PC Hardware in a Nutshell, 3nd Edition

11.4 Buffer Underrun Protection

In the past, anyone who used a CD writer sometimes made coasters, the common term for a ruined CD-R blank. Although packet writing and UDF effectively eliminated coasters, packet-writing software was useless for batch-mode tasks, such as duplicating CDs. Those tasks demanded premastering software, which unfortunately was by no means immune to generating coasters.

Originally, CD premastering was inherently an isochronous (time-dependent) process because data had to be delivered to the write head in a continuous stream from the time the write began until it completed. If that stream was interrupted long enough for the data stored in the writer's buffer to be exhausted—an accident called a buffer underrun—the blank was ruined. Buffer underruns were particularly common with IDE/ATAPI CD writers, although they were by no means rare even on SCSI burners.

Sanyo effectively made buffer underruns a thing of the past by developing a technology called BURN-Proof (Buffer UnderRuN-Proof) and licensing it to CD writer makers. In simple terms, BURN-Proof turns off the writing LASER when it runs out of data to write (duh), and then, when data is again available, restarts the burn exactly where it left off. In effect, BURN-Proof converts CD writing from an isochronous process to an asynchronous one.

BURN-Proof works by constantly monitoring the status of the CD writer's buffer to detect a potential buffer underrun condition. If the amount of buffered data falls to a critically low level, the BURN-Proof firmware finishes writing the current sector and then turns off the writing LASER until the amount of buffered data returns to an acceptably high level. When that occurs, the BURN-Proof firmware repositions the writing LASER to begin writing where it left off.

If BURN-Proof kicks in, the resulting disc is not literally identical to a disc that was written without a buffer underrun occurring. Rather than restarting the write on the sector immediately following the last sector written before the buffer underrun, BURN-Proof must leave a short gap—the length of which varies with recording speed—between the last pre-underrun and the first post-underrun data sectors. Error correction circuitry on nearly all CD drives and players eliminates this minor hiccough, but very old CD drives and players may deal with it poorly—for example, by playing back an audible pop on an audio CD burned with BURN-Proof. Note that this condition applies only if BURN-Proof kicks in during the burn. If no buffer underrun occurs during the burn, discs produced on a BURN-Proof drive are indistinguishable from those produced on any other CD writer.

Our first experience with BURN-Proof was with the superb Plextor PlexWriter 12/10/32A IDE CD writer. With standard IDE CD writers we hesitated even to move the mouse while burning. We walked softly and worried about vibration from passing trucks. But the Plextor 12/10/32A and later PlexWriters eliminated those worries. Plextor CD writers with BURN-Proof are so robust that we successfully burned a disc from an image file on the hard drive while we were defragging that hard drive—a guaranteed way to ruin a disc with any CD writer that lacks buffer underrun protection.

The success of PlexWriter drives with BURN-Proof made buffer underrun protection a must-have feature, so other manufacturers soon followed with similar technologies. Nowadays, even inexpensive CD writers have some form of buffer underrun protection, whether the original BURN-Proof or a competing technology. Yamaha, for example, calls its method SAFEBURN. Sony calls it Power Burn, and LITE-ON calls it Buffer Underrun Free. The various technologies differ in minor respects, but all of them work.

It takes much longer to write a disc if the buffer underrun protection kicks in repeatedly, but the write does complete normally, and we have never had a problem reading the resulting discs. Unfortunately, buffer underrun protection cannot be retrofitted to an existing CD writer. If you want buffer underrun protection (and you should), the only way to get it is to buy a CD writer that supports it. Fortunately, CD writers are very inexpensive, and if your current writer is old enough to lack buffer underrun protection you probably need a new, faster writer anyway.

Buffer underrun protection works only if the burning software explicitly supports the specific buffer underrun technology used by the drive. For example, software that supports BURN-Proof for a Plextor drive may not support SAFEBURN on a similar Yamaha drive. If the software does not support the specific type of buffer underrun protection used by the drive, the feature is disabled, and you're as likely to burn coasters as if the drive had no buffer underrun protection. Most drives are bundled with burning software that fully supports their features, but you may not want to use the software provided with the drive. If you use third-party software, such as Nero Burning ROM, make sure your specific drive model appears on the compatibility list for the software.