Newsgroups: comp.dsp,comp.speech
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From: DSP hotline <dsph@msg.ti.com>
Subject: Re: Signal strength from mu-law sound file?
Message-ID: <1995Jan23.210220.2986@newshost.micro.ti.com>
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Organization: Texas Instruments, Houston
References:  <3g0eu0$7q1@news.cs.tu-berlin.de>
Date: Mon, 23 Jan 1995 21:02:20 GMT
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Xref: glinda.oz.cs.cmu.edu comp.dsp:16091 comp.speech:4432

kay@cs.tu-berlin.de (Kay Neuenhofen) wrote:
>
> Hello,
> 
> I was wondering one thing... Is there a way to derive 
> the signal's amplitude (in mV or dB) from a given 8-bit 
> mu-law sound file?
> 
> Thanks for your help, Kay

Kay,

  The equation for u-Law compressed signals is given by the following
equation.  The transfer function is very similar to a regular logarithm
except for input values near zero(this can be corrected, but probably
not needed).  EG... conversion to dB is very easy and only requires a
scalar multiply and maybe an offset!
                             
               ln(1 + u*|x|)      F(x)=compressed output
F(x) = sgn(x) ---------------     x=input sample
                  ln(1+u)         u=compression factor (255 in US)

Regards, Keith Larson
         TMS320 DSP Applications 
         Texas Instruments Inc
