1. A system for detecting recognizability of an input signal, said system being a front stage of a speech recognition device or a dialog device, and comprising: an environment parameter generator to generate at least one environment parameter from an input signal by using a voice activity detection (VAD) method and a missing feature imputation (MFI) method wherein the MFI method comprises a step of calculating a clean speech spectrum feature parameter, said at least one environment parameter including a confidence index of said system processing said input signal; a signal recognition verifier to verify whether said input signal is recognizable in accordance with said at least one environment parameter, said signal recognition verifier being trained with environment parameters in advance; and a strategy response processor; wherein said confidence index is generated based on a probability distribution of a spectrum parameter of said input signal and the probability distribution of the spectrum parameter of a system model, and said input signal is passed to said speech recognition or dialog device when said input signal is verified as recognizable; while said strategy response processor is triggered to respond with a plurality of strategies when said input signal is verified as unrecognizable.