Super Filter = No filter

[Bissell, Harry]

There have been some studies that indicate that phase is important (and some
that say that its not) To get a particular waveshape i.e. square it is
important to sum the right phase of harmonics. But could you hear the
difference... I guess you could if the fundamental was at a reasonably low
audio frequency. A scope would definatly show the wave as distorted. In the
case of the square wave, if harmonics were shifted in phase, the rise time
would be affected (maybe trashed). As far as the perception to human hearing
???  :-) Harry


> -----Original Message-----
> From:	Gene Zumchak [SMTP:zumchak at cerg.com]
> Sent:	Friday, March 05, 1999 9:53 AM
> To:	Bissell, Harry
> Subject:	Re: Super Filter = No filter
> 
> Harry,
> 
>     Thanks for the comments.  Harry, is phase important.  If you have a
> waveform of some fixed recipe of harmonics, can you distinguish it from
> another
> waveform with the same amplitude recipe but different phases?  This may be
> well
> known, but I don't happen to know it.  Thanks.
> 
> Gene Z.
> 
> 
> "Bissell, Harry" wrote:
> 
> > Yeah, it's called additive synthesis. To finish the job, the phase of
> each
> > harmonic (or non-harmonic) needs to be controlled also. I think the
> > Synclavier and Synergy either used this approach, or it was an option.
> The
> > Yamaha DX series were capable of a limited form of this, one "operator"
> had
> > all wave generators in parallel. The down side is how would you control
> it,
> > and how hard is it to program sounds.   :-) Harry
> >
> > > -----Original Message-----
> > > From: Gene Zumchak [SMTP:zumchak at cerg.com]
> > > Sent: Wednesday, March 03, 1999 12:13 PM
> > > To:   DIY
> > > Subject:      Super Filter = No filter
> > >
> > > List,
> > >
> > >  A recent thread about waveform synthesis reminded me of an  idea I
> had
> > > about 25 years ago when I was into synths. I wanted to build a single
> > > voice channel (ala Moog).  Since I had visions of commercial
> > > possibilities, I needed to make it without a (patented) Moog VCLPF.
> My
> > > background is digital, and the things I did for Moog (like the 960
> > > sequencer) were digital.  I was into microcomputers from the
> beginning.
> > > (I owned a KIM-1 (6502) serial #000005, which at $250, cost less than
> an
> > > 8080 chip set at the time.)
> > >
> > >  My goal was to make the perfect filter.  Imagine a waveform, like a
> > > sawtooth, with a particular spectral recipe, and a filter bank in
> which
> > > each filter affects only one harmonic.  Each filter would have its own
> > > ADSR generator to control its harmonic.  If the waveform is the sum of
> > > the fundamental and some number of coherently generated harmonics,
> then
> > > a VCA/ASDR  for each harmonic, prior to the summing will give the
> > > identical effect as a filter bank with individual time control of each
> > > harmonic.  That is, the ideal LP filter would be no filter at all, but
> a
> > > bank of coherent harmonic generators each with its own VCA and
> envelope
> > > generator.
> > >
> > >  To make the coherent fundamental/harmonic generator, I planned to use
> > > EPROMs tied to DACs for table lookup of sines.  Muxing data  out of a
> > > single EPROM modulo eight, I could supply waveforms for the first
> eight
> > > harmonics (enough?).
> > >
> > >  Needless to say I never built it.  Today it would be a lot easier and
> > > cheaper.  In fact, I might be tempted to calculate everything in the
> > > micro (a 16 MHz 68HC12) and generate the already enveloped, filtered
> > > waveform directly.  Has anyone ever attempted this approach to a
> single
> > > voice channel?
> > >
> > > Gene Zumchak
> > >
> > >