Re: [matplotlib-devel] Pull request

[Pierre H. <pie...@cr...>]

Hi,

Le 21/10/2013 15:58, Todd a écrit :
> On Mon, Oct 21, 2013 at 3:13 PM, Pierre Haessig
> <pie...@cr... <mailto:pie...@cr...>> wrote:
>
>     1) is the terminology "phase" vs. "angle" spectrum standardized ?
>     I must
>     say I've never heard of one meaning "wrapped" and the other
>     "unwrapped".
>     I didn't find similar terms in Matlab docs, but I didn't search that
>     thoroughly.
>
>
> The "angle" function in numpy returns the wrapped angle, while the
> "unwrap" function documentation talks about phase, so it is consistent
> with the usage in numpy.   Further, in signal processing, phases can
> have any value, while "angle" often refers to the angle between two
> lines, which must be wrapped.
>
> There may be some ambiguity, but I made sure to explain it in the
> documentation and provide links between the two functions so people
> know what they should do if they want to use the other approach.
>  
>
>     2) Should there be two separate functions for these two, or just one
>     function, with a switch argument `unwrap` ? (I guess it would be
>     True by
>     default)
>
>
> I originally was going to do that, but decided against it.  The
> problem is with specgram.  Here, I thought it would be needlessly
> complicated to add an "unwrap" parameter that is only useful for one
> "mode".  To make it obvious to users, I wanted to keep specgram as
> similar as possible to the other plot types, and that involved keeping
> the parameter.
>
> Further, this approach is simpler to code and easier to maintain.
> Having to deal with the "unwrap" parameter would have been more
> difficult to program.  Dealing with both an "unwrap" parameter in some
> cases and a separate "mode" in others would have been even more
> complicated.
>
> Further, _spectral_helper and specgram already have a huge number of
> arguments.  This way I was able to get away with just adding one new
> argument rather than two.
>
Thanks for the feedback. I agree that your documentation does make clear
the distinction between "phase" and "angle" and that it has a
consistency. I just feel that this distinction does not exist "outside" ...

But beyond this question of phase vs. angle, I must say I don't see that
big a use case for phase/angle spectrums[*] (as opposed to magnitude
which are much used).  Also, in many cases, "spectrum" is synonymous
with spectral density, which implies "magnitude". In the end I wonder
whether the notion of phase even makes sense for a spectrogram ?

That's the reason why I'm a bit skeptical with the many new "mode
switches" in the spec helper, along with the new phase/angle_* functions.

best,
Pierre


[*] On the other hand I do see a usecase of magnitude and phase for
plotting transfer functions (i.e. Bode diagrams). Those are not fft
based plots, so it's a different topic I guess. Bode/Nyquist/Black
diagrams could be nice to have.