add volume up and volume down option

[ryan00davis]

Any chance you could add an option to run as minidsp.exe -vol-up ?
That way, i could link it to a keyboard shortcut or a dial of some kind to step the volume up and down (either by the default amount, or by a set db step). Having to set the specific volume level makes it not easy to work with incrementally.

[mrene]

That can be done, but only for the master volume, since it's not possible to read individual gain values for inputs and outputs. (Added #230 to track this)

[ryan00davis]

That would solve all of my problems, master volume is the only one I'd care to adjust this way. Thank you.

[mrene]

I added a relative option to the gain subcommand. If you want to increase by 5dB you can do minidsp gain -r 5 or minidsp gain --relative 5.

To decrease, you have to use -- to separate the options from the values. minidsp gain -r -- -5