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No More Defaults: Auto-Selecting Japanese Subtitles & Stereo, with help of AI

One of the ways I study Japanese is the immersion method. What it basically is that you consume a content in target language without any aid, and try understanding it. It may sound ridiculous but thats literally how I, along with many people I know, learned English in the first place! The easiest way to do it is watching anime with Japanese subtitles, or no subtitles at all. The problem is though, as far as I know, MPV does not provide a way to do this at the time of writing. I can only nudge it to give me Japanese subtitles if they're available, else it just defaults to whatever subtitle there is. While trying to troubleshoot this with ChatGPT, it offered me to write a plugin for MPV to handle this, I figured why not. It gave me a short script and told me to put it in my mpv config, so I did. And to my surprise, it actually worked!

~/.config/mpv/scripts/subtitle.lua


local function choose_audio_and_subs()
    local tracks = mp.get_property_native("track-list") or {}

    local sid = nil
    for _, t in ipairs(tracks) do
        if t.type == "sub" and t.lang == "ja" then
            sid = t.id
            break
        end
    end
    if sid then
        mp.msg.info("select-tracks: selecting JA subtitle #"..sid)
        mp.set_property_number("sid", sid)
    else
        mp.msg.info("select-tracks: no JA subs, disabling")
        mp.set_property("sub", "no")
    end
end

mp.register_event("file-loaded", function()
        local tracks = mp.get_property_native("track-list") or {}
        for _, t in ipairs(tracks) do
            if t.type == "sub" and t.lang == "ja" then
                mp.set_property_number("sid", t.id)
                return
            end
        end
        mp.set_property("sub", "no")
end)

After seeing the potential, I figured I should also ask one of the pet peeves I have; I do not have and do not plan to have any surround setup. I am strictly stereo only. I want stereo audio. But the thing is, video files often have several audio channel tracks for a given language, and they almost always default to the one with most channels. This causes the media player to downmix the audio, resulting in a subpar experience compared to the stereo track. I can't hear them people speaking! So I asked ChatGPT about it, and after some trial and error, it also managed to do it, incredible!

~/.config/mpv/scripts/stereo.lua


-- mpv script: if the selected audio track for a given language is multichannel, switch to its stereo counterpart with detailed logging
local mp = require("mp")

-- Get the native track list
local function get_tracks()
    return mp.get_property_native("track-list") or {}
end

-- Find an audio track by id
local function find_audio_track_by_id(id)
    for _, t in ipairs(get_tracks()) do
        if t.id == id and t.type == "audio" then
            return t
        end
    end
    return nil
end

-- Attempt to switch to stereo for the same language
local function switch_to_stereo_if_needed()
    local tracks = get_tracks()

    -- Get current audio track id
    local aid_prop = mp.get_property("aid")
    local current_id = tonumber(aid_prop)
    if not current_id then
        return
    end

    local sel = find_audio_track_by_id(current_id)
    if not sel then
        return
    end

    local ch = sel["audio-channels"] or 0

    -- Only act on multichannel
    if ch <= 2 then
        return
    end

    -- Find stereo counterpart
    for _, t in ipairs(tracks) do
        local tchan = t["audio-channels"] or 0
        if t.type == "audio"
        and t.lang == sel.lang
        and tchan == 2
        and t.id ~= current_id then
            mp.commandv("set", "aid", tostring(t.id))
            return
        end
    end
end

-- Hook into MPV events
mp.observe_property("aid", "string", function(name, value)
    switch_to_stereo_if_needed()
end)
mp.register_event("file-loaded", switch_to_stereo_if_needed)

You can also see it in action when I launch MPV from command line.

Switched to stereo audio and japanese subtitle:

Switched to stereo audio and disabled subtitles:

The takeaway is that the AI helped me avoid digging through MPV documentation to write these simple scripts, so I'm grateful for that. It feels like a slippery slope though, as if you depend on it too much, you will be at a loss when it doesn't work. My embedded software developer friend abused Claude for a week and got rate limited, now he's struggling to write any code.