Blu-ray Disc recordable

Did You Know?

• BDXL is separate from the original BD format; existing Blu-ray drives that predate mid-2010 do not support BDXL, and later drives may not support it unless explicitly stated.
• Each version of BD-RE and BD-R includes three Parts: Basic Format Specifications, File System Specifications, and Audio Visual Basic Specifications.
• BD-R is the first write-once media with defect management functionality similar to DVD-RAM and Mount Rainer-supporting disc drives.

Capacity

A single-layer Blu-ray disc (BD-R and BD-RE) holds 25,025,314,816 bytes, equivalent to 23,866 MiB. A dual-layer disc (BD-R DL and BD-RE DL) holds exactly twice that: 50,050,629,632 bytes, or 47,732 MiB, unlike dual-layer DVDs which offer less than double the capacity. BDXL discs store roughly 30 GiB per data layer, enabling 100 GB with three layers and 128 GB with four layers; no single-layer BDXL exists because a first-generation BD-R DL already exceeds one BDXL layer.

The 100 GB BDXL has 100,103,356,416 bytes (95,466 MiB), which is 2 MiB less than twice a BD-R(E) DL, while the 128 GB BDXL (write-once only) has 128,001,769,472 bytes (122,072 MiB). This storage area is called the "Volume Space" in the UDF specification and holds the file system, file names, folders, and file contents; on CDs the same area is the program area. Information about disc sessions, tracks, and their lengths is stored outside this area.

If the spare area is enabled, 256 MiB (268,435,456 bytes) are reserved from the Volume Space for defect management. Within the Volume Space, file system overhead and slack space slightly reduce usable capacity, though slack space is minimal because optical discs use a 2 KiB logical sector size matching the physical sector size. Packet writing increases file system overhead. The spare area stores addresses for unreadable sectors and replaces them with new data, a process called defect management handled internally by the drive, unlike CD-RW and DVD±RW where the UDF file system (the host system) manages it.

Speed

As of December 2018, all Blu-ray R and RE formats must support 2x writing speeds. For non-XL BD-R discs, 4x and 6x speeds are optional. Starting with BD-RE 5.0 and BD-R 4.0, a 4x read speed is required for UHD support. Write verification, officially called Defect Management, checks data right after writing.

If a section is hard to read, it can be rewritten to spare blocks, but this halves the writing speed because only half of each disc rotation is used for writing. This feature can be turned off using Stream Recording, which restores full speed. BD-RE discs do not need erasing before reuse; data can be directly overwritten. However, they must be formatted before first use, and some burning programs will do this automatically.

Whether Defect Management helps depends on the disc and drive firmware. It works well for isolated bad spots but often fails more than Stream Recording when the whole disc has poor readability. On undamaged media, turning off write verification saves time, especially when making many copies, since errors are unlikely.

Recording mechanisms

BD-R and BD-RE discs use grooves with a wobble frequency to locate the laser position, unlike the pits and lands of prerecorded discs. BD-R includes an Optimum Power Calibrations (OPC) / Test Zone for fine-tuning laser power before and during writing, and a Drive Calibration Zone (DCZ) at the outer edge for optional high-speed calibration, reducing rejected discs and waste. "Normal" BD-R discs use a composite that decreases reflectivity on recording ("High To Low"); Sony uses an inorganic composite that splits into two laminar low-reflectivity components.

Composites may include BiN, Ge3N4, and Pd-doped tellurium suboxide, or a pair of copper alloy and silicon layers that combine on recording. BD-RE discs use a phase transition alloy (often GeSbTe or InAgTeSb; copper silicate or other alloys like Verbatim's MABL are also used). Melting the material with a high-power beam creates an amorphous low-reflectivity state, while lower-power heating returns it to a crystalline high-reflectivity state. In BD-RE discs, data layers are surrounded by dielectric Zinc Sulfur-Silicon Dioxide layers; multi-layer discs use an adhesive spacer layer and a semi-reflective layer.

Recording and dielectric layers are deposited via sputtering. On multi-layer BD-RE discs, each GeSbTe recording layer is progressively thinner: L0 is 10 nm, L1 is 7.5 nm, L2 is 6 nm, and so on. The silver alloy reflective layers also thin progressively: L0 silver layer is 10 nm, L1 is 9 nm, L2 is 7 nm, and so on. Separation layers between recording layers also become progressively thinner.

Recording modes

A BD-R supports three recording modes: SRM (sequential recording mode), SRM+POW (sequential recording mode with pseudo-overwrite), and RRM (random recording mode). The mode is set once before any data is written. SRM, also called SRM-POW, is the fastest mode and does not reserve a 256 MiB overhead. SRM+POW simulates overwriting on write-once media by mapping physical data blocks to different logical blocks, a feature used by UDF version 2.60; the drive handles this mapping, not the operating system. A BD-RE is randomly writable.