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MMD > Archives > November 1998 > 1998.11.03 > 16Prev  Next

Rachmaninoff CD - Mechanics versus Mathematics
By Julian Dyer

Regarding the injured sensibilities that Wayne Stahnke's 'tinkering'
with the Ampico rolls for the CD has produced, perhaps it should be
noted that a rigorous mathematical model of something can be 100%

Forget the 'improved' _mechanical_ devices from the 1960s onwards,
because these produced bastardised performances, their analogue
representations inevitably having performance traits at variance with
the way the rolls were coded.  (The mechanical equivalent of two wrongs
not making a right.)

A computerised mathematical model theoretically can provide a 100%
accurate representation of the way a mechanism works, although the
minutest details of the system must be right, which can be hard to
achieve rigorously (i.e. by mathematical equations).  In practice,
the rigorous approach inevitably has a degree of heuristic (rule-based)
modelling as well, which is much more open to individual judgement.
The beauty of all this is that if some particular aspect isn't right,
you just add another subroutine to correct it!

If it's all a bit theoretical, consider the LP and the CD.  An LP
is an analogue model of a sound, with a wiggly groove representing the
waveform.  A needle slops its way along the track to read it, picking
up all sorts of mechanical distortions along the way, vibrations as the
mechanism resonates, rumbles from the turntable bearings, that sort of
thing.  A CD is just a string of numbers, representing measurements of
the position of the waveform 44 thousand times a second.  The sound you
hear from the CD player is from a mathematical model of what a series
of such numbers should sound like - it is not the original sound

In general, it is now agreed that the distortions and inaccuracies of
the CD are less unpleasant than those of the LP, although it took some
refinement to get that far.  It's the same sort of thing between using
a mechanical device and a mathematical model to play a roll - although
of course the roll itself is digital to start with, LP vs. CD just being
an illustration that may have some real-life meaning!  The difference
is that a (digital) roll is edited to suit the original playback
mechanism, while an (analogue) LP cannot be edited this way.

What all this really means is I can't see anything wrong with the
approach Wayne's taken.  It works because he has put enough, correct,
detail into the model.

Julian Dyer

 [ Early in the development of the IMI Cassette Converter in the 1970s,
 [ Wayne asked the 'best ears in town' to evaluate the performance of
 [ his device (electric magnet valves) in comparison with a roll-playing
 [ reproducing piano.  He learned a lot: the 'best ears' were the
 [ old-time piano techs and player piano techs of Los Angeles, who
 [ worked at, or frequented, Dick Carty's piano shop.  They all loved
 [ and thoroughly knew the classical pieces being played, and they
 [ taught Wayne the subtleties.  You could say that Wayne's mathematical
 [ studies were ultimately confirmed independently by 'the best ears
 [ in town.'  -- Robbie

(Message sent Tue 3 Nov 1998, 14:43:47 GMT, from time zone GMT.)

Key Words in Subject:  CD, Mathematics, Mechanics, Rachmaninoff, versus

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