Ryan May wrote: > Fago, Matt - AES wrote: >> I cannot really compute the example without the pad_to support in svn. >> Nevertheless, using something similar (nfft=128, noffset=64) gives similarly >> erroneous results. >> >> Did you add 'pad_to'? If so, thanks! > > Good to know. I recently (within the last month) did a bunch of work on > psd, based in a large part on work done by one of my colleagues, Sean > Arms. I was worried some of this had broken existing code, but it > appears more likely that this was already a problem. > > After much playing, and reading the Matlab docs, it looks like the > difference is that Matlab normalizes by the sampling frequency. Also, > if a one-sided psd is returned, it multiplies everything by 2. If I > apply these two scaling factors, I get results in line with those > produced by Matlab. > > Now, this scaling was not performed by the old code, so this is not a > new incompatibility (bug?) with Matlab. Also, while I have not reviewed > the reference specified in the docstring (Bendat and Piersol 1986), the > book I have handy (Stoica and Moses 2005) does not mention scaling by > the sampling frequency, nor does the included Matlab code perform any > such scaling. > > So what should be done here? I would be opposed to making such scaling > the default, as IMHO it tries to do too much and I would have to work > around it to get the more "raw" numbers. However, I am not opposed to > adding (yet another) option to do such scaling.
Hi Ryan, I appreciate the work you're doing on this, and while I don't have any very strong opinions on the API questions you raise, I would request that you include in the docstrings information at least at the level of the above, listing the scaling issue, the sources you've followed and how and why you've chosen to follow them or deviate, and differences with the default behavior of other software. I recently helped a colleague use Matlab's PSD/pwelch functions and had to spend some time sending in sine waves of various frequencies and amplitudes to figure out what the results meant. If matplotlib gives different results by default, I think it should 1) justify why and 2) indicate what option(s) may be set to get results equivalent to other systems, including Matlab. Thanks again, Andrew ------------------------------------------------------------------------- This SF.Net email is sponsored by the Moblin Your Move Developer's challenge Build the coolest Linux based applications with Moblin SDK & win great prizes Grand prize is a trip for two to an Open Source event anywhere in the world http://moblin-contest.org/redirect.php?banner_id=100&url=/ _______________________________________________ Matplotlib-users mailing list Matplotlib-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/matplotlib-users
