| Device | Hard Drive | Compact Disc | DVD |
| Media | Magnetic | Optical - infra-red | Optical - red-orange |
| Surface | One or more discs (platters). Both sides used. Only one surface read at a time. |
Single surface | Several formats - surface can be semi-transparent Single Sided Single Layer SS/SL Single Sided Double Layer SS/DL Double Sided Single Layer DS/SL Double Sided Double Layer DS/DL |
| Size | 10 Meg to Terabyte range | 650 Meg |
SS/SL 4.7 GByte SS/DL 8.5 GByte DS/SL 9.4 GByte DS/DL 17 GByte |
| Track Layout | Parallel concentric tracks. The set of same track on all surfaces is called a cylinder | Single track starting from hub | Single track starting from hub. On duel layer, inner layer may be read from the rim back to the hub |
| Data layout | Data layed out in sectors. All tracks have same number of tracks New designs allow the outer set of tracks to have more than inner set | Data layed out in sectors. There is only one track but the number of sectors in each rotation of the disc vary continuously. | Same as cdrom |
| Addressing Sectors | Data accessed by the cylinder (track), head (surface), and sector. On newer designs, software support allows sectors to be accessed by logical ids, but hardware on drive still uses chs. | Sector location approximated and then sequential read of sector ids until desired sector located. | Same as cdrom |
| Accessing Data | Sector accessed randomly. Data in a sector must be accessed sequentially. | ||
| Read/Write properties | Infinite read/write capabilities |
Standard CDROM - read only memory - data stored
by creating actual pits and lands on disc.
Requires manufacturing hardware. WORM - write once read memory - dye is used to create dark spot that simulates pit. Home units capable of writing to disc. Compatible with CDROM CDRW - can be erased and re-written. Media will eventually fatigue with continued re-writing. May not be readable by all standard CDROM readers |
Currently Read Only, but WORM and CDRW style technologies are practical |
| Disc Speeds |
Hard drives use a constant rotational speed.
This combined with the constant number of
sectors on each track means that data near the
hub is much more compressed than data near the
rim. To alleave this, many modern drives divide
the disk into regions and change the number of
sectors on a track for each region.
Current speeds To achieve these speeds,the read/write head is designed not to touch the surface of the disk. Rather it flies on/in the air current generated by the spinning disk. To achieve this feat, the disk must be manufactured and sealed in a very clean environment. |
Compact Disks were origionally designed to deliver
a constant stream of audio digitial data. As a result,
their linear speed (the speed at which each segment
passes under the read head) is constant. This means
that the rotational speed of the cd must slow down as
the read head gets closer to the rim. Since reading data does not have the same speed restrictions, newer CD readers are capable of faster speeds than are needed for audio. Like the hard drive, the read head does not touch the surface. But because it is optical, it is much further away from the media and thus can be speed up without having to seal the mechanism |
Like the CD, the DVD must deliver the data stream at constant speed. The default throughput for the DVD is around 10Mbps and the current need to increase that is not critical. |