david@daisy.UUCP (David Schachter) (01/10/88)
In article <1972@ncr-sd.SanDiego.NCR.COM> greg@ncr-sd.SanDiego.NCR.COM (Greg Noel) writes: >In article <17309@topaz.rutgers.edu> ron@topaz.rutgers.edu (Ron Natalie) writes: >>CD ROM roots would be bad because CD ROM's are blindingly slow. > >Currently true. But don't take the current state-of-the-art as an intrinsic >limit. They'll get faster. According to "CD-ROM: The New Papyrus" by Microsoft Press, faster data transfer from CD-ROMs is unlikely because the frequency of the data starts to approach the frequency of the servo mechanisms used to keep the optics on track and in focus. -- David Schachter (daisy!david or well!davids) Disclaimer: I'm wrong about nearly everything. Cute quote: That's not funny.
tainter@ihlpg.ATT.COM (Tainter) (01/12/88)
In article <791@daisy.UUCP>, david@daisy.UUCP (David Schachter) writes: > According to "CD-ROM: The New Papyrus" by Microsoft Press, faster data > transfer from CD-ROMs is unlikely because the frequency of the data starts > to approach the frequency of the servo mechanisms used to keep the optics > on track and in focus. > -- David Schachter Yes. But, that is a discussion of data transfer rate, which is not what makes CD roms slow. What makes them slow is seek times. Optical disks give you very dense packing of tracks, this means you need very fine advances, which means careful movement, which means slow (relatively, have you ever used a cassette with a C64?). Also, what is to say we can't remork CD roms with alternative servos? --j.a.tainter
david@daisy.UUCP (David Schachter) (01/13/88)
In article <4603@ihlpg.ATT.COM> tainter@ihlpg.ATT.COM (Tainter) writes: >In article <791@daisy.UUCP>, david@daisy.UUCP (David Schachter) writes: >> According to "CD-ROM: The New Papyrus" by Microsoft Press, faster data >> transfer from CD-ROMs is unlikely because the frequency of the data starts >> to approach the frequency of the servo mechanisms used to keep the optics >> on track and in focus. >> -- David Schachter >Yes. But, that is a discussion of data transfer rate, which is not what makes >CD roms slow. What makes them slow is seek times. Optical disks give you very >dense packing of tracks, this means you need very fine advances, which means >careful movement, which means slow (relatively, have you ever used a cassette >with a C64?). > >Also, what is to say we can't remork CD roms with alternative servos? > >--j.a.tainter The surface of a CD-ROM isn't too flat. And the rotation rate of a CD-ROM varies as the head moves in and out. (Unlike magnetic disk drives which have constant _angular_ velocity, the Compact Disk format has constant _linear_ velocity.) And the CD-ROM isn't centered on the spindle too well, either. These factors combine to make it difficult to seek on a CD-ROM. To move to a particular sector, you must be able to read the sector id. To read the sector id, you must be rotating at the correct rate. To rotate at the correct rate must know what sector you are on. Combine this with a non-flat surface and an off-center disk and you have a non-trivial engineering endeavor. Fast seeking is hard and not needed for consumer _audio_ applications. Therefore, the price break due to volume production won't be as great as you might want. Note that changing tracks (and actually, CD-ROM doesn't have "tracks"-- it is a single 3-mile long spiral groove, on a full disk) requires changing the rotation rate of the disk, if you want to keep transferring at the full rate of the CD-ROM. There is a limit to how fast you can speed up or slow down a CD-ROM, due to inertia. (The full speed transfer rate is pitiful-- less than seventy kilobytes a second, after taking out the error correcting code for CD- ROM.)
norman@a.cs.okstate.edu (Norman Graham) (01/13/88)
in article <4603@ihlpg.ATT.COM>, tainter@ihlpg.ATT.COM (Tainter) says: > What makes them slow is seek times. Optical disks give you very > dense packing of tracks, this means you need very fine advances, which means > careful movement, which means slow [...] We must remember that a track on a CD-ROM is very different than a track on a conventional magnetic hard disk. Tracks may be different sizes on the same disk (there may even only be one track for the whole disk (or maybe there has to be two, I don't remember)). What makes CD-ROM slow is that even if you know the minute, second, and sector of the data that you want, all the drive can do is get close to where it thinks it might be and start streamming off data until it gets to what you wanted. > Also, what is to say we can't remork CD roms with alternative servos? We could, but it would defeat one advantage of CD-ROM drives... they're cheap (or at least they will be by the 2nd quarter of this year :-). Since to make a CD-ROM drive you take a CD-Audio drive (which has a HUGE market as compared to CD-ROM), take out a lot of electronics, and put a little bit of electronics back in, CD-ROM drives should approach the price of CD-Audio drives (< $200 US). > --j.a.tainter -- Norman Graham Oklahoma State University Internet: norman@a.cs.okstate.edu Computing and Information Sciences UUCP: {cbosgd, ihnp4, 219 Mathematical Sciences Building rutgers}!okstate!norman Stillwater, OK 74078-0599