From: kludge@panix.com   
      
   Liz Tuddenham  wrote:   
   >Tobiah  wrote:   
   >   
   >> > I could start a thread on an analogue declicking computer for 78s,   
   >> > which I am currently designing, but the number of participants would   
   >> > probably be very limited.   
   >>   
   >> You might be surprised.  Is it working?  How does   
   >> its performance compare to digital solutions.   
   >   
   >I'm pleased to report that the MkIII version which I have been working   
   >on recently has now worked for the first time.  The results are very   
   >promising and it is extremely easy to operate.   
   >   
   >Unfortunately, in trying to track down a common-earth hum problem, I   
   >accidentally blew up one of the power supply regulators and I didn't   
   >have a spare.  A new one is on order, so I hope to resume testing in a   
   >few days.   
   >   
   >The basic principle is to split the sound spectrum into ten bands, each   
   >about an octave wide.  In each band a click or crackle is identified by   
   >comparing the vertical and horizontal vectors of the stylus movement -   
   >if there is more than a certain proportion of vertical movement, a   
   >switch opens and momentarily disconnects the signal.  The band is then   
   >filtered again, so that the harmonics due to switch clicks are removed   
   >and damped resonance in the filter covers any short gaps in the sound.   
   >All ten bands are then recombined to restore the original spectrum.   
      
   That's not a declicker!  That's a broadband noise reduction system!   
   Compare with the Dolby Cat 43 system.   
      
   The declicker has a delay and looks at the first derivative of the input   
   signal, and when it is too high it replaces the signal with the delayed   
   signal or it holds the existing level, or it fades from one to the next,   
   in order to blend around a single impulse.  Compare with the SAE 5000.   
      
   I think that the Audio Cyclopedia has schematics of both designs, both   
   pretty primitive but showing the techniques.   
   --scott   
      
   --   
   "C'est un Nagra. C'est suisse, et tres, tres precis."   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)