Update with a zero-transcoding example for AAC input files.

README.md

@@ -36,6 +36,12 @@ You can find all artifacts in the "out" directory:
 
  ls out
 
+If you know you're operating on an input file that's 44.1KHz, you can also split up the file without transcoding. Try a command like this to test it out:
+
+ NO_TRANSCODE=1 ruby run.rb 1.0 <your-file-with-an-aac-input-stream>
+
 On a Mac, you may want to examine out/stitched.mp4 with a visualization program. The author uses [Audacity](https://www.audacityteam.org/) for that.
 
  open -a Audacity out/stitched.mp4
+
+NOTE: This repo hardcodes the sample rate as 44.1KHz to simplify the demo code. But this method should work with any sample rate.

calculations.rb

@@ -75,7 +75,19 @@ def generate_command_and_directives_for_segment(input_file, index, target_start,
 
  puts "inpoint: #{inpoint}, outpoint: #{outpoint}"
 
- command = "ffmpeg -hide_banner -loglevel error -nostats -y -ss #{start_time_with_padding}us -t #{padded_duration}us -i #{input_file} -c:a libfdk_aac -ar 44100 -f adts out/seg#{index + 1}.aac"
+ command =
+ if ENV["NO_TRANSCODE"]
+ # If we know the input file is AAC and we're not changing the sample rate,
+ # we can create the segments without transcoding too. This works because,
+ # if we cut at exactly the AAC frame boundaries, then we can just slice
+ # out portions of the stream. Note, however, that -ss and -t flags are moved after
+ # the input file so they're applied after the input file is read. Without that,
+ # you'll get some funky output.
+ "ffmpeg -hide_banner -loglevel error -nostats -y -i #{input_file} -c:a copy -ss #{start_time_with_padding}us -t #{padded_duration}us -f adts out/seg#{index + 1}.aac"
+ else
+ "ffmpeg -hide_banner -loglevel error -nostats -y -ss #{start_time_with_padding}us -t #{padded_duration}us -i #{input_file} -c:a libfdk_aac -ar 44100 -f adts out/seg#{index + 1}.aac"
+ end
+
  directives = [
  "file 'seg#{index + 1}.aac'",
  "inpoint #{inpoint}us",