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  »  New  A proper implementation of low-pass filters..  Low Pass...  Melquiades Amplifier  Forum     4  33317  02-18-2008
  »  New  The FM Stereo and Multiplex MPX decoders..  The new life of Rohde & Schwarz decoder....  Off Air Audio Forum     12  107940  05-02-2008
09-06-2008 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,540
Joined on 05-28-2004

Post #: 1
Post ID: 8184
Reply to: 8184
Correcting Filters and the quantum theory.

Ok, can somebody to elaborate on the judgment which filter is better in case it is a corrector that acts in the middle of auditable range, let say from 1000Hz? It is the same filter but let look at the theory of judgments.

1)      Filter A is conceptually better because it acts as shunt filter, kind of subtraction corrector… The signal in band-pass (below 1000Hz ) does not see filter at all and whatever is being filtered out is shunted to ground. Therefore the signal that is passing through does not exposed to the nastiness of capacitor’s dialectic.

2)      Filter C is conceptually better because “indictors are better” and there is no transmission line reflections form the capacitor’s dialectic like in the filter A. The fact that the band-pass signal below 1000Hz flows unnecessary through the 120 feet of copper in the coil is irrelevant.

3)      It is all depend of the quality of coil and the quality of capacitor and if both of them are conceptually “perfect" then there is no difference.

I would like to hear your take, not just the take but also your justifications

Thanks, the Cat


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
09-07-2008 Post does not mapped to Knowledge Tree
hagtech


Hawaii
Posts 117
Joined on 02-13-2006

Post #: 2
Post ID: 8195
Reply to: 8184
What about memristors?
My first answer would be "3".  But then I think some more...

Is "A" really a shunt?  Where is the power dissipated?  Certainly not in the capacitor.  The signal is burned off (converted to heat) in the series element!  In circuit "B" it is the shunt element that burns off the power above 1kHz.

Why is an inductor better?  It has permeability issues (akin to permittivity in a dielectric), magnetic hysteresis, interwinding capacitance, etc.

When investigating questions like this I often take things to the extreme.  For example, what if the input signal was a kilovolt?  Now you think about volts per meter and insulation and arcing through the atmosphere and humidity and electrode spacing of the capacitor and heat dissipation in the resistor.  What if the frequency was 1GHz?  What if the frequency was 0.1Hz?  That coil starts to get pretty big.  What practical issues show up at the extremes?  Now re-apply them to the question at hand.  All the things one has to worry about at the extremes are still existent at 1 volt and 1kHz, just to a different degree.  But it does get you to think about theoretical limitations.

The other thing is that you can never appropriately ask such a question in isolation.  The circuitry before and after this filter changes the reality.  Often, the answer is more dependent upon the driving or receiving circuits than anything else.

So I guess I stick with "3" for now.

jh
09-14-2008 Post does not mapped to Knowledge Tree
CO
Posts 37
Joined on 11-18-2005

Post #: 3
Post ID: 8263
Reply to: 8195
The difference is in the damping
Filter B will have equal damping of the driver for all frequencies while setup A has "none" until the cap is full.
10-05-2008 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,540
Joined on 05-28-2004

Post #: 4
Post ID: 8451
Reply to: 8184
The LR filter in Rohde & Schwarz’s decoder.
I got a pair of good Pultec shielded coils that I am planning to stick in the Rohde & Schwarz’s multiples decoder to replace the Schwarz’s original RC based 75ms FM de-emphasis. I still am contemplation if to stick the filter in open loop where the signal would flow across the coil or to use the subtraction filter in feedback what the filter will be shunting and the signal would not flow across a few dozen feets of the coils’ wire.

R&S_decoder_output.jpg


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
Page 1 of 1 (4 items) Select Pages: 
   Target    Threads for related reading   Most recent post in related threads   Forum  Replies   Views   Started 
  »  New  A proper implementation of low-pass filters..  Low Pass...  Melquiades Amplifier  Forum     4  33317  02-18-2008
  »  New  The FM Stereo and Multiplex MPX decoders..  The new life of Rohde & Schwarz decoder....  Off Air Audio Forum     12  107940  05-02-2008
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