The answer lies in a type of laser known as a semiconductor laser, specifically a laser diode. They are commonly used in CD players, DVD players, and other optical disc. A CD is essentially a digital storage medium that contains audio or data information encoded in the form of tiny pits and lands on a spiral track. The pits and lands reflect light differently, allowing the CD player to read the digital information and convert it into sound waves. I decided to open this part to have a look at the laser diode and phototransistor components. This early technology was pioneered by companies like Philips and Sony, who worked together to develop the CD format. The main beam uses the four (A to D) lacated at the middle, while the side beams use the other two. In 1978, Magnvox introduced the LaserDisc (LD) system, called "DiscoVision", although it wasn't until around 1980 with Pioneer's VP1000 that it became reasonably reliable for the general public.
