Maybe these have changed a bunch since I looked at them, but the way they *used to* work was there was a lead screw that did coarse positioning, then a mirror and lens under audio frequency servo feedback loops looking at the reflection pattern to see how close to the center of the pattern it is. The light impacts the top surface at an almost 45 degree angle and only focuses at the plane of the pits (mechanically presses in mass production disks.) This helps prevent marks on the disk surface from blocking data read back. The pit pattern is assumed to be neither concentric nor flat, and the servo system is continuously adjusting to find the pattern center.
When you seek to an area of the disk, the lead screw positions to the approximate area and you hope the first stop has the location wanted within the mirror reach. The servos start locking in on the signal, then they read whatever they find. In there are the time codes and chapter marks. It then jumps the mirror a guessed amount, locks in more quickly since they have a much better guess of the initial servo locking. Lather, rinse and repeat till you get there. The long and short of this is that the actual positioning accuracy of the system is only moderate, but the servos are very cleverly built, signal lock up is fairly quick and the data encodes a great deal about where you are in the stream. None of this sounds good for what was asked. jerry On Thu, Feb 18, 2016 at 10:47 PM, Nicklas Karlsson < [email protected]> wrote: > On Thu, 18 Feb 2016 20:47:28 -0800 > Chris Albertson <[email protected]> wrote: > > > On Thu, Feb 18, 2016 at 11:23 AM, Nicklas Karlsson > > <[email protected]> wrote: > > > > > I guess it would be possible to position an ordinary disc read head > quite accurate. Either one for DVD/CD or magnetic. How many bits are stored > per turn? > > > > > > > Those devices use the data that is recorded on the spinning platter to > > determine where the read head is. The head is then served to follow > > the track. It works well enough that is the spindle bearing is loose > > the head can follow the track as it wobbles > > > > The CD/DVD track is a long spiral (alike on a vinyl record) so the > > head must continually move if it is to read. All of these are not > > very good at absolute positioning > > They would be good at absolute positioning if a suitable pattern is added. > It should also be possible to figure which bit is at position right now. At > standstill I guess there would be a need for moving disc head. > > How good could it be? There have been extensive research on packing bits > as tight as possible ==> how many bits are there per turn? > > Then in practice it is a matter of what could be bought. > > > Nicklas Karlsson > > > ------------------------------------------------------------------------------ > Site24x7 APM Insight: Get Deep Visibility into Application Performance > APM + Mobile APM + RUM: Monitor 3 App instances at just $35/Month > Monitor end-to-end web transactions and take corrective actions now > Troubleshoot faster and improve end-user experience. Signup Now! > http://pubads.g.doubleclick.net/gampad/clk?id=272487151&iu=/4140 > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users > -- Jerry Scharf FINsix IT 650.285.6361 w 650.279.7017 m ------------------------------------------------------------------------------ Site24x7 APM Insight: Get Deep Visibility into Application Performance APM + Mobile APM + RUM: Monitor 3 App instances at just $35/Month Monitor end-to-end web transactions and take corrective actions now Troubleshoot faster and improve end-user experience. Signup Now! http://pubads.g.doubleclick.net/gampad/clk?id=272487151&iu=/4140 _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
