The issues is not just occured in the editor, but also in the real playing situation. just like what all the good guys in this thread says, The volume pedal only opens about 30 / 40% when half pedal engaged.... but it quickly turns from 40 to 100 in a small forward pedal movement/ travel. the "logarithmic" theory make sense, because the expresion pedal seems behave like a volume/gain pedal on your guitar....... the issues is more obvious if we see it at the calibration page.... when the value of the pedal move from 0 value to 127 value.  btw tx for the quick response miller.. hope this issues could be resolved. btw, musicalcircuitry explanation is well explained the whole issue.

Thx for the quick respone, mr.miller. with ur expalation, maybe i could conclude that the POD HD expression pedal is "actually" design to works logarichmically, so there are no error on our unit. (correct me if im wrong:). But i still think that it was a faulty design, cause minimal pedal travel length is not what lots of user prefer. We like to have a lot of options to control our pods. Personally , i like a linear, long pedal travel range , just like an ordinary volume pedal/ dunlop wah wah,   If i could make a suggestion, in the next update, is it possible to include a choice of expresion pedal behaviour setting, so users could choose linear, logarithmic or anti-log seting. i think zoom already does that in their G.2/7/9u unit. btw, i dont now how exactly i could reach line6 for feedback.....is it in another forum/ via email? thx

Since this discussion seems to have gotten a bit confused, here is a very simple way to illustrate the problem:

1. select a "New Tone" and select an amp
2. double press the move button while the amp is selected to enter expression pedal assignment
3. change the parameter to "Output" (ie. channel volume), the controller to be EXP 1, min value to 0% and max value to 100%
4. press view to get back to the signal chain view, then double press the enter button to get into the AMP:EDIT screen
5. press the expression pedal all the way forwards (toe position)
6. turn the physical volume knob (amp volume, not master) to its maximum setting, then look at the AMP:EDIT view, it should reflect the physical setting and show 100%
7. play your guitar a bit and note how loud it seems
8. turn the physical volume knob to its halfway point, then look at the AMP:EDIT view, it should reflect the physical setting and show 50%
9. play your guitar a bit and note how loud it seems - it should seem to be about half of the full volume
10. return the physical volume knob to its maximum setting
11. adjust the expression pedal so that it's at about half of its range of motion, then look at the AMP:EDIT view, it will read somewhere around 15% - 25%
12. play your guitar a bit and note how loud it seems - it will sound significantly less than half of the full volume

The question then is clear - why is it that the volume knob behaves the way I expect, but the expression pedal does not? They are both physical controls that can be used to adjust volume - shouldn't setting each of them to equivalent points of their physical range have the same effect?

I understand the point of the audio / logarithmic taper -  our ears don't respond to volume changes linearly, and the audio taper  accounts  for that. To me, it seems like the scale of values for volume is already accounting for this logarithmic nature. The volume knob has a linear taper, so that at 50% of its range, the volume level reads 50%, which has already been adjusted for the logarithmic volume issues. On the other hand, the expression pedal uses a logarithmic taper, so that in effect the logarithmic adjustment is happening twice.

For a real world example of why this is a problem, say you are playing a song, and want to back off a little bit on the volume. If you have the expression pedal controlling volume, the immediate suggestion would be to ease off a bit on the expression pedal. Unfortunately, with the current taper, even a small movement could drop you to 50 - 60% volume, which is a drastic change.

I contacted Line 6 by phone earlier to ask about this. The guy I spoke to told me that my expression pedal is working as intended, and that there is nothing wrong with my HD500. This might not be affecting all users, but all HD500s should be reacting the same way. He told me that all Line 6 expression pedals have worked this way, and that people haven't seemed to mind. Taking all of that into account along with the lack of communication from Line 6 about firmware updates, it doesn't make sense to wait for a fix.

Is my real world example above a bizarre scenario that most guitarists don't care about? Am I approaching this the wrong way?

Does anyone have any good ideas for working around this? One thought is to have two volume pedal slots in your signal chain, one with min at 0% and max at 100%, and the other with min at say 75% and max at 100%, then switch between the two. There are obvious downsides to this though - first off you're wasting yet another FX slot on a volume pedal, then you either need to use a footswitch to change between them or give up the ability to switch to wah with the toe switch. Even if this works, it seems quite kludgy and not at all user friendly.

Any thoughts or advice would be appreciated!