8.1 How fast are BD discs written and read?
8.2 What rotational schemes do BD discs employ?
8.3 How much time does it take to write a BD-R and BD-RE disc?
8.4 How might reading and writing speeds progress in the future?
8.5 Can writable BD discs recorded at different speeds be read back at any speed?

8.1 How fast are BD discs written and read?
At single speed (1x) information is written to or read from a BD disc at 4.29 MB (4,495,625 bytes or 35,965,000 bits) per second. Spinning the disc at higher velocities increases these rates, which are then expressed as multiples of these figures (e.g. 2x, 4x, 6x, 8x, 12x, etc.).

To accommodate its extended set of features (high bitrate video and audio encoding, out-of-mux streams, etc.), a two-dimensional (2D) BD-ROM AV (HDMV/BD-J) disc is played at 1.5x speed (6,743,438 bytes or 53,947,500 bits per second) to achieve a normal viewing rate, a three-dimensional (3D) disc at 2x speed (8,991,250 bytes or 71,930,000 bits per second), a 50GB Ultra HD disc at 2x-2.5x speed and a 66/100GB Ultra HD disc at 2.5x-4x speed (see table).

Putting BD performance in perspective, 1x CD transfers 0.15 MB (153,000 bytes) per second while DVD transfers 1.32 MB (1,385,000 bytes) per second. In BD terms 1x is equivalent to roughly 29.4x CD or 3.3x DVD (see table).

8.2 What rotational schemes do BD discs employ?
Depending upon the capabilities of the player, drive or recorder and the nature of the application, BD-ROM/R/RE discs can be read or written using Constant Linear Velocity (CLV), Constant Angular Velocity (CAV), Zoned Constant Linear Velocity (ZCLV) or Partial Constant Angular Velocity (PCAV) methods.

A single-layer (SL) disc, or the first layer (L0) of a dual-layer (DL) disc, is read or written from its inner to outer diameter (ID to OD) whereas the second layer (L1) of a dual-layer disc is always configured as Opposite Track Path (OTP) and thus read or written from its outer to inner diameter (OD to ID).

Constant Linear Velocity
Constant Linear Velocity (CLV) maintains an even data transfer rate as the optical head of the recorder, drive or player reads or writes from the inner to outer diameter (ID to OD) of the disc. At reference velocity (1x), the rotational speed of a BD disc decreases from roughly 1957 RPM to 810 RPM (ID to OD) to maintain a constant linear velocity of 4.917 m/s (5.280 m/s for obsolete 23.3 GB discs).

For example, discs authored in the BD-ROM AV (HDMV, BD-J) format used for commercial movies play at 1.5x CLV while BD-R/RE AV discs play at 1x CLV speed (see figure).

Constant Angular Velocity
Constant Angular Velocity (CAV) spins the disc at a constant RPM throughput the entire reading or writing process. Thus, the data transfer rate continuously increases as the optical head reads or writes from the inner to outer (ID to OD) diameter of the disc.

For example, a 6x CAV BD recorder might begin writing at 2.5x at the inner diameter (ID) of the disc accelerating to 6x by the outer diameter (OD) of the disc (see figure).

Zoned Constant Linear Velocity
Zoned Constant Linear Velocity (ZCLV) divides the disc into zones (regions) and employs progressively faster CLV reading or writing speeds in each.

For example, a 4x ZCLV BD recorder might write the first third of the disc at 2x CLV and the remainder at 4x CLV speed (see figure).

Partial Constant Angular Velocity
Partial Constant Angular Velocity (PCAV) spins the disc at a lower fixed RPM when the optical head is reading or writing near the inner diameter (ID) but then shifts to CLV part way further out on the disc. As a result, the data transfer rate progressively increases until a predetermined point is reach and thereafter remains constant.

For example, an 8x BD recorder might accelerate from 4x to 8x over the first half of the disc then maintain 8x CLV writing for the remainder of the disc (see figure 8.1).

8.3 How much time does it take to write a BD-R and BD-RE disc?
BD-RE was first introduced to market with 1x (CLV) writing but BD-RE and BD-R specifications now define 1x-2x and 1x-6x operation respectively. Since 1x BD writing transfers 4.29 MB (4,495,625 bytes) of data per second it takes, roughly, 93 minutes to write a full 25 GB disc or 46 minutes at 2x [(25,025,315,816 bytes ÷ 4,495,625 or 8,991,250 bytes per second) ÷ 60 seconds = 92.78 or 46.39 minutes].

In practice, recording time depends upon many factors including the writing speed supported by the recorder and disc, the amount of data to be written, if defect management or read-back verification is employed and the transfer rate of the data source.

8.4 How might reading and writing speeds progress in the future?
BD technology is in its infancy but if past optical storage industry experience is any indication, read and write speeds will increase rapidly. For example, manufacturers have already publicly discussed possible BD-R increases up to 8x.

At this early stage anticipating anything is merely speculation but it’s possible to make some informed predictions. From a practical perspective, spinning an optical disc at 10,000 RPM has long proven the realistic limit for half-height drives and 5,000 RPM for slim-types. Since a 12 cm BD disc rotates between 1957 to 810 rpm (ID to OD) at 1x speed, it is possible that 12x CAV (5x to 12x) will be the maximum potential ceiling for computer and 6x ZCLV or PCAV (2.5x to 6x) for portable BD units. Generally speaking, rewritable and multi-layer discs are trickier so their maximum speeds will most likely be lower.

8.5 Can writable BD discs recorded at different speeds be read back at any speed?
The speed at which a BD-R or BD-RE disc is written has nothing to do with the speed at which it can be read back in a BD player, recorder, combo or ROM drive.