One of the most common causes of bad CD burns.
A buffer underrun occurs when the machine doing the burning cannot keep the pre-burn buffer full enough to keep up with the rate at which information is recorded.
If you should suffer from frequent buffer underruns, there are a couple possible solutions.
1. Make sure there are no other processes running that would suck large chunks of CPU time, and avoid doing anything that requires extensive HDD use.
2. Take your burn speed down a notch.
3. Get a better machine.

More generally, a buffer underrun happens when a process that needs a steady, continuous flow of data puts a buffer between the disk or network, and the disk or network can't keep up, resulting in an empty buffer. The results can be as harmless as a few lost frames in a video stream, or as annoying as a coaster from a CD writer as explained above.

If your CD-Burner is doing a fine job of keeping up with your coaster collection, you should look at some of these items to help determine the cause of the buffer underrun. Note that the steps outlined are for Windoze boxes, but the concepts are sound for other operating systems.

First off, watch your hard drive light when burning a CD. Should the hard drive suddenly get accessed for another process in the middle of your burn session (the light goes from a rhythmic blinking to a frenzied spazm of flashes, accompanied by enough rattling noises from your hard drive to make you believe it has been haunted by Orville Reddenbacher popping some popcorn), then you need to work on reducing the background processes or adding additional memory. "Why memory?", you may ask. Your hard drive is used as virtual memory, and as real physical memory is used, the operating system shunts some of the lower-rated processes onto the hard drive. When the operating system needs the information stored temporarily, it goes back to the hard drive and digs through the paging file for the required information. Unfortunately, your CD burner was expecting data at a relatively constant stream, and when that stream runs dry, you get a coaster. Some of the more annoying background programs include virus scanners, some of which start scanning after a certain amount of time elapses between user inputs. Disable or close the virus scanner until you're done burning. Another item that runs in the background are mail programs that scan to see if any new mail is waiting. Shut them down, and also close things like ICQ, AIM or whatever flavor of Instant Messaging you use.

The next thing to look at is your hard drive. If you're burning files off of the hard drive, or making an image of a CD to the hard drive to burn on a new CD, make sure you have sufficient room for a paging file for temporary memory usage, as detailed above. Trying to burn CDs using a drive that is 90% full is just begging the Gods of CD-Burning to curse your session. I personally recommend having a half-gig available on your root (C:) drive, but your mileage may vary. Next, take a look at when you last defragmented your drive. If all of those MP3s you're burning are spread all over the surface of the hard drive, the system has to track down each piece and put it in the pipeline to the CD-Burner. Defrag your hard drive at least weekly, more if you're downloading large volumes of files. Also, from personal experience, try not to burn from your C: drive if you can help it. I set up a separate 1.5gig partition for use as my storage section for burning CDs.

OK, now let's look at hardware. This one caused me the most headaches, and when I figured out why I made so many coasters I smacked myself in the noggin with a trout. Look in the control panel/system properties/cdrom and click on the settings tab (in Win98 for this example, since it's the current most used home Windoze platform). There is a little checkbox called DMA, which stands for Direct Memory Access. Make sure it is checked so the CD can get data directly from memory rather than going through a bunch of software hoops, which will increase the data pipeline. Other hardware items to check include making sure your CD-Burner has the latest ROM version installed (Yes, CD-Burners have ROM updates too!). Then check to make sure your motherboard has the latest drivers, especially the IDE controllers.

Hopefully this will help you avoid having more CD coasters than can be supplied by AOL.

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