Background noise estimate ?

Started by February 10, 2001
 In QCELP-8 (IS96A) speech coder, there is a process to determine rate for current frame of speech. Determination of the rate is based on 3 threshold of energy and average energy of current frame (R0). Formulation to count threshold energy is a function of background noise of current frame i, Bi. But I am confuse with this meaning, because from a simulation (source code in C), Bi for initial frame or frame 1 (B1) is set to a value (the value is HIGH_THRESHOLD_LIM = 5059644, where is this value come from ?). If the meaning of background noise is energy of noise that lies in the speech signal, why it is set to a value. Why don't we calculate this noise energy directly ? Please give me an explanation about this ... Second : Actually, I am still learning in this speech coding, so I meet some problems here. The problem may be simple for you, I don't understand why we flatten the spectrum of speech signal, with the use of preemphasis filter ? Thanks for your answer. Mohamad Alifikri ----------------