FFT and Decibels

Randy Yates <yates@ieee.org> writes:

> Jerry Avins <jya@ieee.org> writes:
>
>> cincydsp@gmail.com wrote:
>>>>> Don't. Either use x[n] = (L+R)/2 or FFT the two channels individually
>>>>> and then compute the average.
>>>> Why bother dividing by 2? That's just a scale factor.
>>> If you're implementing this in a fixed-point system (although it
>>> sounds
>>> like the original problem is on a PC), then you could have overflow
>>> issues if L and R were both full-scale. Dividing by 2 avoids this and
>>> gives a more natural "average" of the two channels.
>>
>> (L + R)/2 overflows just as easily as L + R. 
>
> Not in most DSP architectures, which have 2N+G bits in the accumulator,
> where N is the native datapath length. The extra G bits keep you from
> overflowing. 

Nix that! You only need N+1 bits to do the sum, so any GPP that has
at least this many bits in the accumulator will do. I guess I was
thinking of two multiply-accumulates.
-- 
%  Randy Yates                  % "With time with what you've learned, 
%% Fuquay-Varina, NC            %  they'll kiss the ground you walk 
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%%%% <yates@ieee.org>           % '21st Century Man', *Time*, ELO
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Jerry Avins skrev:
> Rune Allnor wrote:
> > Fred A. skrev:
> >> I would like to have a amplitude as well as a decibel display in my program
> >> of 16-bit stereo audio.
> >>
> >> 1) First I take the data from both channels as L,R,L,R,etc and feed it into
> >> the FFT as one signal.
> >
> > Don't. Either use x[n] = (L+R)/2 or FFT the two channels individually
> > and then compute the average.
>
> Why bother dividing by 2? That's just a scale factor.

Others already covered the overflow issue. Not that it matters; I only
realized after I posted the reply that the OP needs to process the
channels separately. 

Rune