Linear-tracking arms bypass this difficulty. Because they move in a straight radial line, the angle stays constant and the cartridge remains at a true tangent - at least in theory.

That's not the only benefit. Linear tracking also eliminates the side-thrust - called the skating force in hi-fi lingo - which makes conventional arms lean harder against one side of the groove than the other. This side-thrust - normally compensated by special antiskating adjustments - is caused by the offset angle for the cartridge holder in conventional arms. Linear arms have no such offset angle, so the problem never arises in the first place.

On top of all that, linear arms can be a lot shorter than the pivoted kind. This makes them lighter and less likely to get all jumpy when stirred by powerful bass wiggles in the groove or the upand-down swings on a warped record. Their lightness makes them impervious to such disturbances, and they can ride out smoothly those tricky passages that might derail ordinary arms. Finally, linear-track arms don't depend on gravity and counterweights to provide the correct stylus pressure (using constant spring tension instead) so they can play in any position, even standing up vertically.

With all this going for them, one would expect everyone to be cheering for these innovative items. Yet some engineers have second thoughts, balancing the advantages of linear tracking against certain drawbacks.

For one thing, it takes a complex mechanism to guide these arms across a record. They don't just get pushed along by the record groove, like conventional arms. Rather, linear-tracking arms are propelled by a device similar in principle to the autopilot which keeps airplanes on course. The autopilot senses deviations from the charted course and then takes corrective action. Exactly the same happens with these tonearms. Whenever a sensor in the arm registers a misalignment (deviation from true tangent) between the arm and the groove, it activates a mechanism which then pushes the arm the tiny distance needed to bring it in line with the groove. The process is repeated again and again as the cartridge moves from one groove to the next. Thus, in a series of small corrective steps, the arm moves across the record.