The present invention provides a method of hierarchically modulating first and second digital data streams in a broadcast system such as an SDAR system that is compatible with legacy receivers. The second digital data stream, but not the first, is provided with a non-binary pseudo-random encoding. A carrier is first modulated, e.g. QPSK, by the first digital data stream; and a second modulation is performed with the encoded second digital data stream. The encoding includes a multiplication of a matrix formed of 2, 3 or 4 consecutive bits of the secondary data stream by a matrix, e.g. a Hadamard matrix to form a product matrix having non-binary values that are used to modify consecutive symbols of the first modulation. The second modulation appears as Gaussian noise to legacy receivers over time.