tangential drive
I just had to look it up, and found this:...................(1981-1982) Stereo turntable
http://beocentral.com/beogram8000
I just had to look it up, and found this:...................(1981-1982) Stereo turntable
http://beocentral.com/beogram8000
The Beogram 8000 used a new system called “Tangential Drive”, a form of direct drive that used “linear motor” technology to give smooth, silent and controllable power to the turntable. In simple terms, the motor coils placed a magnetic field at a tangent to a metal drum under the platter. This had the effect of pulling the drum through the coils in a manner that was smooth and free of “cogging” and “stepping” effects. This layout was at odds white the techniques used in the typical Japanese direct drive turntable, which tended to use hall-effect motors based around rotating magnetic discs and multiple electronically-commutated coils in an arrangement that was fundamentally not “stepless”.
While B&O’s tangential drive could have been regulated by analogue means alone, the presence of the microcomputer presented an opportunity to control the process digitally and so add the precision of the microcomputer’s quartz crystal to the formula. By sensing the rotating speed of the platter with an encoding ring fitted to the turntable shaft, the microcomputer could be included in a “closed loop” system, permanently ensuring perfect speed accuracy. As B&O’s turntables were frequently bought by musicians, it was desirable to include a “pitch” adjustment. This was easily included in the Beogram 8000 and for the first time a speed display was fitted so that the amount of pitch correction could be recorded and easily returned to. The system allowed the speed to be adjusted in 0.05 RPM steps at 33 1/3 RPM and 0.07 RPM steps at 45 RPM. The display also showed when the speed control servo could not lock by flashing all the digits on and off.
A quirk of the tangential drive system was that when the driving coils were not energised the platter was left to turn freely. Because of the high quality bearings used, this would have meant that after the machine was stopped it would have taken a very long time (about half a minute) for the record to stop turning, making it difficult to remove. To counter this possible nuisance, an electronic brake was fitted that used the driving coils to bring the platter to a swift halt. The braking force was applied progressively to give smooth results and the overall effect was similar to that observed when a conventional belt-drive turntable is switched off.