adaptive spectral entropy coding (ASPEC)

Adaptive Spectral Entropy Coding (ASPEC) is an audio compression for signals with bit rates of up to 64 kbit/s. In the ASPEC process, broadband signals are divided into subbands. In contrast to the Musicam process, these are formed by a modified cosine transform( MDCT). In ASPEC, the transformation into the frequency domain is performed with a maximum of 512 coefficients. One therefore works with 512 sub-bands and thus has an exact frequency division.

In ASPEC compression, blocks are cut out of the input signal, which are then transformed. The length of each block is varied depending on the signal, with the size controlled by pulse detection. After the blocks are transformed into the frequency domain using the Modified Discrete Cosine Transform (MDCT), the listening thresholds are calculated.

ASPEC's quantization is driven by psychoacoustic ratings. For coding, ASPEC uses Huffman coding with variable length code. In this process, frequently used elements are assigned a shorter codeword, while less frequently used elements are assigned longer codewords.