Runs a child process to play a second audio stream. Synced through named pipe. Recommend using with [cycle-adevice](https://gist.github.com/bitingsock/ad58ee5da560ecb922fa4a867ac0ecfd) to change the output device of the parent process.

audio-dupe.lua

--New child with start with the current 'aid' and 'audio-device' of the parent
-- non-Windows environments require the use of the 'socat' package
local platform_is_windows = (package.config:sub(1, 1) == "\\")
local options = require 'mp.options'
local o = {
    pipe_template = platform_is_windows and "\\\\.\\pipe\\mpvDupedAudio" or "~/mpvDupedAudio",      --windows format
    new_child_key = 'Ctrl+A',                           --Start a new child: 'Ctrl+Shift+a'
    cycle_child_control_key = 'Ctrl+Alt+A',             --Cycle child control: 'Ctrl+Shift+Alt+a'
    cycle_child_aid_key = 'Ctrl+Alt+a',                 --Cycle 'aid' of child: 'Ctrl+Alt+a'
    child_increase_volume_key = 'Ctrl+Alt+WHEEL_UP',     --Change volume of child: 'Ctrl+Alt+wheel'
    child_decrease_volume_key = 'Ctrl+Alt+WHEEL_DOWN',     --Change volume of child: 'Ctrl+Alt+wheel'
}
options.read_options(o)

local piper = platform_is_windows and " >" or " | socat - "
local mpvexe = "mpv" .. (platform_is_windows and ".com" or "")
local pipeInstance = {}
local control = 0

local function sync(prop, newVal)
    local timestamp = mp.get_property("audio-pts") or mp.get_property("time-pos")
    for k, v in pairs(pipeInstance) do
        if v then
            os.execute("echo no-osd set time-pos " .. timestamp .. piper .. o.pipe_template .. k)
            if prop == "speed" then os.execute("echo no-osd set speed " .. newVal .. piper .. o.pipe_template .. k) end
            if prop == "pause" then os.execute("echo no-osd set pause " .. newVal .. piper .. o.pipe_template .. k) end
        end
    end
end

local function cycle_aid()
    os.execute("echo cycle aid" .. piper .. o.pipe_template .. control)
end

local function switchControl()
    local lastInstance = 0
    for k, v in ipairs(pipeInstance) do
        if v then
            lastInstance = k
        end
    end
    if lastInstance == 0 then
        control = 0
        return
    end
    if control >= lastInstance then control = 0 end
    for k, v in ipairs(pipeInstance) do
        if k > control and v then
            control = k
            return
        end
    end
end

local controlling = false
local function quit()
    os.execute("echo quit" .. piper .. o.pipe_template .. control)
    pipeInstance[control] = false
    switchControl()
    print(control)
    if control == 0 then
        print(">>> Removed last child <<<")
        mp.remove_key_binding("quit")
        mp.remove_key_binding("cycle_aid")
        mp.remove_key_binding("volumeUP")
        mp.remove_key_binding("volumeDOWN")
        mp.unobserve_property(cycle_pause)
        mp.unobserve_property(syncSpeed)
        mp.unregister_event(sync)
        controlling = false
    else
        print(">>> Now controlling child: " .. control .. " <<<")
    end
end

local function start_child()
    table.insert(pipeInstance, true)
    control = #pipeInstance
    print(">>> Now controlling child: " .. control .. " <<<")
    mp.commandv("set", "msg-level", mp.get_script_name() .. "=warn")
    if controlling == false then
        mp.register_event("playback-restart", sync)
        mp.add_forced_key_binding("q", "quit", quit)
        mp.add_forced_key_binding(o.cycle_child_aid_key, "cycle_aid", cycle_aid)
        mp.add_forced_key_binding(o.child_increase_volume_key, "volumeUP", function() os.execute("echo add volume 5" .. piper .. o.pipe_template .. control) end)
        mp.add_forced_key_binding(o.child_decrease_volume_key, "volumeDOWN", function() os.execute("echo add volume -5" .. piper .. o.pipe_template .. control) end)
        mp.add_timeout(0.25, function()
            mp.observe_property("pause", "string", sync)
            mp.observe_property("speed", "string", sync)
        end)
        controlling = true
    end
    mp.command_native_async({
        name = "subprocess",
        args = {
            mpvexe,
            mp.get_property("path"),
            "--speed=" .. mp.get_property("speed"),
            "--pause=" .. mp.get_property("pause"),
            "--no-video",
            "--no-sub",
            "--osd-level=1",
            "--aid=" .. mp.get_property("aid"),
            "--audio-device=" .. mp.get_property("audio-device"),
            "--input-ipc-server=" .. o.pipe_template .. control,
            "--start=" .. (mp.get_property("audio-pts") or mp.get_property("time-pos"))
        },
        playback_only = false
    }, function() end)
    mp.add_timeout(0.25, function()
        mp.commandv("set", "msg-level", mp.get_script_name() .. "=status")
    end)
end

mp.add_forced_key_binding(o.cycle_child_control_key, "switchControl", function()
    switchControl()
    if control == 0 then
        print(">>> No active children <<<")
    else
        print(">>> Now controlling child: " .. control .. " <<<")
    end
end)

mp.add_forced_key_binding(o.new_child_key, "start", start_child)

mp.register_event("shutdown", function()
    for k, _ in pairs(pipeInstance) do
        os.execute("echo quit" .. piper .. o.pipe_template .. k)
    end
end)

I am on windows and do not exhibit that behavior. IPC is the most direct way to control other processes. there surely is a different windows specific way to modify wasapi apps volume but I would prefer to address the real problem. Do you use a third party terminal emulator or something? Can you show a -v log with these "flickers"? Arbitrary calls to the shell should not create a console window, normally.

I have this problem, constant flickering of a cmd window on all commands passing through.
(Windows 10)
But more importantly i have something like a 3 second lag between the primary and secondary processes... I can't figure this out for the life of me. I even tried controlling the timing offset of the secondary process but it's not moving around (plus no matter what, i dont think a negative offset would even compute, if the secondary one is a dependant stream to the primary, right? IDK tbh.)

Oh and natural Q for Quit just flickers harder instead of quitting anything.

If i hardcode delay by adding
"--audio-delay=-3.6"
at line 107

it syncs perfectly at launch, but will go back to desync if i ever pause or skip.
Maybe useful to reveal that all mpv things like runtime keybinds to change delays, volume change, etc, are allowed from within the child process window, could help someone figure out that they can tune things on the fly... But ofc it is a less than ideal solution, just a workaround. (uuhh wait, this SEEMS to work, it shows my delay is correct at launch, but after a pause/skip, tweaking the delay actually doesn't change anything other than the HUD/UI values. Sigh sorry for the messy comment.)

As for the flickering issues (which I haven't been able to reproduce) you can try specifying the command to spawn in a minimized window to see if it helps. as a test, try replacing line 85:
mp.add_forced_key_binding(o.child_increase_volume_key, "volumeUP", function() os.execute("echo add volume 5" .. piper .. o.pipe_template .. control) end)
with
mp.add_forced_key_binding(o.child_increase_volume_key, "volumeUP", function() os.execute('START /MIN CMD.EXE /C "echo add volume 5' .. piper .. o.pipe_template .. control .. '"') end)
then see if volume up (default: Ctrl+Alt+WHEEL_UP) produces the issue.

---

as for the delay issue, I can only assume your systems are under heavy load. I ran some stressed scenarios and it was a bit out of step but nowhere near 3 seconds. Unfortunately, the only real fix without dependencies would only result in 1 second latency at best as lua's os.time() only produces seconds.

Another possibility is Windows Defender (or some other AV) is analyzing system calls from mpv and delaying it.