Alexander Graham Bell stored sound on a glass disc with light in 1884
A century before the compact disc reached shops, Alexander Graham Bell, his cousin Chichester Bell and Charles Sumner Tainter used a light beam to record sound onto a disc of glass. It is the earliest recorded optical disc, and it rests on the same principle as every CD, DVD and Blu-ray since.
That principle is simple: information is written as tiny physical variations in a surface and read back by light. On modern discs the pattern runs as one continuous spiral from the centre outward, sitting on a thick polycarbonate base about 1.2 millimetres deep. The rainbow shimmer on the data side is not decoration. The closely packed grooves act as a diffraction grating, splitting white light into colours.
Several inventors paved the way. David Paul Gregg devised an analogue video disc in 1958, a distant ancestor of the DVD, and sold his patents to the Music Corporation of America in the early 1960s. James T. Russell filed in 1966 for a system that recorded digital signals on a transparent foil lit from behind by a halogen lamp; Sony and Philips ended up licensing his patents in the 1980s after a legal fight. Both designs were floppy and read by shining light through them, which proved impractical. Reflective formats followed: LaserDisc in 1969, the compact disc in 1984, DVD in 1995 and Blu-ray in 2005. LaserDisc lost the home video market to VHS tape, largely because it cost more and could not record.
Capacity comes down to the colour of the laser. The smallest spot a laser can focus is proportional to its wavelength, so the infrared lasers of early drives could only pack data so tightly; a 12-centimetre CD holds around 700 megabytes. Shorter wavelengths allow finer tracks. The gap between neighbouring tracks shrinks from 1.6 micrometres on a CD to 320 nanometres on a Blu-ray.
Rewritable discs use a different trick: an alloy of silver, indium, antimony and tellurium that switches between physical states. Write-once discs rely instead on organic dyes, and each dye gives the disc its own tint. Because a disc contains no circuitry, it cannot carry malicious hardware, though its data can still hide software threats. Sony showed that in 2005, when some of its music CDs installed a hidden rootkit on buyers' computers.
Source: Optical disc
