a cd player has a laser and an optical sensor. a cd has tracks and bumps on the tracks. the cd player passes a laser beam along the track of the cd. the areas that are reflected (without the bumps) are 1s to the player and the areas that are reflected (with the bumps) are 0s to the player. when the 1's and 0's are added to the cd a complicated coding is used that enables error corrections to prevent the cd "skipping" in the event of a small scratch. the cd player converts the series of 1's and 0's into sound waves by sending a varying voltage into an amplifier which then drives a speaker. the conversion of digital data into an analogue signal is achieved through the use a matrix(maths not the movie), imaginary numbers and trigonometric functions.