cess involved. The tape moved at 18.9 mm a second over a read/write head rotating at 9000 rpm. The head laid down tracks diagonally across the tape, 12 tracks per frame for PAL and 10 tracks per frame for NTSCC. This means that the tape was quite bulky (the cassette was 12.2 mm by 48 mm by 66 mm) and the recording sequential - that is, random access requires the tape to be mechanically wound forward or back. It is thus not suitable (if used on its own) for mixed media (video, audio and data) applications. 9.3.2 Optical Storage DVDs in comparison can be configured for random access (DVD-RAM), or sequential access (DVD-RW and/or DVD+RW). DVDs come in three sizes, four and a half inch, three-inch mini DVDs and Sony's two and a half inch universal media disk. Data is stored in a spiral of pits (up to 48 kilometres long for a double-sided DVD) embedded within the disk. The reflectivity of the pits can be changed by the application of heat from a red (640 nanometre) laser that Making Telecoms Work is also used to read the data. A DVD RAM can be rewritten about 10 000 times, a DVD-RW or DVD+RW can be rewritten about 1000 times (the whole disk gets erased on the write cycle). The disks rotate at between 200 and 500 rpm. Although DVD-RW and DVD+RW are optimised for sequential storage, there are zone storage and search techniques that mean they can be used for storing and reading a mix of video, audio and data. The spin speed changes to keep the read speed constant as the laser moves out from the centre of the disk. The transfer rate is typically 10 Mbps. The seek time on a DVD RAM disk is typically about 75 ms. A standard four and a half inch DVD holds 4.7 GB of data, (9.4 GB on a double-sided disk), a mini DVD holds 1.4 GB and a universal mini disk holds 1.8 GB. And as stated in an earlier chapter moving to a blue laser at 405 nanometres rather than a red laser (a Blu Ray DVD) increases available storage to close to 30 gigabytes, enough for a two-hour HD movie. The Sony Universal Media Disk was used (as a read-only devic