
> Sent:	Tuesday, January 26, 1999 1:32 PM
> To:	synth-diy at mailhost.bpa.nl
> Subject:	Filter Design
> 
> For my Synth(its a Uni. project), my lecturer has suggested these filters:
> 
> MF10
> MF5
> LMF100
> LMF100C1
> LTC1068C
> 
> Should I use any of these or are they not suitable for this purpose?
> If not any suggestions for readily available alternatives?
> 
> Also How many filter banks should i use/are commonly used?  Do i need a
> number of diffrent types i.e low,high and band pass?
> 
> Cheers
> 
> Allan