Frequently Asked Questions About DVD Discs

Please contact Media Sciences if your questions are not answered on these pages.

Why do low PIE discs fail in some user drives?

DVD discs contain Reed Solomon Product Codes (RS-PC) that are used by the read drive for error correction. All DVD information is organized into 32KB ECC blocks, each containing identification data, user information, and associated Reed-Solomon parity bytes structured in Inner Parity rows and Outer Parity columns.

Small errors are corrected by the read drive at the row level. Inner parity (PI) can correct up to five bad bytes in one PI row containing 172 data bytes and ten PI bytes. PI errors measure the number of PI rows containing one or more errors contained in eight sequential ECC blocks. A small, 3 mm long tangential defect can obliterate nine sequential frames, creating errors that are uncorrectable by PI parity. Resulting PI error values would only be nine while a maximum of 280 is allowed by standards.

Large errors are corrected by columns that each contain 192 data bytes and sixteen PO parity bytes. Errors from the small 3 mm tangential scratch approach the uncorrectable, even though PI error rates are low. Such defects must be detected by PIF and large defect error length tests.

Read drive servos utilize information in the reflected laser beam to maintain proper beam focus, radial position, and disc linear velocity. Read errors can result from improper disc parameters or visual defects that diminish or distort the beam and adversely affect these servomechanisms. In the absence of standards for drives, such discs may be readable in some drives but not in others.

Discs can also fail because of logical format problems, even though their physical characteristics are acceptable. Improper descriptors and directories are often observed.

Although PIE is one of many useful quality indicators, predictable readability can only be assured when full electrical parameter and error evaluation, mechanical and visual inspection, and logical format tests are utilized to assure full compliance with the requirements of all standards for interchange.