An audio/speech encoding apparatus/method and an audio/speech decoding apparatus/method are provided. The audio/speech encoding apparatus includes a memory that stores instructions, and a processor that performs operations. The operations include transforming a time domain input audio/speech signal to a frequency spectrum, dividing the frequency spectrum to a plural of bands, calculating norm factors, and quantizing the norm factors. The operations also include calculating differential indices between an Nth band index and an (N−1)th band index, and modifying a range of the differential indices for the Nth band when N is 2 or more. The operations further include replacing the differential index with the modified differential index, and not modifying a range of the differential indices for the Nth band when N is 1. The apparatus encodes the differential indices using a selected Huffman table, and transmits the encoded differential indices and a flag signal over a communication network.