Abstract: A transmission system is provided having a sender side apparatus, a receiver side apparatus, and a transmission line connecting the sender and receiver side apparatuses. The sender side apparatus provides a selector for receiving image data signals of a plurality of channels in parallel and, frame by frame, selects an image data signal of one of the plurality of channels for each frame. The selector discards the image data signals of the other of the plurality of channels. A coding unit codes the image data signal which is selected for each frame by the selector, by interframe difference coding. A monitoring unit monitor data amounts of interframe differences in the image data signals of the respective channels. A control unit determines which one of said plurality of channels is to be selected by the selector, based on the data amounts of interframe differences in the image data signals of the respective channels, and adaptively controls the selector in accordance with this determination.