文件名称：信源编码-华为CDMA2000
文件大小：3.95MB
文件格式：PPT
更新时间：2024-05-13 05:51:59
cdma
信源编码 话音编码器 8K QCELP 13K QCELP EVRC 特点 支持语音激活 在典型的双工通话中，通话的占空比小于35%,不通话的时候降低发射速率，有效提高系统容量。 CDMA 核心技术 Adopted in CDMA system is a QCELP vocoder with variable rates, which is actually a device converting a sound signal into the signal which can be transmitted in a circuit. The method adopted generally in a wire communication system is to first sample (8,000 sample values generated per second) a voice signal with a 8KHZ signal and then implement 8-bit quantization coding for each sample value. Therefore, each voice channel in a wired system has the rate of 64K. However, because the air resource in a wireless system is very precious, a more effective coding mode is needed to use a rate as low as possible in the case where voice quality is guaranteed. QCELP vocoder with variable rates is such a device. The main principles of it are to extract some voice feature parameters when a person speaks and transmit these feature parameters to the peer party. Then,the peer party will recover the voice with these parameters based on the promise between the two parties. Thus, a far lower rate is needed. Let’s give an example. The information of a triangle can be transmitted from one place to another in two ways: one is to obtain some points by means of sampling and transmit the information of these points to the peer party. The two parties connect these points to obtain a triangle. The other is to transmit the length of a side and the degrees of two angles of this triangle to the peer party, who can likewise recover this triangle based on these pieces of information. Obviously, there is far less information to be transmitted in the second method. What a vocoder does is similar to the latter method, but what a vocoder actually does is more complex than this. But the principles are the same. Meanwhile, the codes transmitted from the transmit end to the receive end and describing voice feature parameters vary with the rhythm or loudness of a speech. In summary, variable rates mean that a vocoder can change its own code rates based on the loudness or rhythm of a speech to further reduce a code rate.Thus, a code with a higher rate will be adopted when there is a high voice while a code with a lower rate will be adopted when there is a low voice. In a silent period (when a person makes no sound during a speech), the lowest code rate will be adopted. Thus, a code rate can be decreased to reduce the interference with other users.