Study and design of a digital interphone based on dPMR446

In order to meet the need of the market and the interphone´s transition from analog to digital, this paper presents the design of a digital interphone based on dPMR446 communication protocol and MELP (mixed excitation linear prediction) vocoder. The system based on the scheme of combining MCU (micro control unit) with DSP (digital signal processor) has good practicability and expansibility. The emphasis of the article is on the introduction of the overall system architecture, the design of MCU software, the section of phase locked loop in hardware circuit and the sending and receiving process. Besides, the digital baseband process including the communication protocol and the speech coding algorithm is illustrated. Realizing the digital baseband system by the DSP can not only simplify the hardware circuit, lower cost and power consumption, but also is convenient to choose different kinds of baseband processing algorithm as needed.