[sdiy] Filter ICs

[Josh Landau]

Another IC you may want to take a look at is Maxim's MAX274/275 filter 
chips - 2 or 4 2nd order filter sections on a chip.  Unfortunately, they 
don't seem to be voltage controllable (haven't called Maxim to make sure, 
but I will one of these days), but if you used 1 chip per channel and then 
ran the filters in series/parallel configuration, you have a 8th order 
filter per channel.  Resonance is specified up to 50 for frequencies in the 
audio range.  The chip is resistor-programmed, although an application 
example for bus-programming is included in the datasheet.

One thing to note is that the Maxim chip allows you to choose between 
Butterworth, Bessel, and Chebyshev filter types, whereas the AMI chip 
appears to be fixed as an elliptical filter.  Also, the Maxim is a 
continuous time chip - the AMI is an SC filter, which leads to all sorts of 
high frequency noise.

I'd consider Maxim's chip for an analog vocoder design before anything else 
- seems to almost designed for the purpose.

Josh Landau