Supplementary information transmitting arrangement for a digital data transmission system

A supplementary information transmitting arrangement for a digital data transmission system comprising a transmitter means and a receiver means;

• said transmitter means (2) receiving an incoming digital data signal and a digital supplementary information signal having a low frequency wave representative of a rate which is much lower than the frequency of said incoming digital data signal for the purpose of inserting digital parity check words into said incoming digital data signal and to produce thereby a resulting digital data signal;
• said transmitter means further comprising parity check word inserting means (11, 7) controlling the logic levels of the digits of said digital parity check words being inserted into the incoming digital data signal as a function of the logic levels of said supplementary information signal so that the accumulated parity of said resulting digital data signal is constant between two successive parity check words which are inserted from the supplementary information signal;
• said receiver means (70 or 80) receiving said resulting digital data signal, said receiver means comprising:
• means (72) for detecting the parity modification in said resulting digital data signal;
• and
• a band pass filter (75 or 86) connected to said parity modification detecting means and having its band pass centered on a frequency which is the same as or a multiple of said frequency of said low-frequency wave representative of said supplementary information signal, and restoring said supplementary information signal.