To provide a method and an apparatus of data transmission which are capable of transmission of additional information by utilizing a change in duty ratio in addition to the transmission of a primary original signal without causing deterioration in transmission quality.

The duty ratio of the primary original signal to be transmitted is changed by a quantity corresponding to "1" or "0" of the additional data so that the change in duty ratio comes within a range permitted by digital audio interface standards equivalent to the standards of transmission system of the original signal, for example, IEC 958, and the additional data are transmitted together with the original signal from a transmitter side to a receiver side.