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- Newsgroups: comp.dsp
- Path: sparky!uunet!super!cfreese
- From: cfreese@super.org (Craig F. Reese)
- Subject: Re: Format of .AU Sound files
- Message-ID: <1992Dec31.185650.13748@super.org>
- Sender: news@super.org (USENET News System)
- Nntp-Posting-Host: super
- Organization: Supercomputing Research Center (Bowie, MD)
- References: <memo.834974@cix.compulink.co.uk>
- Date: Thu, 31 Dec 1992 18:56:50 GMT
- Lines: 149
-
- In article <memo.834974@cix.compulink.co.uk> rspence@cix.compulink.co.uk writes:
- >christos@kuhub.cc.ukans.edu () writes:
- >
- >> Hello there,
- >
- >> I am using the Public Domain program Sound Tools on the Sun station
- >> to record speech. The speech samples are saved in a mu-law 8bits/sample
- >> encoding rule with a standard ".au" header precceding the data. What I
- >> would like to know is what the format of the speech samples is? In other
- .
- .
- .
- >
- >So much for background. If I remember correctly, the most significant bit is
- >a sign bit, indicating positive (0) or negative (1) value. The remaining 7
- >bits are the encoded amplitude of the sample. Manipulation will require you
- >to decode the sample correctly, and I don't have the details of mu-law
- >encoding to hand. But, I know where it is described. Try and get hold of the
- >CCITT's G.7xx series recommendations which is one large volume. Try a
- >technical library of a university. Alternatively, you could ask very nicely
- >and I will try to mail the relevant pages when I'm over in the US in January.
- >
-
- Here's some code I wrote many moons ago. It should help.
-
- Craig
-
- =============================================================
-
- TWIMC,
-
- Below are two routines I wrote for converting between ulaw and linear.
- I use them with the SparcStation and they seem to work fine.
- I am pretty sure (99.9%) that they implement the standard as specified
- in the references.
-
- Note that the standard deals with converting between 12 bit linear
- and 8 bit ulaw. These routines assume 16 bit linear. Thus, some
- bit shifting may be necessary.
-
- craig
-
- ------------------------------------------------------------------
- /**
- ** Signal conversion routines for use with the Sun4/60 audio chip
- **/
-
- /*
- * This routine converts from linear to ulaw
- * 29 September 1989
- *
- * Craig Reese: IDA/Supercomputing Research Center
- * Joe Campbell: Department of Defense
- *
- * References:
- * 1) CCITT Recommendation G.711 (very difficult to follow)
- * 2) "A New Digital Technique for Implementation of Any
- * Continuous PCM Companding Law," Villeret, Michel,
- * et al. 1973 IEEE Int. Conf. on Communications, Vol 1,
- * 1973, pg. 11.12-11.17
- * 3) MIL-STD-188-113,"Interoperability and Performance Standards
- * for Analog-to_Digital Conversion Techniques,"
- * 17 February 1987
- *
- * Input: Signed 16 bit linear sample
- * Output: 8 bit ulaw sample
- */
-
- #define ZEROTRAP /* turn on the trap as per the MIL-STD */
- #define BIAS 0x84 /* define the add-in bias for 16 bit samples */
- #define CLIP 32635
-
- unsigned char
- linear2ulaw(sample)
- int sample;
- {
- static int exp_lut[256] = {0,0,1,1,2,2,2,2,3,3,3,3,3,3,3,3,
- 4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,
- 5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,
- 5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,
- 6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
- 6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
- 6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
- 6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
- 7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
- 7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
- 7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
- 7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
- 7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
- 7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
- 7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
- 7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7};
- int sign, exponent, mantissa;
- unsigned char ulawbyte;
-
- /** get the sample into sign-magnitude **/
- sign = (sample >> 8) & 0x80; /* set aside the sign */
- if (sign != 0) sample = -sample; /* get magnitude */
- if (sample > CLIP) sample = CLIP; /* clip the magnitude */
- /** convert from 16 bit linear to ulaw **/
- sample = sample + BIAS;
- exponent = exp_lut[(sample>>7) & 0xFF];
- mantissa = (sample >> (exponent+3)) & 0x0F;
- ulawbyte = ~(sign | (exponent << 4) | mantissa);
- #ifdef ZEROTRAP
- if (ulawbyte == 0 ) ulawbyte = 0x02; /* optional CCITT trap */
- #endif
- /** return the result **/
- return(ulawbyte);
- }
-
- /*
- * This routine converts from ulaw to 16 bit linear
- * 29 September 1989
- *
- * Craig Reese: IDA/Supercomputing Research Center
- *
- * References:
- * 1) CCITT Recommendation G.711 (very difficult to follow)
- * 2) MIL-STD-188-113,"Interoperability and Performance Standards
- * for Analog-to_Digital Conversion Techniques,"
- * 17 February 1987
- *
- * Input: 8 bit ulaw sample
- * Output: signed 16 bit linear sample
- */
-
- int
- ulaw2linear(ulawbyte)
- unsigned char ulawbyte;
- {
- static int exp_lut[8]={0,132,396,924,1980,4092,8316,16764};
- int sign, exponent, mantissa, sample;
-
- ulawbyte = ~ulawbyte;
- sign = (ulawbyte & 0x80);
- exponent = (ulawbyte >> 4) & 0x07;
- mantissa = ulawbyte & 0x0F;
- sample = exp_lut[exponent] + (mantissa << (exponent+3));
- if (sign != 0) sample = -sample;
- return(sample);
- }
-
- -----------------
- Craig F. Reese Email: cfreese@super.org
- Institute for Defense Analyses/
- Supercomputing Research Center
- 17100 Science Dr.
- Bowie, MD 20715-4300
-