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 

g.711 音频编码 #ifndef __G711_H_
g.711 音频编码
#define  __G711_H_
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g.711 音频编码
g.711 音频编码#ifdef __cplusplus
g.711 音频编码
extern   " C "   {
g.711 音频编码
#endif    /* __cplusplus */
g.711 音频编码
g.711 音频编码
g.711 音频编码unsigned 
char    linear2ulaw(short pcm_val);
g.711 音频编码
short            ulaw2linear(unsigned char u_val);
g.711 音频编码
g.711 音频编码unsigned 
char    linear2alaw(short pcm_val);
g.711 音频编码
short            alaw2linear(unsigned char a_val);
g.711 音频编码
g.711 音频编码unsigned 
char    alaw2ulaw(unsigned char a_val);
g.711 音频编码unsigned 
char    ulaw2alaw(unsigned char u_val);
g.711 音频编码
g.711 音频编码
g.711 音频编码#ifdef __cplusplus
g.711 音频编码}

g.711 音频编码
#endif     /* __cplusplus */
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#endif     /* __G711_H_ */
g.711 音频编码

一般工程中A律用的比较多,所以调用

g.711 音频编码 unsigned  char     linear2alaw( short  pcm_val);
g.711 音频编码
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

 

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