Your car has an AUX input but no Bluetooth. You also have an old Android phone sitting in a drawer. Install a sink app on that old phone, plug it into the car’s AUX port, and hide it in the glovebox. Now you have a dedicated Bluetooth receiver for your car. Your new iPhone or Samsung can stream music wirelessly to the old Android, which pipes it perfectly into the car’s stereo.
Synchronization is the biggest hurdle. Bluetooth audio naturally suffers from delay; a Sink app must implement sophisticated buffering to ensure that if you are watching a video on the Source device, the audio on the Sink device remains lip-synced. The Future: From Apps to Native Features android a2dp sink app
The device that receives and plays that stream (e.g., Bluetooth speakers, headphones, or a car's head unit). Your car has an AUX input but no Bluetooth
Many older cars have aux-in ports but no Bluetooth connectivity. Instead of buying a generic Bluetooth dongle that you have to charge and which often has poor audio quality, you can mount an old Android phone in the car. It stays permanently connected to the car’s aux port. You pair your daily driver phone to this "car phone," turning your vehicle into a modern Bluetooth-enabled ride. Now you have a dedicated Bluetooth receiver for your car
The defines two specific roles in a Bluetooth connection:
Using an Android device as a bridge to play music from an iPhone or a PC into a wired car auxiliary port.
With the advent of , the distinction between Source and Sink is becoming more fluid. Newer versions of Android are beginning to support features like "Auracast," which allows for more flexible broadcasting. However, for the millions of legacy devices currently in drawers, A2DP Sink apps remain the primary method for breathing new life into old silicon.