4.0700 CD-ROM Drives and Readers (4/150)

Elaine Brennan & Allen Renear (EDITORS@BROWNVM.BITNET)
Wed, 7 Nov 90 11:46:20 EST

Humanist Discussion Group, Vol. 4, No. 0700. Wednesday, 7 Nov 1990.

(1) Date: Mon, 5 Nov 90 09:50 CST (81 lines)
From: John Baima <D024JKB@UTARLG>
Subject: CD-ROM drives

(2) Date: Mon, 05 Nov 90 12:56:28 EST (14 lines)
From: brad inwood <INWOOD@vm.epas.utoronto.ca>
Subject: Re: 4.0684 Qs: ... CDROM/TLG Readers

(3) Date: Sun, 4 Nov 90 22:01 EST (35 lines)
From: John LaCure <LACUREJ@IUBACS>
Subject: CD-ROM drives

(4) Date: Mon, 5 Nov 90 10:29:37 PST (20 lines)
From: 6500rms@UCSBUXA.BITNET
Subject: Re: 4.0684 Qs: ... CDROM/TLG Readers

(1) --------------------------------------------------------------------
Date: Mon, 5 Nov 90 09:50 CST
From: John Baima <D024JKB@UTARLG>
Subject: CD-ROM drives

There have been a couple of vague comments about the speed of CD-ROM
drives so I would like to offer my two cents about how CD-ROMs work.

First, the data on a CD-ROM is stored in a spiral rather then
in concentric circles like floppies and hard disks. The data is stored
with a constant linear density so that the drive has to change the spin
of the disk depending on where the data is on the spiral.

This makes the "random access time" rather large, typically 350-800 ms.
A typical XT class drive will have a random access time of about 80 ms,
an AT class drive about 35 ms. Many 386 machines have drives with as
little as 16 ms. Some drives with a large controller cache claim even
faster times. Thus, the random access times are 10-20 times slower for
CD-ROMs than hard disks.

There is, however, more to disk speed than the access time. When
reading a large amount of data, the data transfer rate is actually more
important. Here, the speed of a CD-ROM is not quite so bad. The data
transfer rate of CD-ROMs is 150k bytes/sec. This transfer rate is part
of the "High Sierra" standard. Until fairly recently, most PC hard
disks were MFM drives and they have a transfer rate of 500k/sec. The
RLL drives have a transfer rate of 750k/sec. SCSI drives have a
variable data transfer rate because SCSI is really an external bus
standard and a "SCSI" drive can be either a MFM, RLL or other type.
There are other types, of course. ESDI is popular with high performance
PC and IDE is popular with laptops and newer PCs.

Anyway, hard disks cannot sustain their transfer rates for a couple of
reasons. The first is interleave. When the interleave for a drive is
optimally set, it represents the number of rotations necessary to read
one track. Thus, the effective transfer rate of an XT with a 3:1
interleave (typical for XTs) is 500/3 k/sec. Also, most hard disks lose
at least one rotation when they have to move from track to track. Also,
if only are partial track is read, then usually another rotation is
lost. Disk caches which buffer whole tracks can reduce this problem.

Most CD-ROM drives, however, since their data is stored in a spiral,
can indefinitely keep up their transfer of data. The architecture of
PCs does not allow the continuous flow from PCs so that the effective
rate will typically be about 10% slower, perhaps 140k/sec.

The bottom line for hard disks is that they have a hard time getting an
effective transfer rate that is 1/2 of the theoretical limit after
accounting for the interleave. Thus, a typical XT with a 3:1 interleave
will only be able to sustain about 80k/sec, barely 1/2 the speed of a
CD-ROM drive.

The implications for CD-ROM applications is that they must be designed
to read large continuous chunks rather than skipping around. A CD-ROM
application that, for example, uses a lot of indexes and does a lot of
"random access" will indeed be quite slow. However, if the application
is something like reading the TLG, the performance will be somewhere
between an XT (3:1) and AT (1:1) hard disk. The Greek New Testament is
a nice sample because it is almost exactly 1 MB. The GNT can be read
and searched for a phrase in less than 8 seconds on my 386sx--hardly
glacial, IMHO.

Most CD-ROM drives use a SCSI interface and the connection is easy for
most PCs and trivial for Macs. The only problem usually comes when the
CD-ROM drive is mixed with other SCSI drives on a PC. The SCSI
"standard" is inadequately defined (SCSI II is much better) and one
must be careful that all the devices can be interconnected. It is
usually possible to connect almost any combination, but I would not
buy without the option of returning the hardware if it does not.

Finally (whew!), I think that CD-ROM drives are good technology,
especially for those who want to distribute a large amount of texts. It
is a pity that no "killer" business CD-ROM app has appeared and
spread CD-ROMs all over, but the US Govt is making CD-ROMs like crazy
and it is only a matter of time before the prices drop more (they have
been, but more slowly then hoped).

John Baima
Silver Mountain Software

(2) --------------------------------------------------------------23----
Date: Mon, 05 Nov 90 12:56:28 EST
From: brad inwood <INWOOD@vm.epas.utoronto.ca>
Subject: Re: 4.0684 Qs: WP Greek; HP->PS Greek; CDROM/TLG Readers

Re: Which CD-ROM.

The Toshiba XM-2200A is a SCSI CD-ROM reader which can be shifted from a
DOS machine to a Mac. We have been running it from an AT clone for
several months now, searching both TLG and PHI/CCAT discs, with no
problems. I strongly recommend it.

Brad Inwood
Dept. of Classics
University of Toronto
(3) --------------------------------------------------------------41----
Date: Sun, 4 Nov 90 22:01 EST
Subject: CD-ROM drives

TO : Jamie Hubbard.
SUBJ : Which CD-ROM?

> Does anybody have any good advice on which model CD-ROM reader to
> buy?

Funny you should ask. This isn't a direct answer to your question,
since there is not mention of an SCSI interface. But it is certainly an
interesting development.

Apparently along with polka hits of the 50's, the discounters have now
gotten into the CD-ROM business. DAK's Winter 1991 catalog is carrying
a CD-ROM drive for DOS machines ($499 for internal model, $699 for
external) with 6 free disks. The disks include:

Disk 1. 21 vol encyclopedia (with EGA/VGA illustrations)
Disk 2. 8 vol. reference library (Webster's New World
Dictionary, Thesaurus, etc.)
Disks 3 & 4. World and US Atlas with 320 maps and charts.
Disk 5. 450 title library with search software (includes
Shakespeare, Doyle, Milton, etc.)
Disk 6. a number of foreign language dictionaries.

If anyone is interested they have a toll free number: 1-800-325-0800,
or for technical questions: 1-800-888-9818.

Jon LaCure | East Asian Languages and Cultures | Indiana University
Bloomington, Indiana 47405 | (812) 332-3101 | lacurej@iujade

(4) --------------------------------------------------------------33----
Date: Mon, 5 Nov 90 10:29:37 PST
From: 6500rms@UCSBUXA.BITNET
Subject: Re: 4.0684 Qs: WP Greek; HP->PS Greek; CDROM/TLG Readers

... [eds.]

As for the CD-ROM reader, we are using a Toshiba XM3201B, a SCSI unit
which works, fairly quickly, at least for CD-ROM, with both Mac and PC.
We also have a search and browse software pacakge for TLG and PHI
CD-ROM's if those are the CD's you are going to be using. Many other
CD's come with their own retrieval software. There is also a package
licenses by Lotus which several CD vendors are using.

Randall Smith
Classics Department
University of California
Santa Barbara, CA 93106
Tel: 805-893-3556
Email: 6500rms@ucsbuxa.bitnet