Filter
                                Ver 1.01
                                   By
                             Jamie Mueller

(Items new to 1.01 are listed at the end of this file.)

  Filter is a simple program based on the principles of digital signal
processing. The program uses 256 point FHTs (Yes 'H' for Fast Hartley
Transforms, but more on that later) to filter out different frequencies
according to a filter you design. You can load (in fact you have to, to get
any results out! :) an IFF sound file and filter out unwanted frequencies
(i.e. noise) and save it back to another file. The loaded file MUST be an
IFF sound file, not raw or otherwise.

  To design the filter just click in the "Filter Characteristics" window and
draw in a filter (the mouse pointer will draw wherever you move the mouse,
but the graph will only be updated when you let go of the left mouse
button). The filter is designed in decibels; from 0 to -60 db (no
amplification because if the sample already uses all 8 bits of the sound
where would it put the 'extra' information??) NOTE: A db is 20 times the log
of the 'gain' so a gain of -60 db would be equal to multiplying that
frequency by 0.001 (if it sounds complicated, just play with the program,
you'll figure it out! :).

  In addition to the Filter Characteristics; a plot of the original and
filtered data can be 'opened' (under windows on the menu). These plots are
also in db (NOTE: These plots will never show a negative db because that
corresponds to a decimal and the Amiga stores the audio information in
integers).

  The FHT transform used in the program computes the same information as a
FFT would except that the 'butterfly' routine has half as many multiplies as
a comparable FFT! This translates to a little extra speed. For comparison,
my PrintSpec program uses 256 point FFTs and it gets about .88 sec per
transform. This program, however, realizes about .55 to .60 sec per
transform. (NOTE: Both this program and the PrintSpec program use the FFP
library for math).

  If you find this program useful (or otherwise) please let me know! I can
be located on GEnie as C.MUELLER. (sorry about the other services as I'm not
rich!).

Items new to version 1.01: Version 1.01 just 'removes' the small file
limitation imposed by version 1.00. Filter can now filter files up to
(almost) 1/2 the size of available RAM (both chip and fast). (Only 1/2
the size becuase two buffers are allocated: 1 for the original data and
1 for the filtered data). To give you an idea as to how long a 'large'
file can take: I filtered a 360k file on my standard 1 Meg A500:
the initial pass at loadup took 20 minutes, and applying a filter took
40 minutes! (@ 2845 transforms to do)! I anyone uses Filter on a 68020
system please let me know how fast it runs on your system (I'm just
curious).

                                     Jamie C.MUELLER