[linux-audio-dev] jamin and FFT filtering

[Alfons Adriaensen]

On Tue, Aug 10, 2004 at 06:13:33PM +1000, Erik de Castro Lopo wrote:

> There was a discussion on the LAU list about jamin using FFT
> based filtering. I I missed much of the discussion but that
> particular point just jumped out at me.
> 
> Has anyone thought of trying linear phase FIR filters instead of
> FFT methods? Any filter that can be specified in the frequency
> domain can be implemented in the time domain and vice-versa.
> Often (but not always), the time domain version is significantly
> cheaper in CPU cycles.

ISTR (from Karlsruhe) the FFT method was chosen because it was
convenient for two reasons:

- the bandsplitting and EQ can be combined,
- the filter response curves can be 'interpolated' easily
  during scene changes.

But I'd like to raise another point. Why should these filters
be 'linear phase' ? The widespread belief that LP filters
always sound 'better' has been questioned long ago (by Michael
Gerzon and others).

The reason seems to be their symmetry in the time domain,
giving a long 'pre-ringing zone' before the central main peak,
It seems human hearing is very sensitive to this 'pre-ringing',
in particular on transient and percussive sounds.

The impulse response of a typical non-LP analog filter only has
a tail at the end, which seems more 'natural'. 

-- 
FA