Which 'performances'? We can review and tell you why.

'FM Toy Piano' is a one part FM performance and sets the sliders to control operator level rather than the one part. 

Thanks for the reply...

On my MODX on 'FM Toy Piano', control slider 1 is set to Part1 volume, and the others to Part2, 3 and 4 volume (even though there's only one Part).

Anyway, I found a Preset that displays the behavior I'm talking about. It's 'Early 80s DiscoPop'. Sliders 1 and 4 control Part1 and 4 volume, but the volume for Part2 and 3 are fixed at 76 and 0, and the sliders 2 and 3 don't seem to do anything. I'm just trying to understand how they are disabled and how to enable them.

Thanks for the reply...

On my MODX on 'FM Toy Piano', control slider 1 is set to Part1 volume, and the others to Part2, 3 and 4 volume (even though there's only one Part).

Anyway, I found a Preset that displays the behavior I'm talking about. It's 'Early 80s DiscoPop'. Sliders 1 and 4 control Part1 and 4 volume, but the volume for Part2 and 3 are fixed at 76 and 0, and the sliders 2 and 3 don't seem to do anything. I'm just trying to understand how they are disabled and how to enable them.

Thanks for the reply...

The only preset I can find that displays this behaviour is 'Early 80s DiscoPop'. Control sliders 1 and 4 control the volume of Parts 1 and 4 (as you would expect), but the volume of Parts 2 and 3 is fixed at 76 and 0, and moving sliders 2 and 3 does nothing. I can't find how sliders 2 and 3 are disabled, or how to enable them.

Thanks for the reply...

The only preset I can find that displays this behaviour is 'Early 80s DiscoPop'. Control sliders 1 and 4 control the volume of Parts 1 and 4 (as you would expect), but the volume of Parts 2 and 3 is fixed at 76 and 0, and moving sliders 2 and 3 does nothing. I can't find how sliders 2 and 3 are disabled, or how to enable them.

I've worked it out. On the 'Receive SW' page, the Vol/Exp switch was turned OFF for these parts!

Yep, item 2 in my response.

Sometimes users turn off Vol/Exp hoping this only will impact the expression pedal and are surprised to see the slider LEDs go dark.   But this setting impacts the sliders and expression pedal equally for a given Part. 

. . . so I'm not sure why it's even configurable

The only preset I can find that displays this behaviour is 'Early 80s DiscoPop'. Control sliders 1 and 4 control the volume of Parts 1 and 4 (as you would expect), but the volume of Parts 2 and 3 is fixed at 76 and 0, and moving sliders 2 and 3 does nothing. I can't find how sliders 2 and 3 are disabled, or how to enable them.

. . .

I've worked it out. On the 'Receive SW' page, the Vol/Exp switch was turned OFF for these parts!

It is configurable so users have more control over when/if/how a slider is used.

That switch is the one Jason mentioned in his reply to your other thread.

On my M8X the Sustain switch is also set to OFF for that 'Early 80s DiscoPop' performance. That allows a user to use the sustain pedal for other parts but prevent the pedal from affecting parts whose switch is OFF.

Sometimes a user wants to control certain parts as a 'group' and change the volume for the entire group of parts without affecting other parts. Those switches help do that.

CAVEAT - the behaviour of that 'Early 80s DiscoPop' performance is a prime example of the type of issue that MANY of the presets can present. Presets are mostly intended to serve as examples of what CAN be done and not just as 'canned' performances ready to use as-is.

It is important to thoroughly examine ALL areas of a preset to become familiar with everything it does and how it does it. 

Why did the sound designer lock part 3 at a volume of zero? They didn't. If you examine the control assignments for part 3 you will find that 'AsgnKnob 2' is controlling volume. Now look at the 'Standard' curve that is being used.

A curve is used to compute a positive or negative 'offset' to a source parameter. On the part 3 screen you will set that the starting level for volume is zero. So offsets will get applied to that value.

Curve

1. X axis (left to right) represents the source value from Min (left) to Max (right) - typically 0-127. Notice that the leftmost end of the curve is at 'zero' - so an initial volume value of 0 will cause the curve to compute an offset of 0 for a result of 0 volume.

2. Y axis (bottom to top) represents the computed 'offset' value from Min (bottom) to Max (top) - typically 0-127 for 'Uni' Polarity. Change polarity to 'Bi' and watch how the curve changes. That 'Bi' lets you apply both positive AND negative offsets to a parameter. Since the starting level for part 3 volume was set to 0 you can, obviously, only apply positive offsets. Change that start volume to non-zero value (e.g. 64) and polarity to 'Bi' and you can now use a knob to both increase and decrease the value from that starting point.

A. part 3 volume default is 0

B. part 3 knob 2 is assigned to control volume using a standard curve with 'Uni' polarity

C. the SuperKnob control settings show that it is assigned to control part 3 knob 2 and thus control part 3 volume

D. turn the super knob and you are changing part 3 volume by applying an offset to the source volume

BUT WAIT! There's more!

I've worked it out. On the 'Receive SW' page, the Vol/Exp switch was turned OFF for these parts!

That OFF setting prevents the user from accidentally changing the default 'source' volume from 0 and interfering with the super knob control.

If you set that switch to ON and move the slider it will change the baseline volume level. Then the superknob (or part 3 knob 2) will apply offsets to that new value.

Set the switch to ON and the slider to MAX and the super knob (part 3 knob 2) can no longer affect the volume since it is already at MAX.

Unless you change polarity to 'Bi' then the super knob (part 3 knob 2) can decrease the volume.

BUT WAIT! There's more!

Did you notice that part 3 has a note range of C-2 to B2? That means that part won't sound at all for C3 (middle C) or above.

Did you notice that part 3 has 8 elements? Each elements 1-4 is assigned to a different group (2-5) and use 'Normal' XA control. And elements 5-8 use 'Legato' XA control and are assigned groups to pair them up with the 1st four elements; that is elements 1 and 5 are group 2, 2 and 6 group 3 and so on. 

So elements 1 and 5 work together when you play legato. Normally only element 1 sounds but if you press another key while still holding down the 1st key that second key will use element 5 for legato.

I mention all of the above to drive home just how much can be going on 'under the hood' in just ONE part of a preset. Multiply that by 8 parts and include all of the other things that might be happening (effects, arpeggios, motion control sequences) and you can understand why you can't always use an 'off the shelf' preset without unexpected things happening when you move sliders or knobs.

Hope that helps.

Thanks to Jason and Toby for your answers. Shame I can only see some of them in 'Topic overview'... what's with this forum at the moment?

Forum has been broken for months. No word on when, or if, it will be fixed or replaced.

It does seem weird to me that turning off expression pedal control for a part also disables that part's slider. I mean, it's obvious to me why you might want to disable the volume pedal on some parts, if you want the pedal to control some parts but not others (e.g. to bring in strings uner a piano)... since the pedal can do multiple things (control multiple parts) simultaneously, it's valuable to be able be more selective about which things you do and do not want it to do. But the slider has no multi-part functionality. What is the value in stopping it from doing anything? Is there some other function it can perform that I'm not thinking of? Otherwise, my guess would be that it had to work that way due to some underlying board architecture (i.e. it was due to an engineering consequence rather than having been designed that way to be musically useful).

https://yamahasynth.com/community/montage-series-synthesizers/how-to-turn-off-foot-controller-1-to-designated-parts/

Yeah, said something similar in 2017.  The message thread also has a "schematic" of how I pictured the various on/off features functioning with respect to sliders and the expression pedal.