Odp: [sdiy] SVF & phase

[Roman Sowa]

----- Original Message -----
From: jhaible <jhaible at debitel.net>
To: patchell <patchell at silcom.com>; Grant Richter
<grichter at asapnet.net>
Cc: mark s <n0nspaz at mindspring.com>;
<synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, November 27, 2002 5:32 PM
Subject: Re: [sdiy] SVF & phase


> There is a remarkable filter effect that attenuates high
frequencies and
> might not result in a phase shift: The reproduction of an
analogue recording
> over a mis-aligned tape head (wrong azimuth):
>
> Questions:
>
> (1) Does this really cause no phase shift?
>
> (2) Of course it does *not* go against the above rules,
even
>       if it doesn't cause phase shift. Why ?
>

IMHO what we have here is convolution of audio signal from
tape
and head's reading function of 'y' position. Assuming the
head's read
capability is the same along whole gap, the convolution is
with function
of constant value and sharp edges. The more wrong azimuth,
the wider
convoluting function, as a result more filtering effect.
To have linear phase response (i.e. delay only, no phase
shift),
convoluting function must be symetrical, i.e. must fall
identically at both
edges of head's gap.If it isn't so, or tape was poorly
recorded, or tape
is not evenly placed against the head, such a filter will
not have linear
phase response. This I derived from digital filter theory.
Now tell me I'm wrong. It's almost midnight here.

Roman