or threshold of a sound and an increment caused by the presence of other tonal and non tonal components of the audio signal the smr j 39 s are then fed to the second bit allocation unit 50 while p j 39 s and m j 39 s are coupled to the second perceptual parameter estimator 30 the second perceptual parameter estimator 30 serves to estimate a perceptual entropy pe i for the input digital audio signal of an ith frame based on the sound pressure levels p j 39 s and the masking thresholds m j 39 s from the first perceptual parameter estimator 20 the perceptual entropy pe i for the input digital audio signal of the ith frame as well known in the art may be represented as equ1 wherein i j and b have the same meanings as previously defined eq 2 can be obtained by applying the so called rate distortion theory and corresponds to a perceptual entropy based on the human auditory perception also the second perceptual parameter estimator 30 is adapted to group the estimated perceptual entropies i e pe i pe i 1 pe i 2 and pe i 3 for q e g 4 number of the current and its previous frames in order to adaptively allocate bits among them in accordance with the processing of the first bit allocation unit 40 which will be described.