Abstract: A data transmission apparatus is disclosed, which includes a first terminal for use with variable-rate data, a second terminal for use with constant-rate data, a processing unit for input of data from the first and second terminals, a modulator unit for modulating the output of the processor unit, a control unit for controlling the modulator, and a transmitter unit. The processor has a first buffer for writing thereinto data from the first terminal, a second buffer for writing therein data from the second terminal, and a selector for reading data from either one of the first and second buffers while switching therebetween at a predetermined rate, thereby allowing the switching of the selector to be performed under the control of the control unit.