| WHY OTHER METHODS HAVE FAILED |
| VOD storage on drives has not succeeded in the past due to following problems: 1. For random access the computer read/ write process must be discontinuous and a drive's tracks are circular and discrete. But TV is continuous, thus a complex buffer (i.e. auxiliary storage) is needed to compensate for the head motion between tracks. (CD's use a continuous spiral track.) 2. Unmodified disk drives must handle data digitally, thus the TV signal is digitized for storing. But digitizing needs 14 to 16 times more space than the analog and natural TV form, or about 100 GB per 2-hr movie. To manage the situation, various data compression methods were tried which remove redundant information and later reinstate it. However, compression cannot recover the excess storage taken by digital conversion without video and sound degrading. Also, we will loose sound synchrony ("lip sync") and depending on the movie quality we will still need 2.5 to 5 GB per 2-hr movie. 3. The worst problem, and still unsolvable, is the enormous buffering needed to handle multiple entries in the same movie. The reason is that in standard drives a single head is active each time, thus only by adding buffers an entry can be created at an earlier epoch than the one being read. Data compression such as of the MPEG type, which is unpredictable and variable, compounds this difficulty further. |