A transparent data transmission system for encoding data to be transmitted by way of the scan lines of a television video signal, includes interface circuitry for receiving data from a number of input devices each operating at a particular data rate, and encoding processing circuitry which takes the data received by the interface circuitry and arranges it into separate data groups so that each group corresponds to the particular input device which originated the data. The data groups are then inserted, by way of timing circuitry, into corresponding data channels defined in a selected scan line of the video signal. Also, a data channel bit map is defined over a portion of the selected scan line, and data identifying each of the input devices which is providing data to the interface circuitry is inserted into the data channel bit map. Accordingly, when the encoded video signal is decoded to extract the inserted data groups together with the channel bit map, the decoding circuitry can determine, in accordance with the data channel bit map, if a null character contained in the extracted data was provided by an input device to the encoding interface circuitry and, if it was, the decoding circuitry can then allow the null character to be distributed for further processing. The overall transmission system therefore exhibits true transparency.