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From: Les Cargill on 15 Jun 2010 15:59
Tim Wescott wrote:
> On 06/15/2010 10:37 AM, Jerry Avins wrote:
> -- snip --
>> Note that the signal was reversed, not the filter. It would make no
>> sense to apply the signal to a shunt capacitor and put a resistor
>> between the signal and the output. If the output impedance of the driver
>> were low enough and the input impedance of the 'scope were high enough,
>> that filter would have no effect.
>
> Actually, to reverse that filter you'd need to find a device with
> negative capacitance. This would be difficult, as any causal linear
> negative-capacitance device would be storing huge amounts of negative
> energy at very high voltages.
>

Yes, but don't you need Hiesenberg Compensators for that? :)

--
Les Cargill
From: glen herrmannsfeldt on 15 Jun 2010 16:16
Jerry Avins <jya(a)ieee.org> wrote:
(snip of question on time reverse filters)

> The math is getting in your way. Set DSP aside for now. Run an analog
> square wave through a low-pass RC filter consisting of a series R with
> the output taken across a shunt C. Tape record the output as you look at
> it with an oscilloscope. Think of the original waveform as a sequence of
> steps of alternating polarity. Note that the leading edges of the steps
> are virtually unchanged, while the trailing edges are rounded. This
> asymmetry is a sign that in addition to attenuating low frequencies, the
> filter has delayed high frequencies more than lower ones.

I think I can imagine that.

> Now play the tape backwards. (If your player won't run in reverse,
> unwind the tape and wind it on again from the other end.) Observe that
> the leading edges are rounded, while the trailing edges show sharp
> corners. This asymmetry is a sign that in addition to attenuated low
> frequencies, high frequencies have been advanced relative to lows. By
> running the tape backward, we have effectively reversed time. (Well, we
> at least reversed sequence.) What was a phase lag before reversal
> becomes a phase lead afterward.

That might be hard these days. I still have cassette decks, but
it isn't easy to reverse them. It would be fairly easy, though,
to write a program to time reverse a WAV file and write it to
a CD. Probably easier than finding a tape deck to reverse it.
Well, if the tape is thin enough, then you can read it through
from the back. That adds a low-pass filter due to the tape
thickness and head gap width.

-- glen
From: glen herrmannsfeldt on 15 Jun 2010 16:24
Tim Wescott <tim(a)seemywebsite.now> wrote:
(snip)

>> Actually, to reverse that filter you'd need to find a device with
>> negative capacitance. This would be difficult, as any causal linear
>> negative-capacitance device would be storing huge amounts of negative
>> energy at very high voltages.

> Oops -- I forgot the :-)

I once had the idea of building a negative resistor. (That is,
an op-amp with the appropriate feedback such that when inserted
in a circuit it gave the appropriate response.)

Well, more specifically there are physics labs where one is given
a box with a voltage source and resistor. By measuring the IV
characteristics, one can determine the value of the voltage
and resistance. (This is to teach the statistics of measurement
errors, not the physics of voltage and current.) So the idea
was to sneak in the battery and negative resistor box, and see
the results.

As to the lab, one measures the voltage and current with different
load resistors. For sufficient accuracy, though, one must include
the series resistance of the ammeter, and the parallel resistance
of the voltmeter. (More the former than the latter, usually.)

Others that I knew complained about these labs, but I liked
doing them.

-- glen

From: Jerry Avins on 15 Jun 2010 16:59
On 6/15/2010 4:16 PM, glen herrmannsfeldt wrote:
> Jerry Avins<jya(a)ieee.org> wrote:
> (snip of question on time reverse filters)
>
>> The math is getting in your way. Set DSP aside for now. Run an analog
>> square wave through a low-pass RC filter consisting of a series R with
>> the output taken across a shunt C. Tape record the output as you look at
>> it with an oscilloscope. Think of the original waveform as a sequence of
>> steps of alternating polarity. Note that the leading edges of the steps
>> are virtually unchanged, while the trailing edges are rounded. This
>> asymmetry is a sign that in addition to attenuating low frequencies, the
>> filter has delayed high frequencies more than lower ones.
>
> I think I can imagine that.
>
>> Now play the tape backwards. (If your player won't run in reverse,
>> unwind the tape and wind it on again from the other end.) Observe that
>> the leading edges are rounded, while the trailing edges show sharp
>> corners. This asymmetry is a sign that in addition to attenuated low
>> frequencies, high frequencies have been advanced relative to lows. By
>> running the tape backward, we have effectively reversed time. (Well, we
>> at least reversed sequence.) What was a phase lag before reversal
>> becomes a phase lead afterward.
>
> That might be hard these days. I still have cassette decks, but
> it isn't easy to reverse them. It would be fairly easy, though,
> to write a program to time reverse a WAV file and write it to
> a CD. Probably easier than finding a tape deck to reverse it.
> Well, if the tape is thin enough, then you can read it through
> from the back. That adds a low-pass filter due to the tape
> thickness and head gap width.

My tape deck is bidirectional. Normally, when the capstan motor is
reversed, another pair of heads (it's stereo) is used. Simple rewiring
can change that.

Jerry
--
Engineering is the art of making what you want from things you can get.
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From: glen herrmannsfeldt on 15 Jun 2010 17:17
Jerry Avins <jya(a)ieee.org> wrote:
(snip)

> My tape deck is bidirectional. Normally, when the capstan motor is
> reversed, another pair of heads (it's stereo) is used. Simple rewiring
> can change that.

I never had a bidirectional tape drive. Most now have a mechanically
rotated head, which I might not trust to maintain alignment.

When I was younger, I had a friend who (well, his parents) had a
stereo tape deck with a switch that would switch to the opposite
set of stereo tracks, though not reverse the tape. Especially
useful to listen to some rock songs backwards while checking for
secret messages.

Otherwise, some of my favorite tape stories from years ago
include tape loops going across the room and back, with stands
to hold up reels to keep the tape from dragging on the floor.

-- glen
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