|In a CD player, how is the digital optical signal transformed into an electrical signal? — IM, Oxford, UK
The ridges and flat regions on a compact disc's aluminum layer determine how laser light is reflected from that layer. As the disc turns and the player's laser scans across ridges and flat regions, the intensity of the reflected light fluctuates up and down. This reflected light is directed onto an array of silicon photodiodes that provide both the signals needed to keep the laser focused tightly on the aluminum layer and the signal that the player uses to recreate sound. The sound is encoded in the lengths of the ridges. A computer monitors the amount of light returning from the disc to determine how long each ridge is and how much spacing there is between it and the next ridge. The computer uses this information to obtain a series of 16 bit binary numbers for each of the two sound channels that are represented by an audio CD. A digital-to-analog converter uses these 16 bit numbers to produce currents that are eventually amplified and used to produce sound.