An audio/speech encoding apparatus, comprising: a processor; a memory; a transformer adapted to transform a time domain input audio/speech signal to a frequency spectrum; a band divider adapted to divide the frequency spectrum to a plural of bands; an norm factor calculator adapted to calculate a level of norm factors for each band; a quantizer adapted to quantize the norm factors for the each band; a differential index calculator adapted to calculate differential indices between an Nth band index and an (N-1)th band index, where N is an integer of 1 or more; a differential index modifier adapted to modify a range of the differential indices for the Nth band when N is an integer of 2 or more, and replace the differential index with the modified differential index and adapted not to modify the range of the differential indices for the Nth band when N is an integer of 1; a Huffman encoder (1304; 1305) adapted to encode the differential indices using a selected Huffman table among a number of predefined Huffman tables; and a transmitter adapted to transmit the encoded differential indices and a flag signal for indicating the selected Huffman table to an audio/speech decoding apparatus, wherein when the calculated differential index of the (N-1)th band is greater than a first value, the differential index modifier is adapted to modify a differential index for an Nth band by adding a subtracted value determined by subtracting the first value from a differential index for an (N-1)th band, wherein when the calculated differential index of a (N-1)th band is smaller than a second value, the differential index modifier is adapted to modify a differential index for a Nth band by adding a subtracted value determined by subtracting the second value from a differential index for the (N-1)th band, wherein the first value is a sum of an offset value and a threshold value, and the second value is a difference of the offset value and the threshold value, and the offset value is 15, and wherein when the calculated differential index of an (N-1)th band is not greater than the first value and is not smaller than the second value, the differential index modifier is adapted not to modify a differential index for the Nth band. a processor; a memory; a transformer adapted to transform a time domain input audio/speech signal to a frequency spectrum; a band divider adapted to divide the frequency spectrum to a plural of bands; an norm factor calculator adapted to calculate a level of norm factors for each band; a quantizer adapted to quantize the norm factors for the each band; a differential index calculator adapted to calculate differential indices between an Nth band index and an (N-1)th band index, where N is an integer of 1 or more; a differential index modifier adapted to modify a range of the differential indices for the Nth band when N is an integer of 2 or more, and replace the differential index with the modified differential index and adapted not to modify the range of the differential indices for the Nth band when N is an integer of 1; a Huffman encoder (1304; 1305) adapted to encode the differential indices using a selected Huffman table among a number of predefined Huffman tables; and a transmitter adapted to transmit the encoded differential indices and a flag signal for indicating the selected Huffman table to an audio/speech decoding apparatus, wherein when the calculated differential index of the (N-1)th band is greater than a first value, the differential index modifier is adapted to modify a differential index for an Nth band by adding a subtracted value determined by subtracting the first value from a differential index for an (N-1)th band, wherein when the calculated differential index of a (N-1)th band is smaller than a second value, the differential index modifier is adapted to modify a differential index for a Nth band by adding a subtracted value determined by subtracting the second value from a differential index for the (N-1)th band, wherein the first value is a sum of an offset value and a threshold value, and the second value is a difference of the offset value and the threshold value, and the offset value is 15, and wherein when the calculated differential index of an (N-1)th band is not greater than the first value and is not smaller than the second value, the differential index modifier is adapted not to modify a differential index for the Nth band. wherein when the calculated differential index of the (N-1)th band is greater than a first value, the differential index modifier is adapted to modify a differential index for an Nth band by adding a subtracted value determined by subtracting the first value from a differential index for an (N-1)th band, wherein when the calculated differential index of a (N-1)th band is smaller than a second value, the differential index modifier is adapted to modify a differential index for a Nth band by adding a subtracted value determined by subtracting the second value from a differential index for the (N-1)th band, wherein the first value is a sum of an offset value and a threshold value, and the second value is a difference of the offset value and the threshold value, and the offset value is 15, and wherein when the calculated differential index of an (N-1)th band is not greater than the first value and is not smaller than the second value, the differential index modifier is adapted not to modify a differential index for the Nth band.