🚀 go-pugleaf

In article <26618@pasteur.Berkeley.EDU>
  maverick@fir.berkeley.edu (Vance Maverick) writes:
>	The SPARCstation plays audio data which has been compressed from a
>12-bit range to an 8-bit range, using a (presumably) nonlinear mapping
>for better signal-to-noise ratio at low amplitudes.  Where can I find
>out just what this mapping is?

A blast from the past...

|From chip Sat Sep 23 02:43:11 CDT 1989
|Newsgroups: comp.dsp
|Subject: Re: converting to/from compressed form
|References: <RUSTY.89Sep21171741@garnet.berkeley.edu>
|
|rusty@garnet.berkeley.edu writes:
|>We have a Sun Sparcstation 1.  The chip inside that it uses for D/A and A/D
|>is the AM79C30 Digital Subscriber Controller.  [...] I would like to take
|>uncompressed digital sound samples, say generated by a [program], and feed
|>it to this chip.
|>
|>Does anybody have any code or know where I can get my hands on some
|>that would convert "normal" (uncompressed) 16 bit digital audio into
|>the 8 bit u-law (or A-law) compressed form that the AM79C30 uses?
|
|    Reference:	|Digital Telephony|, John C. Bellamy, John Wiley & Sons,
|		1982, pp. 90-113.
|
|These compression techniques are used in digital telecommunications to
|try to squeeze reasonable sound quality ("toll quality") into 8-bits.
|
|u-Law (that's "mu-law", not "you-law") compression is defined by:
|
|			ln( 1 + u*|x| )
|    F (x)  =  sgn(x) * ----------------
|     u                   ln( 1 + u )
|
|where -1 <= x <= +1, and sgn(x) is the sign function.
|
|The compressed value is usually represented as an 8-bit signed magnitude
|number:  one sign bit plus seven magnitude bits.  In this scheme, the
|most positive number is 0111111, the most negative number is 11111111,
|00000000 is a positive zero, and 1000000 is a negative zero.
|
|The u-Law expansion formula is:
|
|     -1               1               |y|
|    F (y) = sgn(y) * --- * [ ( 1 + u )   - 1 ]
|     u                u
|
|In modern telephony, a value of u=255 is used.  The first digital
|telecommunication systems (the "D1" channel bank) used u=100, but with
|the introduction of the "D2" channel bank a value of u=255 was selected
|to simplify the conversion process.  This value allows Fu(x) to be easily
|approximated by 15 linear segments.  This feature is not so important these
|days since it's easy enough to build a conversion lookup table into ROM.
|
|Please see Bellamy for more information on the linear approximation
|technique and details on constructing ROM lookup tables.  A-Law companding
|is also discussed there.  u-Law is used primarily in the North American
|and Japanese telecommunications networks, while A-Law is used in Europe.

-- 
Chip Rosenthal                            |  You aren't some icon carved out
chip@chinacat.Unicom.COM                  |  of soap, sent down here to clean
Unicom Systems Development, 512-482-8260  |  up my reputation.  -John Hiatt