| 摘要 |
A method for shaping the signal spectrum of a transmit signal for placing on an output line (3) of a modem (1) wherein a plurality of symbol levels (8) are derived from a user data input bit stream (2) fed to the modem (1), and an alternative symbol level (9) is assigned to each symbol level. The method comprises passing each symbol level (8) and its alternative symbol level (9) through a digital filter (12, 14, 15) having a relatively high output at undesired frequencies and a relatively low output at desired frequencies. The energies of the output signals (20, 21) from the filter (12, 14, 15) corresponding to the respective symbol level (8) and the alternative symbol level (9) are determined and subtracted (24) one from the other. Should the difference between the two energies exceed a predetermined value (V ref ), then the one of the symbol level (8) and the alternative symbol level (9) of lowest energy is selected for the transmit signal and applied to the output line (3). |
