DVD Tracks and Sectors

The pits are stamped into a single spiral track (per layer) with a spacing of 0.74 microns between turns, corresponding to a track density of 1,351 turns per millimeter or 34,324 turns per inch. This equates to a total of 49,324 turns and a total track length of 11.8km or 7.35 miles in length. The track is comprised of sectors, with each sector containing 2,048 bytes of data. The disc is divided into four main areas:

• Hub clamping area. The hub clamp area is just that, a part of the disc where the hub mechanism in the drive can grip the disc. No data or information is stored in that area.

• Lead-in zone. The lead-in zone contains buffer zones, reference code, and mainly a control data zone with information about the disc. The control data zone consists of 16 sectors of information repeated 192 times for a total of 3,072 sectors. Contained in the 16 (repeated) sectors is information about the disc, including disc category and version number, disc size and maximum transfer rate, disc structure, recording density, and data zone allocation. The entire lead-in zone takes up to 196,607 (2FFFFh) sectors on the disc. Unlike CDs, the basic structure of all sectors on a DVD are the same. The buffer zone sectors in the lead-in zone have all 00h (zero hex) recorded for data.

• Data zone. The data zone contains the video, audio, or other data on the disc and starts at sector number 196,608 (30000h). The total number of sectors in the data zone can be up to 2,292,897 per layer for single layer discs.
• Lead-out (or middle) zone. The lead-out zone marks the end of the data zone. The sectors in the lead-out zone all contain zero (00h) for data. This is called the middle zone if the disc is dual layer and is recorded in opposite track path (OPT) mode, in which the second layer starts from the outside of the disc and is read in the opposite direction from the first layer.

The center hole in a DVD is 15mm in diameter, so it has a radius of 7.5mm from the center of the disc. From the edge of the center hole to a point at a radius of 16.5mm is the hub clamp area. The lead-in zone starts at a radius of 22mm from the center of the disc. The data zone starts at a radius of 24mm from the center and is followed by the lead-out (or middle) zone at 58mm. The disc track officially ends at 58.5mm, which is followed by a 1.5mm blank area to the edge of the disc.

DVDs come in both single- and dual-layer as well as single- and double-sided versions. The double-sided discs are essentially the same as two single-sided discs glued together back to back, but subtle differences do exist between the single- and dual-layer discs. The table below shows some of the basic information about DVD technology, including single- and dual-layer DVDs. The dual-layer versions are recorded with slightly longer pits, resulting in slightly less information being stored in each layer.

The spiral track is divided into sectors that are stored at the rate of 676 sectors per second. Each sector contains 2,048 bytes of data.

When being written, the sectors are first formatted into data frames of 2,064 bytes: 2,048 are data, 4 bytes contain ID information, 2 bytes contain ID error detection (IED) codes, 6 bytes contain copyright information, and 4 bytes contain EDC for the frame.

The data frames then have ECC information added to convert them into ECC frames. Each ECC frame contains the 2,064-byte data frame plus 182 parity outer (PO) bytes and 120 parity inner (PI) bytes, for a total of 2,366 bytes for each ECC frame.

Finally, the ECC frames are converted into physical sectors on the disc. This is done by taking 91 bytes at a time from the ECC frame and converting them into recorded bits via 8 to 16 modulation. This is where each byte (8 bits) is converted into a special 16-bit value, which is selected from a table. These values are designed using an RLL 2,10 scheme, which is designed so that the encoded information never has a run of fewer than 2 or more than 10 0 bits in a row. After each group of 91 bytes is converted via the 8 to 16 modulation, 32 bits (4 bytes) of synchronization information is added. After the entire ECC frame is converted into a physical sector, 4,836 total bytes are stored.

Unlike CDs, DVDs do not use subcodes and instead use the ID bytes in each data frame to store the sector number and information about the sectors.