From: liz@poppyrecords.invalid.invalid   
      
   Geoff  wrote:   
      
   > On 16/09/2025 7:25 am, Liz Tuddenham wrote:   
   > > Scott Dorsey  wrote:   
   > >   
   > >> 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!   
   > >   
   > > It was primarily intended to de-click and de-crackle 78s, but it can   
   > > also de-thump cracked ones, so I suppose it is more broadband than I   
   > > originally suggested.   
   > >   
   > >   
   > >> 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.   
   > >   
   > > Finding the clicks and crackle is realtively easy on a mono recording:   
   > > they show up in the vertical component (which obviously wasn't   
   > > recorded).  The big problem is what to do about them.  The Marantz and   
   > > other noise-reduction systems simply faded out a click - which   
   > > substituted a 'bloop' instead; cross-fading between before and after   
   > > could be better if it is done well.   
   > >   
   > > The only system that seemed to do it really effectively and in real time   
   > > was Cedar: it claimed to synthesise a 'gap-filler' from the Fourier   
   > > transform of the sound before and after the click.  Even that wasn't   
   > > really intended to handle the continuous crackle of some of the worst   
   > > gritty solid stock material (particularly inter-wars U.K. HMVs).   
   > >   
   > > I wondered if some sort of resonator could 'ring' and fill the gap, but   
   > > it would need quite a lot of them to cover the whole audio band.   
   > > Octaves seemed the logical way to go but that involved either a large   
   > > number of ferrite pot cores or a lot of op-amps in state-variable   
   > > filters.  I tried the pot-core approach many years ago and gave up   
   > > because it was becoming too unweildy.   
   > >   
   > > This time I opted for state-variable filters and my design finished up   
   > > with 170 op-amps and several other I.C.s.  It was a bit of a gamble   
   > > whether it would be worth building it, as the principle was untested, as   
   > > far as I knew.  It has paid off, the results are stunning; as the   
   > > sensitivity is increased, the crackle just fades into the background and   
   > > the music comes through absolutely unaffected.   
   > >   
   > > There are still a few things that need attention, as I can hear residual   
   > > artefacts at a very low level - but I exhibited it to a group of   
   > > gramophone enthusiasts at the weekend and they were completely bowled   
   > > over by the sound quality.   
   >   
   > Isn't this all now trivial to do, better, in software ?   
      
   In real time?  In a portable record player?   
      
   The only real-time system I know of is Cedar and that isn't too good on   
   continuous crackle.  I may get the opportunity to try the analogue   
   computer alongside a Cedar digital one in a few weeks time, then I will   
   be able to say how they compare.   
      
   Everyone tells me it can be done (better?) in software but I haven't   
   heard anyone do it yet.   
      
      
   >Or is the   
   > project more of an intellectual exercise ?   
      
   It was intended to test out an idea that hadn't been tried before and   
   see if it worked and what sort of results it gave (and which material it   
   *didn't* work on).  If it was successful, it would then be used in   
   portable situations where dismounting the rack-mounted analogue system I   
   already use was too much trouble.   
      
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
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