On Fri, 31 Mar 2006 17:07:23 +0100, Nick Gorham
wrote:

Don Pearce wrote:
On Fri, 31 Mar 2006 16:23:33 +0100, Jim Lesurf
wrote:


In article , Don Pearce
wrote:



But an FFT DOES give you all the information - phase as well as
frequency. THe fact that the normal presentation of the data is simply a
magnitude plot is neither here nor there.

It seems common to plot 'power' versus 'frequency', but in theory all an FT
or FFT does is gives you a method to transform information from one form
into another, nominally with no loss at all. People then may choose to
discard the phase info if they so wish, but the transform can provide it.


It is just as well that phase information is preserved; if it weren't,
then reversing the procedure back into the frequency domain would
result in an unrecognizable signal.


THe FFT is not, though, as you say, a model of reality. It is a model of
what reality would look like if it were repeated for ever - it should
have the word Groundhog in there somewhere.

Actually *an* FFT (since there are various processes which can be called
such) is a computational method that returns the same transformation as an
FT. The advantage is computational speed. The snag is source code that may
cause some head-scratching, and makes finding some typos a nightmare. :-)


FFT, DFT - whatever. As long as the number of calculations doesn't
increase exponentially with the number of points, It'll do for me. The
central looped core of an FFT is fairly small, though - not too much
trouble to debug. Just don't use an in-place algorithm.


Ok, I stand corrected :-)

Actually I have spent much more time dealing with DCT than anyhing else.

What I notice a lot and Jim may be able to explain this, is the HiFi
mags seem to often do FR plots using what they describe as a convolved
inpulse analysis, I have a theory what this means, but it would be nice
to have some references (ok, so I guess I could just look it up).

Its mainly through playing with RIAA stages that I have become aware of
the importance of maintaining phase as well as frequency response, but
maybe this is old hat for you pro's.

The thing about impulse analysis is that it results in something that
is visually very easy to understand. Convolution is just a
mathematical trick for subjecting a signal to some sort of
fequency-dependent transformation. Roughly speaking, convolution of a
signal and a response in the time domain is equivalent to FFTing them
both into the frequency domain, multiplying them together
point-by-point and then re-FFTing them back to the time domain.

As for things like RIAA correction, it uses what is called a minimum
phase response. What this means is that each amplitude value has a
unique and unambiguous phase associated with it. In practice, what
this means is that you don't have to worry about it. Get the amplitude
right, and the phase will follow.

d

Pearce Consulting
http://www.pearce.uk.com