Modified very low bit rate coder for wireless messaging applications

This paper describes a modified variable coder with an average bit-rate of 1200 bps for wireless messaging including voice paging and voice e-mail delivery to a PC or a hand held device. The coder uses LPC based analysis/synthesis with Zinc function excitation for voiced frames, a plosive and pitch detector to achieve good synthetic voice quality at this rate. Further, a frame deemed unvoiced has the RMS value of its LPC residual quantized and sent to the decoder. The proposed system achieved an MOS of 3.1, while conventional MELP achieved an MOS of 3.3 in a subjective quality test. Further, the coder has been simulated on a workstation and a laptop PC running windows NT. Hence the coder can be used from MS Outlook to send and receive voice e-mails. The performance of the coder under random bit errors is also presented. It has been found that only at error rates of 10^-2 and higher does the degradation becomes objectionable.