Reduced complexity signal transmission system

The invention is related to a transmission system comprising a transmitter for transmitting an input signal to a receiver via a transmission channel, the transmitter comprising an encoder with an excitation sequence generator for generating a plurality of excitation sequences, selection means for selecting an excitation sequence from a plurality of excitation signals resulting in a minimum error between a synthetic signal derived from said excitation sequence, and a target signal derived from the input signal, the transmitter being arranged for transmitting a signal representing the selected excitation sequence to the receiver, the receiver comprises a decoder with an excitation sequence generator for deriving the selected excitation sequence from the signal representing the selected excitation sequence, and a synthesis filter for deriving a synthetic signal from the excitation sequence. In a CELP coder a comparison between a target signal and a plurality of synthetic signals is made. The synthetic signal is derived by filtering a plurality of excitation sequences by a synthesis filter having parameters derived from the target signal. The excitation signal which results in a minimum error between the target signal and the synthetic signal is selected. The search for the best excitation signal requires a substantial computational complexity. To reduce the complexity a preselection of a small number of excitation sequences is made by selecting a small number of excitation sequences resembling the most a backward filtered target signal. With this small number of excitation sequences a full complexity search is made. Due to the reduced number of excitation sequences involved in the final selection the required computational complexity is reduced.