Re: [Bluez-devel] Voice setting for U-law and A-law.

[Ivy <ivy@cashette.com>]

--- Andreas Beck <becka-bzdvl@bedatec.de> wrote:
> Ivy <ivy@cashette.com> wrote:
> 
> >    xxxxxx01xxxxxxxx Input Coding: u-law 
> >    xxxxxxxx01xxxxxx Input Data Format: 2"s complement 
> > ( what is the advantage of 2's complement that 1's com ?).
> 
> Actually I don't think this makes sense for ulaw/alaw, as AFAIK these
> are always 8 bit, as they were designed for telephony channels (ISDN)
> that operate at 64kbit, thus giving 8kSamples/s at 8 bit.
> 
> The alaw/ulaw encoding gives a somewhat logarithmic scale which gives
> better dynamic range at the expense of small details the human ear
> doesn't recognize well anyway.
> 
> For linear, the difference is in the coding of negative numbers.
> 1s complement will code negative numbers as their positive equivalent 
> XOR 0xffff (i.e. a word with all-one bits). This is a rater uncommon
> coding scheme. It has the odditiy of having a +0 (0x0000) and a 
> -0 (0xffff). 2s complement will add 1 after the above step, which 
> has the advantage of simple handling of arithmetic operations with
> common ALUs designed for unsigned numbers. It just works - only thing
> you have to adjust is handling of overflow flags.
> Another advantage is a slightly increased range of values (-32768 to
> 32767 at 16 bit as compared to -32767 to 32767 at 1s complement)
> at the expense of an asymmetric value range.
> 
> >  xxxxxxxxxx1xxxxx Input Sample Size: 16 bit   (Can i use 8 bit ?) 
> 
> IMHO you should, if you want alaw/ulaw.

              You meant to use 16 Bit or 8 bit ?..

> 
> >  (what does it  mean "ONly FOR Linear PCM in Documentation ?).  
> 
> I suppose these bits get ignored for formats other than linear PCM, as
> ulaw/alaw are inherently 8 bits.

> 
> > xxxxxxxxxxx010xx # of bits padding at MSB: 2 (if i use 16 Bit, 
> > 2bit's of MSB is needed for padding, coz only 14bits are enough 
> > for u-law) 
> 
> AFAIK 8 bits are enough - could you explain?

              U-LAW takes 14bits of linear PCM data as input and generates 8 bit compressed data. Am i correct ?.. Please correct me if i m wrong !!.. Thatswhy i assume that it needs 16 bits for INPUT Sample size and 2 bit offset in MSB position. 

> 
> 
> CU, Andy

-ivy 

> 
> -- 
> = Andreas Beck                    |  Email :  <becka-sig@bedatec.de>         =
> 
> 
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