blackman and flattopwin cannot be used with pwelch

[David Bateman]

Francesco Potorti` wrote:
> pwelch.m as distributed in the signal package of octave-forge does not
> accept a window with negative values.  However, blackman(4096) and
> flattopwin(4096) generate negative values.
>
> I suspect that as far as blackman is concerned, it is just a rounding
> error, but flattopwin definitely generates negative values.
>
>   
Could you propose a fix?

D.