Hi. I've working with al TMS320C613 DSK building some guitar effects.
>I was suscesfully whit all of these but I had no idea
about build a
>valve guitar preamp on DSP. In other forums, people said I have to
>decimate, interpolate and check out curves in the valve 12AX7's
>datasheet and process the signal with that. The thing is .... I'm
>still have not much idea about it.
>
>Is someone could give me some tips about it?....Thanx a lot
Hi back...
The implementation of accurate, good sounding guitar overdrive models, whether
based on physical circuits, such as vacuum tube amps, waveshaping, or any other
technique is no trivial matter. It requires a thorough understanding of DSP
concepts, electronic circuit analysis, and mathematics. I have a decent working
model implemented on a Windows PC that I'm porting over to a Microchip DSP
board so I can put it into an amp cab and take it on stage w/ me, but it took
me, literally, years of research & analysis before ever writing a line of code
to even get started.
If you'd like I can give you a list of great research papers that explain
the concept and give mathematical solutions to several techniques - however the
implementation is still left up to you.
As far as implementations go, a good tone can be achieved in many ways.
I've had good success with physical models of tube circuits as well as
static and dynamic waveshaping. Regardless of the model used, one thing, which
you mentioned in your post, is critical: oversampling (interpolation &
decimation). Unlike physical circuits, numerical models create harmonics ad
infinitum, which, when above the Nyquist frequency, wrap back into the audio
band - creating a lot of nastiness which no amount of filtering will remove. In
fact, oversampling is so critical and significantly impacts the tonal quality of
the model that simply changing oversampling parameters one can take a model of a
super distorted metalhead amp and turn it into a tame country twanger without
changing the overall tone. The amplitude, spectral spread, and multiples of
harmonics is the defining factor in tone quality and in achieveing a good
sound.
Here's a great intro paper on modeling guitar amps written by David T. Yeh,
formerly of Stanford U., and author of volumes on the subject:
http://www.mitpressjournals.org/doi/pdf/10.1162/comj.2009.33.2.85
Hope that helps. Drop me a line if you want more links to papers.
Michael B.
