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4-1] What does "buffer underrun" mean?


This article is from the CD-Recordable FAQ, by Andy McFadden (fadden@fadden.com) with numerous contributions by others.

4-1] What does "buffer underrun" mean?


It means you have an attractive new coaster for your table.

Generally speaking, the CD recording process can't be interrupted in
mid-session. Once the laser starts writing, any interruption would create
a physical gap on the disc that could confuse CD readers. The recorder
must always have data to write, from the moment the recording starts until
the session ends. To avoid a situation where a temporary slowdown in the
computer causes the write process to fail, the makers of CD recorders
put a write buffer in the drive, usually between 512K and 4MB in size.
Data read from the hard drive, tape, or another CD is stored in the buffer,
and pulled out as needed by the recorder.

If the recorder requests data from the write buffer, but there's none there,
it's called a buffer underrun. The disc is still spinning, but there's no
data to write, so the recording process aborts.

This was a very common and very annoying problem for many years, so
most recorders released in 2001 or later have optional "buffer underrun
protection" features available. See section (2-31).

You can sometimes use a disc that failed during writing by closing the
session and starting another, assuming there's enough space left on the
CD, and assuming your pre-mastering software didn't choose to finalize the
disc for you. If you were using disc-at-once recording, you're probably
out of luck.

Advice for preventing buffer underruns is scattered throughout this FAQ.
A brief summary:

- If your hardware and software support it, enable buffer underrun
protection. Usually this is just a checkbox.
- Use a fast, AV-friendly hard drive (i.e. one that doesn't do slow
thermal recalibrations). Pretty much all drives sold since the
late 1990s fall into this category.
- Record at a slow speed - it takes longer to empty the buffer when
recording at 1x.
- Don't do anything else with the computer while recording. Don't record
from a file server.
- Defragment your HD, especially if you're doing on-the-fly recording.
(But don't defragment *while* you're recording.)
- Record from a disc image file rather than on-the-fly.
- Depending on your setup, putting the recorder and your hard drive on
separate SCSI or IDE controllers may be helpful.
- Keep your CD-R cool. Sometimes the drives fail when they overheat,
with a buffer underrun or an inability to finalize a session. This
is rarely a problem with drives made in 2000 or beyond.

Also watch out for things like anti-virus programs that wake up, virtual
memory settings that cause swapping, screen savers that activate during the
CD creation process, unusual network activity, and background downloads of
data or faxes. One way to check is to run the HD defragmenter in Win9X.
If it restarts every few seconds, it's because something is hitting the

Some game discs use a form of copy protection where bad sectors are
deliberately placed on the original CD. Attempting to copy one of these
discs on the fly may fail, because some CD-ROM drives slow down and
repeatedly try to read the "damaged" blocks. The slowdown may result in
a buffer underrun before the CD-ROM drive reports an error.

A utility included with Microsoft Office, called "FindFast", will
occasionally start up and scan your hard drives. Disabling this by
deleting the shortcut in the Windows\Start Menu\Programs\StartUp folder may
be necessary.

If you're using Windows, see the sub-sections on Auto-Insert Notification
and VCACHE settings, below.

http://www.roxio.com/en/support/cdr/bufunder.html has a comprehensive
collection of buffer underrun info.

http://www.adaptec.com/support/configuration/cdrec.html is interesting
reading for users with CD-Rs attached to Adaptec SCSI cards. They're
pretty far on the conservative side, but if you're having trouble this may
help you.

An article by Dana Parker entitled "CD-R on the Safe Side: Seven Rules of
Successful CD Recording" in the April 1997 issue of Emedia Professional
listed the Seven Habits of Successful CD-R Users:

1. Defragment Your Disk
2. Use a Partition for Staging Input
3. Create a Real Image
4. Test before writing
5. Stabilize Your System for CD-R
6. Shut Down Other Applications
7. After the Burn: Label and Test

If you really want to be careful, you can shut down background stuff under
Win95/98 with WinSolo from http://www.procode.com.au/winsolo/ (the web site
was down at last update, but a search for it on http://www.google.com/
turned up a number of shareware sites that have it). Another option is
WinTasks from http://www.liutilities.com/products/wintasksstd/; see
http://www.liutilities.com/products/wintasksstd/tutorials/tutorial1/ for
a tutorial.

(Side note for search engines: some versions of Ahead's Nero refer to
buffer underruns as "loss of streaming".)


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