The cassette adapter allows another source of music to be played through sound systems with a tape player. This is useful for vehicles without auxiliary (aux) ports or CD players. Usage Patented on March 29, 1988, a cassette tape adapter is a device that allows the use of portable audio players in older cassette decks. Originally designed to connect portable CD players to car stereos that only had cassette players, the cassette tape adapter has become popular with portable media players even on cars that have CD players built in. Today, it is primarily used for vehicles without auxiliary ports built into their stereo systems. For vehicles with AM/FM systems, but no cassette playback, FM transmitters are recommended. A typical cassette adapter uses a single-sided writing tape head (similar to the recording head on a traditional tape deck) connected to a stereo minijack connector with a cord. The cord is connected to the device's output (or headphones) port and the electrical signal is converted into a magnetic signal by the head. This magnetic signal is then received by the tape deck's reading head, converted back into an electrical signal, and amplified by the sound system. Because most cassette adapters use a single-sided head, they only work in one direction. There are, however, some cassette adapters that have double-sided heads that work in either direction. One-way gears within the cassette simulate tape movement from reel to reel, to ensure that the deck does not auto-reverse. A newer type of adapter is a type of MP3 player shaped like a cassette tape, which can be used as a stand-alone MP3 player with headphones or inserted as a cassette into the cassette player, where it can be used with a remote control. These usually have a double-sided head which means they can work in both directions. Another type of cassette adapter is the Bluetooth cassette adapter. It has the shape of a standard cassette, but has a built-in audio Bluetooth receiver module, a simple power supply to allow charging and power and a small battery. Usually, they may power on when the cassette player is set on play, and power off when the cassette player is stopped. They have a switch under the head. Some may as well operate as stand-alone adapters, having also a separate power switch. Charging is done externally. Operation is simple, just like the cassette shaped MP3 player. It