PWM on complex audio signals (really tone controls!)

[Don Tillman]

   Date: Wed, 03 Jan 2001 04:10:56 -0500
   From: Glen <mclilith at ezwv.com>

   In addition to some of the good reasons that you gave (and that I
   snipped out for brevity sake), I thought that Baxendall and other
   "Hi-i" tone controls were simply too complicated and expensive
   compared to some of the tone control circuitry that ended up in
   guitars and guitar amps. After all, the frequency responses of H-Fi
   tone controls could be easily shifted into a more appropriate range
   by simply selecting different component values.  This should either
   eliminate or greatly reduce one of the biggest problems you
   mentioned.

There is one brand of guitar amp that uses passive hifi style tone
controls -- Ampeg.  Ampeg amps have been somewhat popular for
traditional bass guitar sounds but were never used much for electric
guitar.  I would guess that the Ampeg controls were tweaked to have
more of a shelving effect than standard hifi tone controls.

   Date: Wed, 03 Jan 2001 19:28:44 +0100
   From: Magnus Danielson <cfmd at swipnet.se>

   Ah, thank you Don.

   Would that mean a wider range of control (+/- 24 db at peak rather
   than +/- 6 dB)?

Traditional hifi tone controls have a range approaching +/- 20 dB,
that's at the full cut and full boost positions.  The slope approaches
6dB/octave.

   Would that mean wider (low-Q) or smaller (high-Q) peaks/valeys?
   Or do we talk about shelf responces?

It's a problem when the tone control has the most boost outside the
frequency range of the instrument.  So a lot of shelving effect would
be good.  And variations on shelf frequencies.

I think a tone control that changed the slope of the response curve
around the midband and left the slope flat at the high and low
frequency extremes would be very nice.  (Hmmm, how could I do
this...  separate controls for tuning a pole and a zero.)

The Fender tone controls (and variations) did this but also have a
moving notch because of the phase cancellation between the bass and
treble sections.  This gives the amp a specific sonic character, which
may or may not be a good thing depending on your goals.  

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com