MPV script that speeds up playback to maintain synchronization with live streams

keepsync.lua

-- keepsync.lua
--
-- Attempts to maintain initial offset of the live stream in case of
-- network or hardware lags by slightly speeding up playback until
-- synchronization is restored.
-- 
-- Initial offset is calculated right at the start of playback. If playback
-- immediately starts to lag, you can increase offset by pressing Shift+LEFT.
-- If your connection to stream source is good enough, you can also try to
-- decrease offset by pressing Shift+RIGHT. Both hotkeys are changing offset
-- by 1 second.
--
-- This script was tested only with streamlink, both with and without the low
-- latency patch.

local initial_offset
local offset
local sync_timer
local synced

local function get_offset()
  return mp.get_time() - mp.get_property("time-pos")
end

local function get_offset_delta()
  return get_offset() - offset
end

local function keep_sync()
  delta = get_offset_delta()

  if delta > 0.1 and synced then
    mp.msg.info("Speeding up playback")
    synced = false
    mp.set_property_number("speed", 1.2)
  elseif delta < -0.1 and synced then
    mp.msg.info("Slowing down playback")
    synced = false
    mp.set_property_number("speed", 0.8)
  elseif delta < 0.1 and delta > -0.1 and not synced then
    mp.msg.info("Synced successfully")
    synced = true
    mp.set_property_number("speed", 1)
  end
end

local function pause(name, paused)
  if paused then
    sync_timer:stop()
  else
    sync_timer:resume()
  end
end

local function seek_back(name)
  offset = offset + 1
  synced = true
  mp.msg.info(string.format(
    "Increasing offset. New value: %f (%d)", offset, offset - initial_offset
  ))
end

local function seek_forward(name)
  offset = offset - 1
  synced = true
  mp.msg.info(string.format(
    "Decreasing offset. New value: %f (%d)", offset, offset - initial_offset
  ))
end

local function init()
  if mp.get_property_bool("seekable", true) then
    return
  end

  initial_offset = get_offset()
  offset = initial_offset
  synced = true
  sync_timer = mp.add_periodic_timer(0.5, keep_sync)
  mp.observe_property("pause", "bool", pause)
  mp.msg.info(string.format("Initialization complete. Offset: %f", offset))

  mp.add_key_binding("Shift+LEFT", seek_back)
  mp.add_key_binding("Shift+RIGHT", seek_forward)
end

mp.register_event("file-loaded", init)