From: bill@rsphys.anu.edu.au (Bill Alford)
Newsgroups: aus.hi-fi,rec.audio
Subject: Followup to Rotel 955 versus Rotel 855 CD players
Date: 19 Nov 1992 04:16:57 GMT
Organization: SCU, RSPhyScE, Australian National University
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Reply-To: bill@rsphys.anu.edu.au (Bill Alford)
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I've had a query about two CD transport tweaks that I mentioned, which
I'll explain further. The first was mentioned in Stereophile, June 1992,
pp 208-210 about Laser Illusions Spatial Filter. Basically this is an
optical aperture device which sits over the laser designed to reduce
jitter in the recovered datastream from a CD transport. I'll summarise
the Stereophile article.
  Don Snipes, a laser researcher and non-audiopile, took a look at the
laser and optics in CD players and noticed that the optical aperture
(opening) of the lens seemed larger than optimum. He also noticed a
consequential high level of noise (jitter) in the recovered EFM (eight
to fourteen modulation) signal from the disc. The EFM signal is a series
of nine discrete frequency sinewaves that contain the information encoded
on the disc. These nine frequencies correspond to the nine discrete pit
or land lengths embedded in the CD. Although the EFM signal is composed
of sinewaves and appears to be analog in nature, the sinewaves zero
crossings delineate the digital code transition.
  A tiny donut-like disc was machined that would fit over the lens, thus
making the aperature smaller. Looking at the EFM signal recovered by a
CD transport fitted with the Spatial Filter, it was markedly and quite
measurably less noisy, presumably due to the increased depth of focus.
However making the len aperature smaller also reduces tracking ability.
The CD's designers apparently decided that reliable tracking was more
important than a cleaner EFM signal. I have yet to pick up any
mis-tracking with my Rotel 955 CD player and I'm curious to see what
happens with the Pierre Verany drop out tests with and without the
Spatial Filter. According to the Stereophile article, the Spatial Filter
makes a nice difference to the Rotel 855 CD player, which I yet to
verify. Several CD player manufacturers are incorporating the Spatial
Filter in their products.
  I should point out that a similar thing is happening when it was
noticed that recordable CDs (CDR's) played better than the source CD from
which it was made. The CDR's has a much cleaner pit shape than mass
produced CD's which leads to a better EFM signal and hence a lower level
of jitter in the recovered datastream.
  The second CD transport tweak was mentioned in Stereophile, December
1990, p264, which I'll summarise about Simply Physics Isodrive CD
clamping/replacement bearing system for Philips CD transports. The
Isodrive is precision machined to clamp the entire CD (at the periphery
first) and itself into the centre of spin -- the drive hub itself. This
greatly improves the rotational stability, flatness and reduces
vibrational interference which lead to reduced servo corrections and a
reduction in jitter. The flimsy Philips CDM 4 drive in the Rotel 955 CD
player has minimal CD clamping, which Philips admits to in the Marantz
CD52SE when they take the same drive and increase the amount of CD
clamping. And it wouldn't be too hard to do, but the Isodrive does it properly
(although Pioneer's stable platter mechanism is a much better approach).
One correspondent has pointed out that the extra weight of the Isodrive
may tax the motor in a CD transport through its extra weight.
  

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| Bill Alford,                            | Email: bill@rsphys.anu.edu.au |
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