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From: sjc@angband.UUCP (Steve Correll)
Newsgroups: net.audio
Subject: Re: 2 questions, 2 replies
Message-ID: <48@angband.UUCP>
Date: Sun, 3-Mar-85 00:55:46 EST
Article-I.D.: angband.48
Posted: Sun Mar  3 00:55:46 1985
Date-Received: Tue, 5-Mar-85 02:29:53 EST
Distribution: net
Organization: S-1 Project, LLNL
Lines: 57

>  Are there advantages to a linear tracking tonearm other than the
>  obvious removal (you hope) of tracking error?  An article that I copied
>  from Stereo Review states that there is no need for anti-skating
>  compensation.  Is this true?  Can you explain why?...Also, what is the
>  audible manifestation of tracking error?

Look down on your turntable from above, and imagine a line down the
center of your cartridge from front to back. If you have a pivoted
tonearm, you'll notice an angle between that line (I'll call it the
"centerline") and the tonearm: this angle is responsible for skating
force. Friction on the disk surface exerts a force on the stylus, and
because of that angle, the force pulls sideways on the tonearm (or, in
Newtonian terms, the force has two vector components, one of them
parallel to the tonearm and the other perpendicular to it and directed
inward).

Why the angle? Well, the machine which initially cut the groove kept
its centerline tangent to the groove, so one goal of any tonearm is to
keep the cartridge's centerline tangent, too.  The obvious approach
(which nobody uses) is to mount the cartridge with its centerline
parallel to the arm and make the arm just long enough to be tangent
to the outermost groove. A little Euclidian reasoning will show that as
such a tonearm approaches the innermost groove, it must somehow grow
longer to remain tangent.

The popular solution is to make the tonearm longer in the first
place, but mount the cartrige so its centerline is angled inward from
the tonearm, retaining tangency at the outer groove. Given the right
length and angle, such a tonearm will be tangent at the innermost
groove, too, though it will be a little bit wrong in between.
(Manufacturers' hype to the contrary, it doesn't matter whether the
angle is achieved by bending the arm or mounting the cartridge at an
angle at the end of a straight arm.)

Tangential (linear-tracking) arms, whether electrical or mechanical, exhibit
no skating because all the force exerted on them by the record surface is
parallel to the arm.

I have never heard any audible manifestation of tracking error. It rarely
exceeds a degree or so, whereas the vertical angle of the stylus can vary
by 5 or 10 degrees between different brands of cartridge. The minimization
of tracking error is popular, however, whereas the standardization of
vertical angle is not. People don't seem to worry about the 5 to 8dB resonances
which some highly praised moving-coil cartridges exhibit around 15kHz, either.

I'm (overly) fond of citing an article in "Audio" magazine about a year
ago which concluded that tangential arms actually have about as much
error as pivoted arms, because their servos dither a bit. However,
tangential arms can be much shorter, and therefore less massive, and
therefore (with highly compliant cartridges) more immunity to record
warp.  (Even if warps don't bounce the stylus out of the groove, they
make life harder for the stylus by making the downward tracking force
fluctuate.)

-- 
                                                           --Steve Correll
sjc@s1-b.ARPA, ...!decvax!decwrl!mordor!sjc, or ...!ucbvax!dual!mordor!sjc