[Audacity-quality] Issues around Equalization in Graphic EQ mode

[Gale A. <ga...@au...>]

In looking at Bug 627 - "Equalization settings are modified when 
switching between views":
http://bugzilla.audacityteam.org/show_bug.cgi?id=627 

which surfaced somewhere in 2.0.3 alpha, I've found other problems
with Graphic EQ that are also repeatable in 2.0.0 (and probably before).

Both for bug 627 and what I'm reporting below, Graphic EQ Mode 
seems to be misinterpreting the data once it gets "applied", 
though the XML data being written seems correct.  

It also occurs to me these may be undocumented limitations in 
Graphic EQ mode, so here goes anyway.  The following reproduce 
in 2.0.0 or HEAD. 

1 In Draw Curves Mode, "Flatten".
2 Drag the whole bar down to -30 dB. 
3 Save/Manage Curves, Rename "Unnamed", Export that 
   curve. 
4 Quit Audacity, initialise audacity.cfg and delete the EQ* curves 
   in Audacity's folder for application data.
5 Launch Audacity. Generate a tone, amplitude 1.0.
6 Open EQ. Switch to Graphic Mode, Flatten, OK.
7 Reopen EQ. Save/Manage Curves, import your -30 dB curve, 
   select it in the list, OK. 
8 Curve looks fine on the graph. The -30 dB setting does not show 
   on the sliders
   (I don't know if this is part of the same "problem", but for me
    that's "wrong"). Anyway, just click OK.
9 Check the tone in Effect > Amplify. Amplification (dB)  is correct. 
10 Undo the EQ, open EQ dialogue. The curve is corrupted, and 
     applying the EQ does not deamplify the tone to -30 dB.

If instead at step 7 I do not select the -30 dB curve, the first curve in 
the list gets selected (another issue). So if I just click OK I get the 
"AM Radio" curve loaded. If I now click on "Select Curve" and choose 
my -30 dB curve, the curve is corrupted. 

Now, is this issue that Graphic EQ cannot cope with levels in excess
of +/- 20 dB (as per the graphic slider range), or is it something else?  

Consider the attached "15.xml" file. It has just two points and if you 
import it as per step 7 above (selecting it in the list), it looks in
the EQ graph just like it did when I created it in Draw Curves Mode -
straight lines.

If I now apply the "15" EQ then reopen the EQ dialogue, the curve is now
something much more complex than "straight lines". Is what I see after
applying the curve and reopening it "correct" - essentially because a 
curve that looks like "straight lines between points" in Draw Curves
must be represented differently in Graphic EQ mode? If so, why don't 
we show its complex representation when importing it in Graphic EQ
mode?  

Finally, when you drag the Graphic EQ sliders up or down to the limit of
the range (+/- 20 dB), this only seems to apply +/- 15 dB gain in the
EQ graph, whichever Mode you look in. Why is that? 



Thanks


Gale