Low-frequency suppression of baseband coded sequences

Two classes of constrained sequences with favourable low-frequency spectral suppression properties are considered for applications where a conventional analogue speech signal is transmitted on the same channel as a digital signal, but totally independently from it. These are sequences with second-order spectral zeros at zero frequency and a new class which complies to restrictions on the time between crossings of the average value of the running digital sum. General characteristic equations are derived to obtain values of channel capacity for these sequences. The levels of low-frequency suppression obtainable are then evaluated, and the optimisation problem regarding the choice of a code for a specific application is addressed. Finally, the synthesis of a new code with suppressed low-frequency contents is presented