mpv/TOOLS/lua/drc-control.lua

-- This script enables live control of the dynamic range compression
-- (drc) audio filter while the video is playing back. This can be
-- useful to avoid having to stop and restart mpv to adjust filter
-- parameters. See the entry for "drc" under the "AUDIO FILTERS"
-- section of the man page for a complete description of the filter.
--
-- This script registers the key-binding "\" to toggle the filter between 
--
-- * off
-- * method=1 (single-sample smoothing)
-- * method=2 (multi-sample smoothing)
--
-- It registers the keybindings ctrl+9/ctrl+0 to decrease/increase the
-- target ampltiude. These keys will insert the filter at the default
-- target amplitude of 0.25 if it was not previously present.
--
-- OSD feedback of the current filter state is displayed on pressing
-- each bound key.

script_name = mp.get_script_name()

function print_state(params)
    if params then
        mp.osd_message(script_name..":\n"
                           .."method = "..params["method"].."\n"
                           .."target = "..params["target"])
    else
        mp.osd_message(script_name..":\noff")
    end
end

function get_index_of_drc(afs)
    for i,af in pairs(afs) do
        if af["label"] == script_name then
            return i
        end
    end
end

function append_drc(afs)
    afs[#afs+1] = {
        name   = "drc",
        label  = script_name,
        params = {
            method = "1",
            target = "0.25"
        }
    }
    print_state(afs[#afs]["params"])
end

function modify_or_create_af(fun)
    afs = mp.get_property_native("af")
    i = get_index_of_drc(afs)
    if not i then
        append_drc(afs)
    else
        fun(afs, i)
    end
    mp.set_property_native("af", afs)
end

function drc_toggle_method_handler()
    modify_or_create_af(
        function (afs, i)
            new_method=(afs[i]["params"]["method"]+1)%3
            if new_method == 0 then
                table.remove(afs, i)
                print_state(nil)
            else
                afs[i]["params"]["method"] = tostring((afs[i]["params"]["method"])%2+1)
                print_state(afs[i]["params"])
            end
        end
    )
end

function drc_scale_target(factor)
    modify_or_create_af(
        function (afs)
            afs[i]["params"]["target"] = tostring(afs[i]["params"]["target"]*factor)
            print_state(afs[i]["params"])
        end
    )
end

function drc_louder_handler()
    drc_scale_target(2.0)
end

function drc_quieter_handler()
    drc_scale_target(0.5)
end

-- toggle between off, method 1 and method 2 
mp.add_key_binding("\\", "drc_toggle_method", drc_toggle_method_handler)
-- increase or decrease target volume
mp.add_key_binding("ctrl+9", "drc_quieter", drc_quieter_handler)
mp.add_key_binding("ctrl+0", "drc_louder", drc_louder_handler)