transmitted to a packet parser 27 to parse the adpcm digital data the initial value and the scale factor of the data packet in this embodiment the predetermined bit of the header of the rtp packet is parsed to be u201c 1 u201d therefore the packet parser 27 parses and transmits the adpcm digital data e g x n illustrated in fig1 a of the packet and further reads content of the extension field illustrated in fig3 in response to the predetermined bit u201c 1 u201d of the header so as to retrieve the initial value and the scale factor a scale factor adjuster 29 retrieves the initial value and the scale factor parsed by the packet parser 27 and a demodulator 28 retrieves the adpcm digital data parsed by the packet parser 27 a differential calculation unit 30 coupled to the scale factor adjuster 29 and the demodulator 28 then demodulates the adpcm digital data according to the initial value and the scale factor to obtain a plurality of decompressed digital sampled values e g p n illustrated in fig1 a the digital sampled values are de quantized by a de quantizer 31 to reconstruct a raw analog audio output signal s u2032 similar to the raw analog audio input signal s as mentioned in the foregoing description an initial value and a scale factor of a packet according to the present disclosure are transmitted to a.