In digital audio or video, a clock serves as a timing source. Data has to be executed at very specific times for digital audio or video to function properly. An issue that has to be addressed when synchronizing systems with each other. Although audio and video require different references to keep data passing through a system correctly, they both do it with the aid of a clock. This could be black burst, word clock, or clock embedded in a digital data stream, such as AES/EBU or S/PDIF.
(See Black Burst)