[Sv1-devel] [ sv1-Bugs-1507698 ] Time scaling error when spectrogram stepsize changes

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Bugs item #1507698, was opened at 2006-06-17 03:03
Message generated for change (Comment added) made by craigsapp
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Category: None
Group: None
Status: Open
Resolution: None
Priority: 8
Submitted By: Craig Stuart Sapp (craigsapp)
Assigned to: Chris Cannam (cannam)
Summary: Time scaling error when spectrogram stepsize changes

Initial Comment:
(Using 0.9 Windows build of SV)

When the hop ratio of a spectrogram is increased, 
the time axis goes out of alignment as demonstrated 
in attached image #1.

In the attached example, the spectrogram starts at the
default hop ratio of 1/2, then is changed to 1/8.
When the hop ratio is changed to 1/2 again, the time
axis is stretched in the spectrogram display.

When the hop ratio is change to 3/4 the spectrogram
time axis remains stretched, but when the hop
ratio is set back to 1/2, the spectrogram has the 
correct time axis scaling.

In general, going from a low hop ratio to a higher
hop ratio causes the time scaling problem (except
for 1/16 to 1/8 with the attached test sound).  When
going from a higher hop ratio to a lower ratio, the 
time scaling problem does not occur, except
occassionally: when the hop ratio is sometimes 
changed from 1/1 to 3/4, the time scale error
demonstrated in attached image #2 occurs.

Also attached is the sound file used to create
the spectrogram displays in the example images.



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>Comment By: Craig Stuart Sapp (craigsapp)
Date: 2006-06-19 16:19

Message:
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Yes, it must be one of the Windows-only bugs :-).

It happened again just now on my computer 
(after rebooting and several days later).  
Here are the steps that I did before seeing 
the problem:

(1) import audio file
(2) create a new pane
(3) add a spectrogram layer in the new pane
(4) change the max freq of the display to 2kHz.
(5) zoom in so that the sound takes up most 
    of the space in the display
(6) switch to 1/4 hop ratio
(7) switch back to 1/2 hop ratio

Resulting display is shown in attached image A and
demonstrates that I am not crazy, unless I am doctoring
the images in PhotoShop :-)  This display shows 
an interesting error at the central cursor
which I haven't seen the other times that I have 
generated the problem, so don't know if it is a 
coincidence.

I also tried this, minimizing the number of steps
to generate the error:

(1) import audio file
(2) add spectrogram to current pane
(3) change hop ratio to 1/4
(4) change hop ratio back to 1/2

The error in this case is shown in attached image B.  
The error remains if I zoom in (attached image C), 
or click on it, etc.  So repainting the display 
is not fixing it.  Also, changing the max frequency
does not remove the error.  Or switching between
log and linear scales.  Or switching to "Peak Bins"
display.

Changing the windowing method (i.e., doing the transform
again) yeilded interesting results (see image D).  
Switching between windows only sometimes is showing 
the error (at least when it gets into this funny state).


I never noticed it before, but then I have usually 
gone from a large step size to a smaller step size, which
doesn't generate the problem.


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Comment By: Chris Cannam (cannam)
Date: 2006-06-19 07:24

Message:
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user_id=13489

Hm, I can't reproduce this.  Perhaps it only happens on
Windows.  I'm running a debug build at the moment, I'll try
again with an optimised one.


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