As portable telephones have been widely used, a technique for improving voice quality at the time of talking in portable telephones is installed. Main CODEC for each portable telephone system will be shown below, in which system:CODEC, transmission band (Kbit/sec), and voice band (Kbit/sec) are shown in this order.
PHS: ADPCM, 32, 32
PDC Full rate: VSELP, 11.2, 6.7
PDC Half rate: PSI-CELP, 5.6, 3.45
GSM: RPE-LTP, 22.8, 13
CdmaOne: EVRC, 9.6. 8
WCDMA(FOMA): AMR, 38.6. 12/23
Fixed-line telephone (reference): PCM, 64, 64
Note that transmission band means a band of the whole wireless section, and voice band means a partial band where voice signals flow of the transmission band.
For example, encoding processing by EVRC CODEC used in a CDMA network is performed in the procedure shown in FIG. 10. First, in response to a voice analog input (step S101), the voice analog input signal is normalized (step S102). Then, band filtering in analog signals (removal of low frequency components and high frequency components) (step S103) and analog/digital conversion processing (quantization: from analog to linear PCM) (step S104) are performed. Then, among voice waveforms, small amplitude parts are deleted (step S105), and a speaker is recognized, and noise removal processing is performed (step S106). Then, voice signals are encoded (step S107) and converted to codes for transmission (redundant configuration, addition of error correction codes) (step S108).
By encoding voices in accordance with the above-described procedure, in the small amplitude part compression processing in step S105, in the case of no-voice section or noise section, only noise signals are transmitted so as to lower the transmission rate. Thereby, it is possible to reduce power required for transmission and power consumption of a mobile terminal. Further, in the processing of step S106, background noises are reduced to thereby improve the sound quality (clarify) of voice in talking.
Patent Document 1: Japanese Patent Laid-Open Publication No. 11-55716
Patent Document 2: Japanese Patent Laid-Open Publication 2002-247156