Across human history, communication has taken many different forms. Early technical communication started when humans drew images and symbols on cave walls. By around 200 BC, people were using clay tablets and animal skins, and the practice of using parchment paper was beginning. Around 1450, communicating through paper became much easier with the invention of movable type for the printing press. Thoughts, ideas, and complete books could now be produced at far lower cost and greater speed. This printing method led to major innovations in the field of communication. Today, books, newspapers, and flyers can be produced in large quantities for public access. In the modern era, communication trends focus strongly on the expanding field of digital communication. Digital communication is a branch of electronic communication that operates through small units of data. The Internet, mobile phones, e-mail, and DVD players are all examples of digital communication. These developments represent a communication revolution and reflect changing perspectives within mass communication systems.