jeff@tesla.UUCP (05/28/83)
Relay-Version:version B 2.10 5/3/83; site mhuxt.UUCP Message-ID:<81@tesla.UUCP> Date:Sat, 28-May-83 13:39:22 EDT Linear-tracking turntables have always seemed to me to gadgetry for its own sake. Naturally if it could be done perfectly (i.e., without ANY drag on the stylus to keep it normal to the radius of the disc) it should lead to better sound than available with a standard tonearm; but it`s never done "perfectly" and the results would probably be barely audible, if at all audible, anyway. Just too much engineering for too small a result(possibly a negative result in terms of distortion and record wear if proper tangency cannot be maintained) which is in principle a Bad Thing. Nowadays you`d be better off saving your $ for a CD player; prices should hit $300 in a few years. There have been linear-tracking tables on the market, on or off, more more than 20 years; none has managed to stay on the market. Jeff
caf@cdi.UUCP (05/29/83)
I had a Rabco straight line tracking table/arm with a rotating cylinder for several years (still have it in the back room ...). The tracking mechanisim introduced significant noise into the playback process. When I replaced it with a mid-line (best unit that still had pitch control) Sony conventional direct drive semi-automatic, I was amazed at how much quieter my records had become. With the "Dynamic Stablizer" in some of the newer cartridges (Shure V-15/V) the short length/low mass of a linear tracking tonearm is no longer quite so important. -- Chuck Forsberg, Chief Engr, Computer Development Inc. 6700 S. W. 105th, Beaverton OR 97005 (503) 646-1599 cdi!caf
michaelk@tekmdp.UUCP (05/31/83)
The Sony PSX-800 linear tracking turntable is uP based. It has a base horizontal speed for stylus, then when there is a tracking error detected, it does a correction. So far, things are normal, but then, (according to literature) it *corrects* the horizontal speed to match the record! So for most of the record (other than the first few "learning" tracks) it is nominally more or less tracking the groove center rather than being dragged across as in normal tonearms. Of course, even at the correction point, the arm is close to tangential anyway. This turntable also has the Biotracer type mechanism that gives an effective arm mass of zero in both x and y directions. I'd agree that a CD player would be a better choice of purchase though,my Sony PSX-800 went for about $550-600 mail order from a discount place (plus another $125 for the cartridge) Mike Kersenbrock Tektronix Microcomputer Development Products Aloha, Oregon
jeff@tesla.UUCP (06/02/83)
I don`t see how the servo system can be open-looped after "learning the groove". Groove spacing varies according to depth of modulation--i.e., grooves are spaced wider where there`s a lot of high-amplitude bass information--so that continuous error messages MUST be received & processed by the servo. JF
michaelk@tekmdp.UUCP (06/02/83)
Yes the Sony PSX-800 "learning" is not turned off after the first few
tracks, it is continuously learning, and correcting , it is just the first
few track revolutions that some error is guaranteed. The point I was
trying to make is that unlike "dumb" tonearms that are ONLY drug across the
face of the record, or linear trackers that do a sawtooth correction (a simple
correction loop that has a tracking angle sensor, and just "corrects" the
position whenever the sensor trips, thereby giving a sawtooth error), the
SONY does try to establish a horizontal movement, that given a record with
evenly spaced grooves, would sit dead center in the groove, with zero
tracking angle error for the entire record. Now then, this learning
mechanism in addition to the linear tracking construction results in the
first order errors corrected. The error correction closed loop (with
the tracking angle sensor, etc) becomes a second order effect corrector, rather
than first order corrector. I might point out that all words "error" above
refer to very small numbers on an absolute scale, and refer generally to
relative error in a linear tracking environment.
While I am at it, I might explain the SONY biotracer construction. The
linear tonearm has (obviously) freedom of movement on a grand scale
horizontally when the whole arm moves. The arm also has a large vertical
movement freedom (up and down) that is obvious. But, it also has a limited
horizontal movement a little either side of perpendicular to the carriage.
In the arm (near the pivot point) are vertical and horizontal oriented
velocity sensors, and linear motors. It uses these to actively null out
arm resonances, and gives the arm a measurable, effective mass of zero.
Some of the interesting side benefits are that it automatically balances
the arm everytime it picks it up. So if you change the cartridge (that
universal mount) to a different weight one, instant re-balance. Additionally,
it uses a bias on its vertical linear motor to generate the vertical tracking
force. This makes the tracking force knob a potentiometer off to the side,
and you can turn it while playing the turntable to see how the cartridge
tracking is doing "real-time". Likewise, it provides a more consistant vertical
force on warps. The innards of the thing is PC board galore. Scratches
on the records still go pop though.
Mike Kersenbrock
Tektronix Microcomputer Development Products
Aloha, Oregon