Electronics Forum / Basics / May 2006

You make your cables out of vacuum? I tried that once, but they were a
bugger to solder. Kept wicking it up. ;-)

Tim

My first response was a bit foggy.  I think my "analog filters"
brain cell was on coffee break.  Here's an better answer:

There are delay-line analog filters, whose main purpose
is not to remove frequencies but only to delay them.  These
are generally found under "All-Pass Filters" in filter texts.
They may also be found as "Delay Equalizers".
The earliest versions of delay lines were cascaded LC networks,
which is essentially what you'd get with the "really long cable"
approach others have posted.  But with active filters this can
be done much more simply, in only a few stages.

The basic all-pass design (first order) looks sorta like
a differential op amp stage with the incoming signal fed to
both the inverting and non-inverting inputs.  There are
the usual identical feedback and inverting input Rs,
but where the diff amp would have idenitical non-inverting
input and ground Rs (voltage divide by 2), the all-pass has
instead an RC, either low-pass of high-pass.

2nd order stages are a bit more involved, but not much
different from an active low-pass 2nd order.

The basic strategy is to design a low-pass filter with
a (typically) Bessel response, then apply a "transformation"
to get new values for the Rs and Cs in the all-pass
design.  Besides Bessel,  there are Linear Phase and
Equiripple Error approaches to the same goal, which is
to have a phase shift that varies linearly with frequency.

A good reference for this is "Electronic Filter Design Handbook"
by Arthur B. Williams. My copy is 1981; there may be a later
edition by now.  

Best regards,

Bob Masta

An analog delay inherently stores information. If the delay is, say, 1
second and you want CD-audio quality, it stores the equivalent of over
700K bits, whether it's a digital or an analog network. So one way to
evaluate any delay device is to ask if it can store this many bits.

John

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Reely?