2. The speech recognition system according to claim 1 , wherein the cepstral noise subtraction device comprises: a feature mean vector calculator device, for obtaining the first feature vector of the preset voice frame and the first feature vectors of the plurality of voice frames before the preset voice frame, so as to calculate the feature mean vector; a first multiplier, for multiplying the feature mean vector by a negative value of the first scalar coefficient, so as to calculate a first multiplication result; a first adder, for adding the first feature vector of the preset voice frame with the first multiplication result, so as to calculate an addition result; a second multiplier, for multiplying the first feature vector of the preset voice frame by the second scalar coefficient, so as to calculate a second multiplication result; a comparator, for comparing whether the addition result is greater than the second multiplication result, and outputting a control signal; and a multiplexer, for switching the second feature vector of the preset voice frame into the addition result or the second multiplication result according to the control signal.
2. The speech recognition system according to claim 1 , wherein the cepstral noise subtraction device comprises: a feature mean vector calculator device, for obtaining the first feature vector of the preset voice frame and the first feature vectors of the plurality of voice frames before the preset voice frame, so as to calculate the feature mean vector; a first multiplier, for multiplying the feature mean vector by a negative value of the first scalar coefficient, so as to calculate a first multiplication result; a first adder, for adding the first feature vector of the preset voice frame with the first multiplication result, so as to calculate an addition result; a second multiplier, for multiplying the first feature vector of the preset voice frame by the second scalar coefficient, so as to calculate a second multiplication result; a comparator, for comparing whether the addition result is greater than the second multiplication result, and outputting a control signal; and a multiplexer, for switching the second feature vector of the preset voice frame into the addition result or the second multiplication result according to the control signal. 3. The speech recognition system according to claim 2 , wherein when the addition result is greater than the second multiplication result, the second feature vector of the preset voice frame is the addition result, and when the addition result is smaller than the second multiplication result, the second feature vector of the preset voice frame is the second multiplication result.