Re: Amplitude Decibel conversion problem

On Oct 7, 11:07=A0pm, buleg...@[EMAIL PROTECTED]
 wrote:
> On Oct 7, 2:52=A0pm, "jungledmnc" <jungled...@[EMAIL PROTECTED]
> wrote:
>
>
>
>
>
> > Hi,
> > this question might be very trivial, but I haven't found good
explanati=
on
> > anywhere else.
>
> > I have an audio signal - some sine. Use a wavelabl to look at the
> > waveform, and its maximum is at about 0.6. When I change the display
in=
to
> > dB it shows about -5dB at that level.
> > But this does not make sense - the formula is 10*log10(amplitude). So
i=
t
> > should be -2.5dB. I have found, that this is because we use squares
for
> > comparison :
> > 10 * log10(amp1^2 / amp2^2) =3D 20 * log10(amp1 / amp2)
>
> > They say it is often used in electronics to compare voltages, but why
i=
s
> > it used in digital audio? Yeah I know that the digital values are
> > equivalent to resulting electrical power, but we are not comparing
anyt=
hing
> > to anything. So why should we square?
>
> > And if so, then when should we NOT square?
>
> > Thanks a lot.
> > dmnc
>
> dB's always compare one power to another power by the formula that you
> have written down.
>
> In electronics we often do not directly measure power, but rather
> voltage. =A0Most of our measuring equipment measures things in voltage.
>
> So we are stuck in a position where we want to compare one power level
> to another power level, but our measuring equipment measures voltages.
>
> (Also in DSP most of the time the A/D converter is getting a voltage
> equivelent input)
>
> To properly compare one power level when we made the measurement in
> voltage you have to first convert the measured voltage level into a
> measured power level, and the reference voltage level into a reference
> power level.
>
> Here is where the shortcut comes in, but remember, dB's always compare
> power levels. =A0It is the definition.
>
> But when you are comparing voltage levels, it turns out you get to use
> the trick of multiplying by 20*log(ratio) instead of 10*log(ratio),
> becuase power pro****tional to the voltage squared. =A0so 10*log(V1^2/
> V2^2) is the same as
>
> 20*log(V1/V2). =A0This is a shortcut or trick. Always go back to dB's
> compare power per the definition formula 10*log(P1/P2).
>
> The 20 log thing comes into play becuase in the real world you rarely
> measure power directly. =A0You measure voltages directly, and then
> apply the 20* trick to convert the voltages to power.- Hide quoted text
-
>
> - Show quoted text -


PS  I have a website that describes some DSP stuff.  One of the
programs that I wrote gives an overview of dB's as a precurser to
understanding windowing.

Look it over if you like.  The dB stuff is on the first and especially
second page of the program.

http://www.fourier-series.com/fourierseries2/flash_programs/DFT_windows/ind=
ex.html