Volume - expression?? They are the same thing.
Technically yes, but one cant be substituted for the other.
I will try to explain.
[-A-]
A Compatible expression pedal say for a PSR, would have one port for the connection.
In essence, ONE cable runs from the PSR to the Pedal.
[-B-]
The Volume pedal that the shopkeepers show you will have one Input and One output (in some case two inputs and two outputs)
How does this connect ?
A cable runs from your Keyboard's AUX OUT (L/R Mono or never mind) into your pedal.
A second Cable runs from your Pedal into an External Amplifier
So you are basically controlling your entire output volume from your keyboard to your external speakers.
A pedal like this wont work as an expression pedal.
As an Expression pedal configuration [-A-] explained above,
- I can configure my keyboard to say control volume or any set of assignable parameters.
- I can tweak what part to change, E.g. as to control volume, I can set it to change only my R1 volume, while my style / accompaniment and everything else remain unaffected.
OTOH the [-B-] configuration (Volume pedal that they show me) I control the volume of the entire instrument via its output.
On a side note.
Korg EXP has both the functionality.
One Input and Two outputs.
I run my cable from the PSR into the Out1 and this functions as an expression pedal.