Dynamic bit allocation apparatus and method for audio coding

A dynamic bit allocation method for audio coding for determining a number of bits used to quantize a plurality of decomposed samples of a digital audio signal, the plurality of samples being grouped into a plurality of units each having at least either one of different frequency intervals or time intervals, the different frequency intervals being determined based on a critical band of human audio characteristics and the different time intervals including a first time interval and a second time interval longer than the first time interval, said method including the following steps of:

• (a) an absolute threshold setting step for setting an absolute threshold for every unit based on a specified threshold characteristic in quiet representing whether or not a person is audible in quiet;
• (b) an absolute threshold adjusting step for adjusting the absolute threshold of a unit having the first time interval by replacing the absolute threshold of the unit having the first time interval by a minimum absolute threshold among a plurality of units having the same frequency interval;
• (c) a peak energy computing step for computing peak energies of the units based on the plurality of samples grouped into the plurality of units;
• (d) a masking effect computing step for computing a masking effect that is a minimum audible limit with the simplified simultaneous masking effect model based on a specified simplified simultaneous masking effect model and a peak energy of a masked unit when all the units have the second time interval, and updating and setting the absolute threshold of each unit with the computed masking effect;
• (e) a signal-to-maskratio (SMR) computation step for computing SMRs of the units based on the computed peak energy of each unit and the computed absolute threshold of each unit;
• (f) a number-of-available-bits computing step for computing a number of bits available for bit allocation based on a frame size of the digital audio signal, assuming that all frequency bands to be quantized include all the units;
• (g) an SMR positive-conversion step for positively converting the SMRs of all the units by adding a specified positive number to the SMRs of all the SMRs so as to make the SMRs all positive;
• (h) an SMR-offset computing step for computing an SMR-offset which is defined as an offset for reducing the positively converted SMRs of all the units, based on the positively converted SMRs of all the units, a SMR reduction step determined based on an improvement in signal-to-noise ratio per bit of a specified linear quantizer, and the number of available bits;
• (i) a bandwidth computing step for updating a bandwidth which covers units that need to be allocated bits based on the computed SMR-offset and the computed SMRs of the units so as to update the SMR-offset based on the computed bandwidth;
• (j) a sample bit computing step for computing a subtracted SMR by subtracting the computed SMR-offset from the computed SMR in each unit, and then, computing a number of sample bits representing a number of bits to be allocated to each unit in quantization based on the subtracted SMR of each unit and the SMR reduction step;
• and
• (k) a remaining bit allocation step for allocating a number of remaining bits resulting from subtracting a sum of the numbers of sample bits to be allocated to all the units from the computed number of available bits to at least units having an SMR larger than the SMR-offset.