old_contents: # Split the audio channel from a file into its channels
  # Keeps video and subtitles tracks untouched
  import json
  import subprocess
  import sys
  
  if len(sys.argv) < 3:
  	sys.exit(1, "USAGE: python3 %s inputfile outputfile")
  _, infile, outfile, *_ = sys.argv
  
  # Determine the channel layout of the input file
  p = subprocess.run(["ffprobe", "-print_format", "json", "-show_streams", "-v", "quiet", infile],
  	check=True, capture_output=True)
  info = json.loads(p.stdout)
  if not isinstance(info, dict): raise Exception("shouldn't happen - non-dict output from ffprobe")
  map_video = map_subs = []
  audio = None
  # Scan streams. If we have any video streams, make sure we grab them, ditto subs.
  for strm in info["streams"]:
  	if strm["codec_type"] == "video": map_video = ["-map", "0:v"] # If there are multiple, these still only need one map argument
  	elif strm["codec_type"] == "subtitle": map_subs = ["-map", "0:s"]
  	elif audio is None: audio = strm
  if audio is None: sys.exit(1, "No audio stream found")
  
  # Good. There's an audio stream. (We'll use the first if there are multiple.)
  # Ask ffmpeg what channel layouts there are and how the channels should be described.
  p = subprocess.run(["ffmpeg", "-layouts"], check=True, capture_output=True)
  channel_desc, layouts = {}, {}
  cur = None
  for line in p.stdout.decode().split("\n"):
  	if line == "": pass
  	elif line == "Individual channels:": cur = channel_desc
  	elif line == "Standard channel layouts:": cur = layouts
  	else:
  		k, v = line.split(maxsplit=1)
  		cur[k] = v
  
  # Build up a filter_complex that will split the channels out.
  # Also, give each channel a descriptive title eg "[FR] front right".
  filter = "channelsplit=channel_layout=" + audio["channel_layout"]
  map_audio = []
  for i, channel in enumerate(layouts[audio["channel_layout"]].split("+")):
  	filter += f"[{channel}]"
  	map_audio.extend(["-map", f"[{channel}]", "-metadata:s:a:%d" % i, f"title=[{channel}] {channel_desc[channel]}"])
  
  # Let's do this.
  args = (["ffmpeg", "-y", "-i", infile, "-filter_complex", filter]
  	+ map_video + map_audio + map_subs
  	+ ["-c:v", "copy", "-c:s", "copy", "-loglevel", "error", "-stats", outfile])
  # print(args)
  sys.exit(subprocess.run(args).returncode)
  
new_contents: # Split the audio channel from a file into its channels
  # Keeps video and subtitles tracks untouched
  import json
  import subprocess
  import sys
  
  if len(sys.argv) < 3:
  	sys.exit(1, "USAGE: python3 %s inputfile outputfile")
  _, infile, outfile, *_ = sys.argv
  
  # Determine the channel layout of the input file
  p = subprocess.run(["ffprobe", "-print_format", "json", "-show_streams", "-v", "quiet", infile],
  	check=True, capture_output=True)
  info = json.loads(p.stdout)
  if not isinstance(info, dict): raise Exception("shouldn't happen - non-dict output from ffprobe")
  map_video = map_subs = []
  audio = None
  # Scan streams. If we have any video streams, make sure we grab them, ditto subs.
  for strm in info["streams"]:
  	if strm["codec_type"] == "video": map_video = ["-map", "0:v"] # If there are multiple, these still only need one map argument
  	elif strm["codec_type"] == "subtitle": map_subs = ["-map", "0:s"]
  	elif audio is None: audio = strm
  if audio is None: sys.exit(1, "No audio stream found")
  
  # Good. There's an audio stream. (We'll use the first if there are multiple.)
  # Ask ffmpeg what channel layouts there are and how the channels should be described.
  p = subprocess.run(["ffmpeg", "-layouts"], check=True, capture_output=True)
  channel_desc, layouts = {}, {}
  cur = None
  for line in p.stdout.decode().split("\n"):
  	if line == "": pass
  	elif line == "Individual channels:": cur = channel_desc
  	elif line == "Standard channel layouts:": cur = layouts
  	else:
  		k, v = line.split(maxsplit=1)
  		cur[k] = v
  
  # Give each channel a descriptive title eg "[FR] front right".
  audio_titles = []
  for i, channel in enumerate(layouts[audio["channel_layout"]].split("+")):
  	audio_titles.extend(["-metadata:s:a:%d" % i, f"title=[{channel}] {channel_desc[channel]}"])
  
  # Let's do this.
  args = (["ffmpeg", "-y", "-i", infile, "-filter_complex", "channelsplit=channel_layout=" + audio["channel_layout"]]
  	+ map_video + map_subs + audio_titles
  	+ ["-c:v", "copy", "-c:s", "copy", "-loglevel", "error", "-stats", outfile])
  # print(args)
  sys.exit(subprocess.run(args).returncode)
  
subject: Simplify filtergraph creation