William Donzelli wrote: Like eight-track tape? (I still have a few). -- Ward Griffiths They say that politics makes strange bedfellows. Of course, the main reason they cuddle up is to screw somebody else. Michael Flynn, _Rogue Star_

Or 33 and 45 rpm vinyl and cassettes. If you think these three formats, at least two of which are getting rather elderly, are obsolete, think again. But not nearly as popular as more modern mediums. The first two examples date from a time when not every other person on the train has a personal deck. The hard part of a CD drive (either kind) is the head and servo system, and if those parts work already, your almost all the way home. The format for CDs is well documented. You could even decode everything in software, once the entire bitstream of a CD is read. William Donzelli william(a)ans.net

I was thinking of simply reading the whole disk, as if you were going to transfer its contents onto a more modern media. If it does get to the point that CD-ROM drives in working condition are indeed rare things, it is time to move to another format. The servo systems in those things are dead stupid, and should not really cause many problems reverse engineering those bits. William Donzelli william(a)ans.net

Skip that stuff! If you can get the raw signals off the photodiodes, and the characteristics of the motors (focus, disk rotation, and seek), designing a dumb control system would be trivial. You could even skip the focus control, replacing it with a manual system that would need fiddling with each time (although todays CDs are remarkably consistent). Just take outputs of the photodiodes and stick them thru a very simple control system (a PID system would be way overkill!) and feed it to the servomotor. Yes, you might have to fiddle around with a bunch of resistances to get the loop right, but that is not too bad. For a true CLASSICOMP type, a Heathkit EC-1 could be used to get the control system designed! I am not thinking about having a random access device at all. If CD-ROM drives are not available, go to the next medium. Read the old CD-ROMs once and transfer, just like 7 track tapes. William Donzelli william(a)ans.net

That is the easy one to figure. Compare the queue length with a threshold value, and feed a value into a DAC (motor drive circuits are assumed). Something along the lines of Mspeed = Kspeed + (Qoptimal - Qlength). Here, if the queue's length (Qlength) is greater that the optimal queue length (half full, half empty, Qoptimal), a negative value is added to constant (optimal) Kspeed, and the result is Mspeed, a value thrown at a DAC. The DAC then feeds a simple DC motor driver. There are lots of proven circuits for that last bit. Of course, if one reads of the bitstream and stores it on a hard disk or a mountain of RAM, the queue could be considered infinitely large, and the spindle would never race ahead and fill it. I believe that the pits in the aluminum (or gold) are encoded with a clocking reference (probably PCM), so clock recovery might not be too bad if the spindle is kept at a reasonable velocity. One could even vary the velocity by the position of the head, to making the clocking "window" a bit more controlled. Warped? Yes, if the disks are treated poorly. I just took a sample of some of my CDs, and they are definitely not warped - I can stick them face to face and they match perfectly. I would certainly hope that archival copies of things on CD-ROMs would be treated gingerly. Having to deal with scratches and scuffs would be a big hassle. OK, big deal. You could figure out the basic characteristics of the focus system by monitoring the circuit while it does its initial looking for the disk (put no disk in the drawer, and watch the lenses move to its extremes), and while spinning a disk, so you can get an idea of what the optimal signal levels are. I assume that the photodiodes will output the most when things are in focus. If not in focus, some of the light will not hit the detector. As I said before, forget the seek function. Read the whole track and figure out what you want later, with it sitting on a hard disk (or even RAM). Adding seek functionality does complicate things greatly. Yes, I have seen this, where the worm lurches back and forth. Solving this problem would involve adding one more input to the fine tracking controller, basically a value stating how far off the coarse tracker thinks it is. I would condition the input a bit to get rid of any gear chatter and motor "cogging", probably with an integrator. Not doing so might cause the the fine movement to buck around wildly every time the coarse drive motor "coggs" (unofficial term for the unevenness caused by the magnets in motor, much like a stepping motion). Experimentation is called for! I remember making control systems with analog parts (op amps are wonderful things!), and the easiest thing to do was to chuck the equations out the window and place multiturn pots where called for. It involves a bit of trial and error, but a good idea of how things behave quickly emerges. In the above example with the integrator, put a pot on the thing to vary the effect. Find out what value works best, so the fine control system still tracks when the coarse system adjusts itself, yet does not go nuts. William Donzelli william(a)ans.net

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