[linux-audio-dev] Diode wave shaper (LADSPA plugin)?

[Alfons Adriaensen]

On Mon, Aug 16, 2004 at 08:45:49PM +1000, Erik de Castro Lopo wrote:
> On Mon, 16 Aug 2004 12:13:51 +0200
> Alfons Adriaensen <fons.adriaensen at alcatel.be> wrote:
> 
> > This is a bit too simple. The diode circuit does not perform hard
> > clipping when driven at moderate levels.
> > 
> > Something like 
> > 
> >   out = tanh (in)       (or tanhf() if available)
> > 
> > or
> > 
> >   out = in / sqrt (1 + in * in) 
> 
> Both of those would probably sound better than what I posted,
> but the original circuit had a 12k source impedance. As soon
> as the the input overcomes the forward voltage drop of the
> diodes the impedance of the diodes drops to the low hundreds 
> of ohms.

The current in a silicon diode is an exponential function of
the voltage. Consequently, the small signal impedance is 
inversely proportional to the current. A diode does not
have a fixed voltage drop, it still depends on the current.

> If you Spice the given circuit I think you'll find that the
> output looks a lot more like what I posted than either of
> the above.

The transition from the linear to the clipped region would 
indeed be a bit sharper than with either of the functions
I gave. But it's still a function with a continuous
derivative. This makes all the difference for the spectrum
of a 'clipped' signal.

-- 
FA