Saturday, March 26, 2011

Clicks, Pops, and Other Irritants

     I still remember the time I first heard a compact disk (I have an aversion to spelling disk d-i-s-c.).  I knew immediately that I would have to switch to CDs as soon as my financial condition permitted it. That was about twenty-five years ago; and I eventually replaced most of my records.  Indeed I have no interest in even taking up room in my home with a turntable.
    Why do I feel this way? What was it about that first experience that impressed me so much? The short answer is everything. For me listening to music is an experience and anything that detracts from that experience is bad. Someone walking into the room and starting to talk is annoying to me as are background sounds like lawnmowers, traffic sounds, and construction noises. I don’t think I am alone in feeling this way. The problem is that at least 95% of all the music I listen to is classical music. Classical music has something modern music lacks, a wide dynamic range. The loudest and quietest parts of a work easily differ by 60db. There are actually short periods of complete silence. So how is this relevant?
     Roughly thirty-five years ago while working in the PVC industry I was involved in optimizing phonograph record compounds. Phonograph records or as they are called today, vinyl, are made of a vinyl chloride-vinyl acetate copolymer usually with fine particle vinyl chloride homopolymer added as a blending resin. Stabilizers and lubricants are added to help the polymer survive the processing step. A few percent carbon black is added primarily to cover up any contaminant that would be visible inside the record after pressing. During the 1950s RCA produced 45RPM records that were transparent red and I am truly impressed that they always looked pristine.
     The compound ingredients are mixed in a high intensity mixer and then extruded using a twin screw extruder and a special die with a rotary cutter to produce pellets of compound. These pellets are then put into a boom type extruder which forms what we used to call a doughnut. This doughnut is picked up hot and placed on the record label already waiting on the spindle of the press. A second label is placed on the doughnut and the press activated. The press platens come together in the heated hydraulic press and the polymer flows out to fill the record grooves. After a fast cooling step the pressed record is removed by hand and placed on a trimmer that removes the flashing along the edge. At that time a decent operator could produce a record every 20 seconds. Not many years after this we upgraded to an automatic press capable of pressing records at any rate the record compound would permit.
     From the viewpoint of the end user you would think that the primary objective of the record making process would be to produce the highest quality record possible at an acceptable cost. They would be wrong. If you follow the path between recording studio and record store you find a whole bunch middle men. There are sales and marketing people, distributors, shippers, and the retailer all in need of making a profit. The recording artists also get a piece of the action. If you consider that the record complete with dust jacket and album cover cost less than a dollar to produce back in those days and the final product sold for over five dollars you come to realize that the guy pressing the records does not have the highest profit margin.
     Going back to the 20 seconds it takes to produce a record you can see that making one in 19 seconds represents a 5% increase in productivity. That means an increase in profit margin and so a record compound capable of producing a record in one second less will look mighty attractive to the record manufacturer.
     Now imagine a process producing good quality records at a rate of one every 19 seconds when the processing conditions are exactly right and the compound has the optimum rheological properties. Now also imagine a slightly altered situation where the record compound being used is near its upper limit in flow properties and the temperatures of the processing equipment are a little short of ideal. You see what is going to happen. The record compound will not flow as well and some of the grooves will not form properly. This is called non-fill and causes a short burst of buzzing when played. Records can also be removed from the press while a little too hot and the operator if he grasps the record with a little too much pressure will cause a deformation beginning at the record edge and continuing about an inch towards the center. I have seen deformations so bad that the stylus actually loses contact due to the upward momentum of the tonearm when rising up the leading side of one of these deformations. Records with deformations such as this are said to be warped. Records will also warp if you store them at too high a temperature but that is a different issue.
     If you eliminate the records with obvious and unacceptable manufacturing defects you are still left with a record full of clicks, pops, hiss, and distortion. This is where rock has the advantage over classical music. With rock who is going to notice minor clicks and pops? The hiss will be completely buried in the signal and let’s face it, distortion is a part of rock music anyway.
     In the laboratory where record compounds were developed we used record platens with silent grooves and with recorded tones. When you listened to these records you could hear all of the defects in the silent grooves. With a scanning electron microscope you can see various defects in the grooves including an overall roughness. This roughness is probably not an issue if you consider that the radial velocity of the groove relative to the stylus is at least 22cm/sec so that even nearest the record label any defect causing the stylus to move less than 10µm will probably be inaudible.
     At one point in my life I spent the money buying pre-recorded open reel tapes and even though the clicks and pops were gone and the distortion reduced to a much more acceptable level, the hiss seemed to worsen. Dolby systems worked pretty good at eliminating the hiss but tapes were terribly inconvenient. They were also very expensive. And as I found out later they do not have an unlimited life. I also noticed that the magnetic signal seemed to migrate into adjacent layers of tape so that you could hear an extremely weak signal in the silent areas of the music
     With a compact disk the quality of the original recording comes through and as long as the CD is treated well the sound quality does not deteriorate with age. If there is something coming through on records that is missing on CDs it is not worth the price of noise and distortion. I have CDs of the exact same recordings I had on records and have compared the two. There is absolutely no way a record sounds as good as a CD.

3 comments:

Unknown said...

Ageed. :)

Robert~

Unknown said...

Very interesting! I should pass this on to Pam. She likes records a lot. I should bring up the subject at dinner sometime and listen to you two argue. :)

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