[linux-audio-dev] Re: Cheby amp code

Steve Harris S.W.Harris at ecs.soton.ac.uk
Mon Nov 4 13:44:01 UTC 2002

On Mon, Nov 04, 2002 at 10:03:11 -0800, Paul Winkler wrote:
> On Mon, Nov 04, 2002 at 04:06:53PM +0000, Steve Harris wrote:
> > My current guess is different transfer functions. Do you know the phrase
> > "when all you have is a hammer, every problem looks like a nail" ;)
> I'd bet money that they use different pre- and post- filters, too.
> It's very instructive to play with this setup of effects pedals:
>  graphic EQ -> distortion -> graphic EQ

Absolutly. I also remeber discussing speaker modelling with a hardware guy
and he said that EQ was the most important part of that. Tim's bigass IIR
hinted at that too.
> For instance, giving a heavy pre-boost to the lows / low mids
> will make the sound "fuzzier" but soften the attack.
> Think 1970s metal... sabbath, dude!

Interesting, thanks.

Do you have any thoughts on trying to gather transfer functions with saws by
any chance? I'm still vaguely concerned about phase issues, but I guess we
wont know till we try it.

For the record my current guess for an amp process is:

              .-> bandpass -> shaper -.     .--- LP <--.
              |                       |     v          |
input -> EQ? -+-> bandpass -> shaper -+-> delay -> EQ ---> output
              |                       |
              `-> bandpass -> shaper -'

With N bandpass filters, maybe 2 octaves each. The top and bottom could be
shelves, but I think you get DC block and antialiasing for free if you
dont bother. The EQ? could be optional if you have enough bandpasses.

I think it would be pretty fast. I'm hoping there is a proffesional amp
modeller out there who will eventually crack and admit how its done ;)

- Steve

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