[sdiy] Pole-mixing, Highpass filters, and switching the first stage off

[Andrew Simper]

On 6 September 2014 18:59, Tom Wiltshire <tom at electricdruid.net> wrote:

>> Here are some plots I did to show the difference between a true 4 pole
>> cascade (4 1 pole high pass cascaded with feedback) and a 4 pole
>> generated from pole mixing:
>> http://www.cytomic.com/files/dsp/cascade-hp4-vs-lp4hpxtaps.pdf
>
> Very interesting. I don't see anything there that would put me off - like Richie said, "good enough for music".

If you keep the drive levels low then using the pole mixing works
fine, the high pass response does sound a bit different than a regular
high pass cascade, still good though. It's when you increase the drive
the differences really show up.

>
> Perhaps the most significant difference between the lp4 with summed taps and the hp4 normalised is that the peak moves *up* with increasing resonance for the lp4, whereas it moves *down* a little bit for the true hp4. A dead giveaway!
>
> Tom

Yep, well noted! This hasn't made a huge difference in musical
applications that I've found, but the tone of the drive is quite
different between the two, which is a matter of taste I suppose.
Having all those different responses formed by the pole summing is
pretty useful though, and there are some notch + resonant peak + notch
responses you just can't get any other way I know of using a 4 pole
filter core.

Andy