Optical Disc Recording Technologies - Buffer Underrun Protection

Buffer Underrun Protection

Usually, the recorder must perform a complete write without pauses. Once the laser is on, stopping and restarting the recording process may introduce flaws.

A buffer underrun occurs during recording if the supply of data to the recorder is interrupted before the write is complete. Software typically moves the data to be recorded into a buffer; underrun occurs if the recorder processes data in the buffer faster than the software reloads it. Historically, buffer underrun was often caused by writing data obtained from a slow device, or by slowness of the recording software, from a slow processor or a processor executing other tasks concurrently.

Various recorders minimize or cope with buffer underrun in the following ways:

• Nearly all burners can slow the rotation of the disc and record at a slower rate. A burner may do so on sensing that it is drawing down the data in the buffer faster than software is reloading it.
• Recording software maintains larger buffers than when CD recorders were first introduced. Some recorders maintain their own buffer memory independently of the computer. This additional buffering ensures that momentary pauses in the supply of data do not cause buffer underrun.
• Some recorders are, in fact, able to stop writing in the middle of a session, and resume writing whenever the buffer is refilled. Recorders with such buffer underrun protection handle the interruption with an extremely small gap in the recorded track on the disc.

Since the techniques for protecting against buffer underrun are proprietary and vendor-specific, technical details vary.

Buffer underrun is minimized by a strategy in which the recorder burns a packet rather than an entire session or an entire disc. When using rewritable media (CD-RW, DVD-RW, DVD-RAM), the UDF file system organizes the disc into packets that are written individually. The packets are referenced by a single, updated address table.