Path: utzoo!utgpu!water!watmath!clyde!rutgers!cmcl2!brl-adm!adm!moore@UTKCS2.CS.UTK.EDU From: moore@UTKCS2.CS.UTK.EDU (Keith Moore) Newsgroups: comp.unix.wizards Subject: Re: CD-ROM speed Message-ID: <11233@brl-adm.ARPA> Date: 12 Jan 88 23:05:57 GMT Sender: news@brl-adm.ARPA Lines: 46 J. A. 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 > There are two major problems with improving the access times of CD-ROM drives: 1. Difficulties in positioning the laser head. This is due to the "dense packing of tracks", as well as the fact that the CD-ROM is laid out in a spiral, not concentric circles, and uses CLV (constant linear velocity) encoding. So the drive doesn't know exactly where to place the head to access a given frame of the disc; it has to use a successive approximation technique. 2. The disc rotates at a very slow speed. Even if the drive knew exactly where to place the laser, you would have to wait on average 1/2 of a disc revolution. As it is you usually have to wait longer. With frame-to-frame seek times of 1/2 second, a large portion of this delay is due to rotational latency. You can speed this up somewhat for CD-ROMs (it has to be fixed for audio players), but you start running into limitations of the optics, the decoder chips, the servos, and the control systems. These can all be redesigned, but by then your CD-ROM player doesn't even resemble an audio CD player anymore; perhaps it can't even play audio disks. So it's going to be expensive, at least until CD-ROMs become commonplace enough to make it worthwhile to design the players from scratch. Keith Moore UT Computer Science Dept. Internet: moore@utkcs2.cs.utk.edu 107 Ayres Hall, UT Campus CSnet: moore@tennessee Knoxville Tennessee BITNET: moore@utkcs1