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Polishing changes cover layer thickness and affects spherical aberration, a
form of optical distortion where light rays from various radii of the
objective lens do not all converge at the same point. The result is a larger
focused spot (circle of confusion) and a degraded read signal having higher
intersymbol interference and crosstalk.

Polycarbonate has a different index of refraction than air, thus light is
bent when it enters and exits. The degree of bending depends on the angle of
incidence. The objective lenses of CD, DVD, and BD players are specially
shaped (aspheric) so as to predistort the laser beam such that the bending
is corrected and a diffraction limited, minimum diameter spot results on the
information layer. This correction is exact for only one cover layer
thickness, and the focused beam diameter increases for larger or smaller
thickness values.

Most disc manufacturers target a nominal thickness, thus polishing results
in a thinner disc and a degraded read signal because of spherical
aberration. Some manufacturers may ship thin discs because of cost
pressures, and polishing worsens an already bad situation. Conversely,
adding what is thought to be protective material to the entrance surface of
a nominal disc results in excessive thickness and undesirable spherical
aberration.

Non-uniform polishing will also cause tangential thickness variations
leading to mechanical unbalance that results in vibration during playback.
Vibrations coupled to the spring-mounted objective lens cause errors in
radial position and focus that the servos then attempt to correct.

Jerry Hartke
Media Sciences, Inc.

> -----Original Message-----
> From: Association for Recorded Sound Discussion List
> [mailto:[log in to unmask]] On Behalf Of Tom Fine
> Sent: Friday, November 19, 2010 1:55 PM
> To: [log in to unmask]
> Subject: Re: [ARSCLIST] Listening Tests
> 
> I am skeptical about polishing CD's beyond a non-defective finish from the
> factory. I'm not at all
> skeptical about stable mechanical transport. I would suggest that
> regarding the polishing, science
> needs to be done as to measured error rates before and after polishing. I
> can't think of what else
> polishing would effect, it shouldn't effect mechanical jitter. One thing I
> notice -- there doesn't
> seem to be too much controversy anymore that a careful listener can hear
> jitter artifacts, but there
> seems to be much controversy about whether one can hear correction of
> correctable read-errors.
> 
> -- Tom Fine
> 
> ----- Original Message -----
> From: "Clark Johnsen" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Friday, November 19, 2010 11:16 AM
> Subject: Re: [ARSCLIST] Listening Tests
> 
> 
> >I concur most heartily with Tom Fine's remarks. While much is known, much
> > yet is to be discovered -- or brought into production. The real error in
> > RBCD is our thinking that it works exactly as the (frequently appended)
> > textbooks tell us.
> >
> > Which brings me to:
> >
> > "Is anyone suggesting that fluctuation in the rotating of the disc
> itself
> > can have ANY effect on playback?  Do I misunderstand something?  Since
> the
> > data stream is buffered the speed with which it is read from the disc
> cannot
> > possibly have any effect on the playback speed which is controlled by
> the
> > DAC clock.  So what is the above quote intended to convey?"
> >
> > It is being suggested that all manner of things affect playback.
> Speaking as
> > an ex-digital designer (albeit optical) on the Mars Lander Camera, the
> error
> > budget for RBCD has not yet been written nor all the contributors
> identified
> > -- de rigueur for any NASA project but CD has never been systematically
> > analyzed. Hence it should not be surprising that improvements can be
> > discovered. I'll offer a couple of my own: vibration isolation and
> surface
> > cleaning/polishing. Yet it is widely supposed that players are robust
> enough
> > to overcome such difficulties; they are not.
> >
> > clark
> >
> > On Fri, Nov 19, 2010 at 10:27 AM, Alex Hartov <
> > [log in to unmask]> wrote:
> >
> >> Hi Folks
> >>
> >> Sometimes I have to express serious doubts about what I read.  From the
> >> given URL I read the following:
> >>
> >> "Many years ago, an irate musician contacted his CD-DA replicator
> claiming
> >> that his latest release was of poor quality. Extensive electrical,
> >> mechanical, and listening tests by the replicator failed to disclose
> any
> >> flaws while the musician became angrier. The persistent replicator
> finally
> >> identified a defective glass mastering turntable in which one coil of
> its
> >> brushless DC motor had failed, causing an imperceptable once- around
> speed
> >> variation. After repair, the musician declared his satisfaction in the
> >> resulting high CD-DA quality."
> >>
> >> Is anyone suggesting that fluctuation in the rotating of the disc
> itself
> >> can have ANY effect on playback?  Do I misunderstand something?  Since
> the
> >> data stream is buffered the speed with which it is read from the disc
> cannot
> >> possibly have any effect on the playback speed which is controlled by
> the
> >> DAC clock.  So what is the above quote intended to convey?
> >>
> >> Alex Hartov
> >>
> >> On Nov 19, 2010, at 9:30 AM, Rob Poretti wrote:
> >>
> >> > -----Original Message-----
> >> > From: Association for Recorded Sound Discussion List
> >> > [mailto:[log in to unmask]] On Behalf Of Jerry Hartke
> >> > Sent: November 18, 2010 8:10 PM
> >> > To: [log in to unmask]
> >> > Subject: [ARSCLIST] Listening Tests
> >> >
> >> > Input on the subject has been posted at:
> >> >
> >> > www.mscience.com/faq35.html
> >> >
> >> > Jerry
> >> > Media Sciences, Inc.
> >>
> >