g.711 音频编码

     关于a law  and u law 定义如下：A律编码是ITU-T(国际电信标准局)定义的关于脉冲编码的一种压缩/解压缩算法。世界上大部分国家采用A律压缩算法(包括中国)。美国采用u 律算法进行脉冲编码。其英文定义大致如下：

A-law and u-law are companding schemes used in telephone network to
get more dynamics to the 8 bit samples that is available with linear
coding. Typically 12..14 bit samples (linear scale) sampled at 8 kHz
sample are companded to 8 bit (logarithmic scale) for transmission
over 64 kbit/s data channel. In the receiving end the data is then
converter back to linear scale (12..14 bit) and played back.
converted back
u-law definition
u-LAW (pronounced mu-LAW) is
sgn(m) ( |m |) |m |
y= ------- ln( 1+ u|--|) |--| =< 1
ln(1+u) ( |mp|) |mp|
Another definition for mu-law I have seen
ln(1+255 |x|)
output = sgn(x) ---------------------
ln(1+255)

x = normalized input ( between -1 and 1)
255 = compression parameter
sgn(x) = sign (+/-) of x

a-law definition
A-LAW is
| A (m ) |m | 1
| ------- (--) |--| =< -
| 1+ln A (mp) |mp| A
y=|
| sgn(m) ( |m |) 1 |m |
| ------ ( 1+ ln A|--|) - =< |--| =< 1
| 1+ln A ( |mp|) A |mp|
Values of u=100 and 255, A=87.6, mp is the Peak message value, m is
the current quantised message value. (The formulae get simpler if you
substitute x for m/mp and sgn(x) for sgn(m); then -1 <= x <= 1.)
Converting from u-LAW to A-LAW is in a sense "lossy" since there are
quantizing errors introduced in the conversion.
"..the u-LAW used in North America and Japan, and the A-LAW used in
Europe and the rest of the world and international routes.."
References:
? Modern Digital and Analog Communication Systems, B.P.Lathi.,
2nd ed. ISBN 0-03-027933-X
? Transmission Systems for Communications, Fifth Edition, by
Members of the Technical Staff at Bell Telephone Laboratories,
Bell Telephone Laboratories, Incorporated, Copyright 1959, 1964,
1970, 1982

 下面介绍下g.711的c代码的核心　

#ifndef __G711_H_
#define __G711_H_


#ifdef __cplusplus
extern "C" ...{
#endif    /* __cplusplus */


unsigned char    linear2ulaw(short pcm_val);
short            ulaw2linear(unsigned char u_val);

unsigned char    linear2alaw(short pcm_val);
short            alaw2linear(unsigned char a_val);

unsigned char    alaw2ulaw(unsigned char a_val);
unsigned char    ulaw2alaw(unsigned char u_val);


#ifdef __cplusplus
}
#endif    /* __cplusplus */


#endif    /* __G711_H_ */

一般工程中Ａ律用的比较多，所以调用

unsigned char    linear2alaw(short pcm_val);
short            alaw2linear(unsigned char a_val);

注意在音频处理中用的数据类型为　unsigned char 和 short ，其中ITU-T的官方代码是没有问题的，openh323中G.711的编解码器的数据类型都使用的int 和 unsigned 所以造成数据溢出，不能正常的工作，本人在第一次做的时候遇到了这个问题，替换一下就可解决即把相应的int 改为 short 和 unsigned char