[rescue] Bad luck with SCSI cdrom drives

[Geoffrey S. Mendelson]

On Thu, Sep 06, 2007 at 10:56:31AM +0200, Jonathan Groll wrote:
> I would like to know if my experience of one out of five working drives
> is atypical for this list?

There are several issues, 

One is the block size issue. If your SUN is new enough to have a "boot
cdrom" command then the drive does not need to be set to 512 byte
sectors. If it uses the older version "b sd..." command it must be set
to 512 byte sectors. I keep one old CDROM drive like that.

If you can use the "boot cdrom" command, then you need a drive set to
ID 6. It also must accept the SCSI-2 command to change sector size.
This has nothing to do with the interface connector, it's a command
that was added by the SCSI-2 specifications. Toshiba drives, even
as far back as the 3201b which I bought in 1991 support it. So do Sony,
Pelxtor and many others. AFAIK no NEC drives ever have. 

Apple drives with Sony mechanisms in them support it such as the 
CD-300, but from what I understand the later ones, including the more
common Matsusita mechanism CD-300+ do not. 

The second issue is the ability of the drive to read the CD. From my
experimentation, they all read factory pressed CD's properly, unless
they are defective or dirty. Dirty drives can be cleaned with a
CDROM/DVD lens cleaning CD with a small brush on it. If that does
not fix the drive, I would not waste my time with it.

Burnt CD's are a very different matter. Since the drives in question
were made before CDROM burners were common, they often only read one
color of blanks. Usually this is what I call "yellow" ones, they are
more often called golden or silver. 

This is the color of the dyes used on the recording surface, the disc
itself must be clear plastic and not colored. There are other blanks
which I call blue, which range from dark blue to cyan. These are
difficult if not impossible to read on many early CDROM drives.

The next issue is the size and position of the "pits". CDROMS work by
having little pits in them 1/4 wavelength deep. This has the effect of
creating light and dark spots as the 1/4 wavelength reflection appears
black. Burnt CD's mimic this effect.

The specification for the size and location of the pits is quite strict.
However CDROM burners over the years have stretched the standard and
it has been slowly "bent' by faster and faster burners. The buffer
underrun protection schemes also bent the standard. 

Newer CD-ROM drives have firmware to accomodate these variations, but
older ones have not.

If you want to be sure to have your disks read cleanly, burn them at
the lowest possible speed your burner supports. As a rule of thumb,
I never burn a CD faster than the drive I intend to read it on. 

I have also found that DVD ROM burners tend to do a better job of burning
CD's than CDROM burners. For example, a friend brought over a SUN4M computer
and I accidently burnt him a boot CD at 32X using my NEC 16X DVD burner.

Much to my surprise it was perfectly readable on my 512 byte scector
Compaq 2x (Matsusita mechanism) drive. 

As an aside, I have a Yamaha 4x burner that I use on an old Macintosh.
It is very selective of the media it will read, but not as selective
of the media that it will write. With Verbatim media, I can burn a
CDROM on it that it will not read. It reads perfectly well on
Apple 8X CDROM drives. :-)

There is also a problem with some bit patterns that can not be written
by some CD burners. Since a program to copy protected CD's called 
"CloneCD"  uses a sheep as its logo, someone wrote a program to
create these files and called it "sheeptest". There are three levels
of sheep tests. I am only concerend with levels 2 and 3, the more
difficult levels. 

I have only tested IDE burners and none of them passes the level 3 test,
although several have been reported to. 

I do have some, mostly LG manufacture that pass the level 2 test, and
one DVD burner (an old Toshiba 4x DVD-R) that does.

The test is very simple and perplexing. Your download a program called
"sheeptest.zip" and run the bath files in it to create test files.
This is a DOS/Windows program.

Once the files are created, you merely burn them on a CD with your
favorite CD creation program on any computer that can burn CD-ROMS. 
They are NOT ISO images, just plain data files. 

Much to my surprise, often the discs burnt are not readable,

Geoff. 

-- 
Geoffrey S. Mendelson, Jerusalem, Israel gsm at mendelson.com  N3OWJ/4X1GM
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