CDROM Laser Tracking Question

[Web Gecko]

Howdy

I have been checking out CDROM hardware lately and have a question relating to how a basic single beam IDE/ATAPI drive tracks the spiral data track on a standard CD. I have Meuhlers'
Repairing and Upgrading PCs 12th edition (can recommend this) which has good detail on optical drives but the spiral tracking is not detailed right down at the low optic level.

Basically a low powered laser source is reflected onto the CD surface using a mirror. The reflection is captured by a special lense/splitter optical unit which is mounted on a worm gear and moves radially across the spinning disc surface. The data is then fed to the onboard IDE electronics/drive processor and sent back to the host processor.

The basic single speed format is laid out in blocks (or sectors) of
2,352 bytes (304 bytes are used for SYNC, ID, ECC and EDC
error detection/correction code leaving 2048 [2K] bytes for user data. At single speed the blocks are read at 75 blocks per second.
So for a standard CDROM this amounts to standard single speed transfer of 153,600 Bps or 150KB/sec which is also your 1x speed (minimum) transfer rate which must be sustained between
CDR[W] and HDD for burning. If you laid out the data spiral which starts from the inside of the disc out, the pits and lands would stretch for around 3 miles on a standard 650MB disc !

It seems that for best performance with a dual IDE setup you should put any CDROM/CDR[W] drives on the secondary and your HDDs on the primary. Many systems have a master HDD and slave
CDROM or CDRW on the primary IDE port which works but maybe not optimally. The HDD will have to wait while any CD commands on the same port complete. So it makes sense to have your main HDD and CD drives on opposite IDE ports.

Can anyone explain how the laser device tracks the spiral of the data track ? With only radial movement across the surface (like a floppy or HDD which use simple parrallel or concentric tracks) I can't see how this is done. I have some old dead IDE/ATAPI drives and have one apart (an old ACER I think with OAK chip set).
The tray and clamping devices are simple but I think the magic must lie in the optical/splitter unit. I would love to know how it works.

Also I have been told that many drives stop reading because of dirt on the optics and maybe only need some simple adjusments
to realign/recalibrate the optics. Is this correct ? So many people may be throwing out good drives when a simple adjust/clean is all that is required. I know they are cheap and many people don't bother.

If it can be done without too much hassle than I for one am prepared to try.

Any CDROM afficionados out there ? Any ideas welcome.

cheers and thanks

The Web Gecko

[systempat]

Whew!
I think you're getting too heavy for us here. We just install and use them, we don't design them.
I believe the CD-rom design (and remember it was originally designed for audio work with a continuous output) - I believe the mirror or head on the worm gear is steadily advancing at a specific rate so as to follow the spiral track.
As opposed to a HD where it has to step from track-to-track.

[mike breck]

Hi Web Gecko,

I think many people write off CD-Rom Drives when even a simple lens cleaner and cleaning the CDs would get it working.