From: liz@poppyrecords.invalid.invalid   
      
   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.   
      
   My previous attempt (the MkII) used inductors in the resonant circuits   
   and became so cumbersome and difficult to align that I abandoned it.   
   The latest version uses 'state-variable' filters based on op-amps and a   
   cascade filter configuration that makes the centre frequiencies far less   
   critical and allows the use of ordinarly 5% tolerance components.  The   
   overall pass band is within 2 dB of flat from 20 c/s to 20 Kc/s and the   
   ripple due to filter overlap is less than 0.5 dB.   
      
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)