From: ajb8886@ritvax.isc.rit.edu
Subject: Re: Multimedia not need Soundcard
Reply-To: ajb8886@ritvax.isc.rit.edu
Organization: Rochester Institute of Technology
Date: Tue, 26 Jan 1993 14:45:45 GMT

In article <1993Jan26.110430.2966W@lumina.edb.tih.no>, ketil@edb.tih.no (Ketil Albertsen,TIH) writes:
>In article <C1G8Gw.9uo@news.iastate.edu>, wity@iastate.edu (Senopati Pamungkas) 
>writes:
>
>>Get real!  CD-ROM is for read only, and it is damned sloow!
>>The fastest CD-ROM drive is about 200 ms, but it is about 10 ms for hard-disk!
>
>Although your main point is correct, don't make it worse than it is.
>There are several CD-ROM players in the market with a below-100ms access
>time (I believe the fast ones are in the 70-80 ms range). And although
>10 ms hard disks are available, typical speed is rather in the 14-20 ms
>range. So the speed factor is in the 4-5 range, not 20!

As an aside (I'm not involved in this argument) : There are no CD-ROM drives on
the market that I know of with a sub-100ms avg. access time.  The current crop
of "typical" drives is 350-380ms (considered "fast" compared to the older
500-600ms seek times).  The fastest ones typically have 250-280ms average seek
and are equipped with "double speed", meaning the platter is spun at double RPM
to double the transfer rate from 150-160K sec to 300-320K/sec.

Optical assemblies are quite a bit more massive than any hard drive head, and
in attempting to reposition the assembly more quickly you start getting
overshoots (head assembly moves past desired track).  

Even the slowest XT hard drives had an avg seek time about 4 to 9x faster than a
"fast" CD ROM.  In terms of raw transfer rates, a CD's 150K (or 300K/sec for a
"double speed" drive) compares to :
       Old 2:1 IBM AT 286 : 250k/sec   (40ms)
       1:1 MFM (old)      : 500k/sec   (28ms most drives)
       1:1 RLL (old)      : 800k/sec   (28ms most drives)
       1:1 ESDI           : 800K to 1600K depends on drive/bus. (18 to 28ms)
       1:1 SCSI/IDE       : see ESDI.  depends on drive/bus/controller.

However, to be fair most CD ROM disks have been organized in such a way as to
keep related data on adjacent or near adjacent tracks so as to minimize
worst-case seek overhead.

Alex



>
>Another point to be considered is that CD-ROM is read only. So the
>manufacturer may optimize the layout of information with respect to
>the usage pattern, with no danger of it becoming messed up by fragmentation
>and reallocation. To read n Mbytes from a file on a CD-ROM usually
>requires a single seek, while on a Unix system, one seek per, say,
>2 Kbytes might be required. The advantage of faster seeks is easily eaten 
>up by the *number* of seeks required.
>(Obviously, in order to optimize, the manufacturer must have some idea
>of the usage pattern. But as info is read-only, in many cases the
>usage can be pretty well predicted.)
>
>For certain MM applications, the *transfer rate* might be a problem, which
>is less for CD than for harddisk. (Please do not confuse seek time
>with transfer rate - even though both are lower for CDs.) But we have
>already seen double-speed CD-ROM players, moving into the speed range
>where an ISA bus PC might have a bus capacity problems anyway, and you
>are in the range of PC hard disks of only a few years ago.
>
>So, as long as the read-only property is acceptable/desirable, don't
>discount the CD-ROM as an MM medium.
Newsgroups: comp.sys.ibm.pc.soundcard