[PD] noise floor: median in audio signals, for peak extraction

Alexandre Torres Porres porres at gmail.com
Thu Dec 22 17:06:57 CET 2011

> So, why is a mean (average) not good enough, while a median would be good
enough ?
> It's possible, but I'd like to hear an explanation.

I'd need to test and compare, all I know is that I tested [avg~] and
compared it to sethares' objects in MAX, that do this median filter, and
they seem to sound better over there.

But these objects (GNU General Public License v2.0) are actually codes
in java,

for(int i=0; i<framesize; i++) { // set the output magnitude by
if(i % medlen == 0){ // taking moving median of input
for(int ij=0; ij<medlen; ij++) { // this takes median
thesevals[ij] = magformed[i+ij]; // of medlen elements
noisefloor[i] = (float)(Statistics.median(thesevals));
} else { // advance median by medlen steps
noisefloor[i] = noisefloor[i-1]; // keeping same value

2011/12/22 Mathieu Bouchard <matju at artengine.ca>

> Le 2011-12-20 à 21:29:00, Alexandre Torres Porres a écrit :
>  this would be kinda like using the [median] or [median_n] objects, but
>> over audio blocks and not number lists.
> Ok, lots of heavy details below...
> The way [median] and [median_n] are built, they take a lot of time.
> Sorting each window of 32 elements is slow with any full sort. Even the
> usually slow process of putting elements one by one in a sorted array is
> faster than any full sort like qsort() or [sort], in this kind of
> situation, because you need to look at the result of the sort after each
> insertion/removal.
> But a sort based on binary-trees will make your median filter able to
> compete with the speed of FFT, for example. You keep your sorted «array» as
> a sorted binary tree and this makes it fast to insert, delete, and find the
> middle. But you can't do that with C++'s std::map because it offers no way
> to find the middle quickly... you'd need something special.
> There may be ways to bend quicksort so that it can do many similar sorts
> quickly, but you can't do that with qsort() nor anything based on it.
> I don't know how zexy's [sort]. Apparently it doesn't use the quicksort
> method that I assumed it did, it uses the shellsort method instead, which
> is sometimes slower, sometimes faster. I wonder whether it could speed up
> your task if you made a kind of [median_n] that would reuse the
> already-sorted list to speed up the next sort. (This would not improve a
> quicksort, but I wonder whether it'd improve [sort]).
> Generally, median-based methods are harder to work with, as they involve
> lots of comparisons and swaps and such, by sorting or by doing things that
> are like sorting ; whereas mean-based methods involve simple fast passes of
> addition and multiplication. But both give quite different results, in many
> situations.
>  Since there's the need of calculating this in and using the result back
>> in the same block round into the audio chain, I can't put the spectrum into
>> a table, and then calculate the median over bits of it.
> with [tabsend~] and a [bang~], then you can send a whole block into the
> message domain, compute it, and get it back as a signal, one block later.
>  But then, how to do it? Should I be able to pull this out only if I write
>> a "median~" or [noise_floor~] external?
>> Or somehow there's another way to do this with some existing external, or
>> a similar technique, or even some audio math trick using [fexpr~] or
>> something?
> I don't think you can do any reasonable sort using [fexpr~]... except
> perhaps a strange undecipherable network of a hundred [fexpr~] or so. I
> think it's easier to write an external.
>  This has to do with the other post I did about a project that attempts to
>> isolate notes into a chord in a spectrum, something like melodyne is does.
> So, why is a mean (average) not good enough, while a median would be good
> enough ? It's possible, but I'd like to hear an explanation.
>  ______________________________**______________________________**
> __________
> | Mathieu BOUCHARD ----- téléphone : +1.514.383.3801 ----- Montréal, QC
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.puredata.info/pipermail/pd-list/attachments/20111222/5495a438/attachment.htm>

More information about the Pd-list mailing list