Re: [Audacity-devel] Waveform db

[Dennis <den...@gm...>]

Hey Martyn,

thx for your reply. When I see this correctly, you are just "avoiding"
zero values by adding a small number.
This is a simple but effective way to make the calculation of the
logarithm possible for any sample value.
But what I don't understand is the result. If you take a sine for
example, you get almost the same waveform as the original. But actually
this can't be because the energy of the signal doesn't get smaller when
the values are negative, otherwise the energy of a sine would be 0. So
it should actually look like y=|sin(x)|.
Do you know why it is not like that? Or is my thinking wrong here?

Dennis

Am 22.09.2011 02:07, schrieb Martyn Shaw:
> Hi Dennis
> 
> have a look at
> http://code.google.com/p/audacity/source/browse/audacity-src/trunk/src/TrackArtist.cpp
> about lines 758-771
> where dBr is the minimal value to plot (I think, have not spent much 
> time there this evening)).
> 
> You can't really go on scaling to the signal, since one of them 
> samples might be at zero, and then you'll crash.  Having a minimum 
> display level is sensible.  We have done that at the same minimal 
> level that our meters display at, whether that was a good plan is 
> arguable.
> 
> HTH
> Martyn
> 
> On 21/09/2011 13:56, Dennis wrote:
>> Hello!
>>
>> I'm new to the list and first of all I would like to say that audacity
>> is a great project and I really appreciate your work.
>> As I am working on a university project about psychoacoustical stuff at
>> the moment,I came across the requirement to display an audio signal
>> (pcm) in its waveform but in db fs.
>> The first thing that came to my mind was this:
>>
>> x_db = 20log10(x_old/x_ref) with x_ref = highest possible positive
>> value,for example 2^16 for 16bit pcm.
>>
>> But when I do this,the signal looks kinda weird.
>> Of course this is the result of the different position of the samples.
>> if there is a sample very close to 0 the result is  more or less -inf.
>> Now my question is,how are you doing this in audacity in the view mode
>> waveform (db)?
>> I would appreciate any help :-)
>>
>> Dennis
>>
>>
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