Refactor and add comments.

calculations.rb

@@ -0,0 +1,61 @@
+def frame_duration
+ 1024.0 / 44100.0 * 1000000.0
+end
+
+def get_closest_aligned_time(target_time)
+ decimal_frames_to_target_time = target_time.to_f / frame_duration
+ nearest_frame_index_for_target_time = decimal_frames_to_target_time.round
+ puts "target_time: #{target_time}, decimal_frames_to_target_time: #{decimal_frames_to_target_time}, nearest_frame_index_for_target_time: #{nearest_frame_index_for_target_time}"
+ nearest_frame_index_for_target_time * frame_duration
+end
+
+def generate_command_and_directives_for_segment(index, target_start, target_end)
+ puts "--- segment #{index + 1} ---"
+
+ start_time = get_closest_aligned_time(target_start)
+ end_time = get_closest_aligned_time(target_end)
+ puts "start_time: #{start_time}, end_time: #{end_time}"
+
+ real_duration = end_time - start_time
+ puts "real_duration: #{real_duration}"
+
+ start_time_with_padding = [start_time - frame_duration * 2, 0].max
+ end_time_with_padding = end_time + frame_duration * 2
+ puts "start_time_with_padding: #{start_time_with_padding}, end_time_with_padding: #{end_time_with_padding}"
+
+ inpoint = 0
+
+ if index > 0
+ # We ask to also encode two frames before the start of our segment because
+ # the AAC format is interframe. That is, the encoding of each frame depends
+ # on the previous frame. This is also why AAC pads the start with silence.
+ # By adding some extra padding ourselves, we ensure that the "real" data we
+ # want will have been encoded as if the correct data preceded it. (Because
+ # it did!)
+ start_time_with_padding = [start_time_with_padding - frame_duration * 2, 0].max
+
+ # Although we only asked for two frames of padding, ffmpeg will add an
+ # additional 2 frames of silence at the start of the segment. When we slice out
+ # our real data with inpoint and outpoint, we'll want remove both the silence
+ # and the extra frames we asked for.
+ inpoint = frame_duration * 4
+ end
+
+ padded_duration = end_time_with_padding - start_time_with_padding
+ puts "padded_duration: #{padded_duration}"
+
+ # inpoint is inclusive and outpoint is exclusive. To avoid overlap, we subtract
+ # the duration of one frame from the outpoint.
+ outpoint = inpoint + real_duration - frame_duration
+
+ puts "inpoint: #{inpoint}, outpoint: #{outpoint}"
+
+ command = "ffmpeg -hide_banner -loglevel error -nostats -y -ss #{start_time_with_padding}us -t #{padded_duration}us -i out/#{SINE_WAVE_FILE_NAME} -c:a libfdk_aac -ar 44100 -f adts out/seg#{index + 1}.aac"
+ directives = [
+ "file 'seg#{index + 1}.aac'",
+ "inpoint #{inpoint}us",
+ "outpoint #{outpoint}us"
+ ]
+
+ [command, directives.join("\n")]
+end

run.rb

@@ -1,82 +1,40 @@
-require 'fileutils'
+require "fileutils"
+require_relative "./calculations"
 
+# Feel free to change these constants for your own testing.
 SINE_WAVE_DURATION = 10.to_f
 SINE_FREQUENCY = 1000.to_f
 TARGET_SEGMENT_DURATION = 1.0.to_f
 SINE_WAVE_FILE_NAME = "sine-wave-#{SINE_WAVE_DURATION.to_i}-seconds.wav"
 
-def frame_duration
- 1024.0 / 44100.0 * 1000000.0
-end
-
-def get_closest_aligned_time(target_time)
- decimal_frames_to_target_time = target_time.to_f / frame_duration
- nearest_frame_index_for_target_time = decimal_frames_to_target_time.round
- puts "target_time: #{target_time}, decimal_frames_to_target_time: #{decimal_frames_to_target_time}, nearest_frame_index_for_target_time: #{nearest_frame_index_for_target_time}"
- nearest_frame_index_for_target_time * frame_duration
-end
-
-def generate_command_and_directives_for_segment(index, target_start, target_end)
- puts "--- segment #{index + 1} ---"
-
- start_time = get_closest_aligned_time(target_start)
- end_time = get_closest_aligned_time(target_end)
- puts "start_time: #{start_time}, end_time: #{end_time}"
-
- real_duration = end_time - start_time
- puts "real_duration: #{real_duration}"
-
- start_time_with_padding = [start_time - frame_duration * 2, 0].max
- end_time_with_padding = end_time + frame_duration * 2
- puts "start_time_with_padding: #{start_time_with_padding}, end_time_with_padding: #{end_time_with_padding}"
-
- inpoint = 0
- if index > 0
- inpoint = frame_duration * 2
- start_time_with_padding = [start_time_with_padding - frame_duration * 2, 0].max
- inpoint += frame_duration * 2
- end 
-
- padded_duration = end_time_with_padding - start_time_with_padding
- puts "padded_duration: #{padded_duration}"
-
- outpoint = inpoint + real_duration - frame_duration
-
- puts "inpoint: #{inpoint}, outpoint: #{outpoint}"
-
- command = "ffmpeg -hide_banner -loglevel error -nostats -y -ss #{start_time_with_padding}us -t #{padded_duration}us -i out/#{SINE_WAVE_FILE_NAME} -c:a libfdk_aac -ar 44100 -f adts out/seg#{index + 1}.aac"
- directives = [
- "file 'seg#{index + 1}.aac'",
- "inpoint #{inpoint}us",
- "outpoint #{outpoint}us"
- ]
-
- [command, directives.join("\n")]
-end
-
 FileUtils.rm_rf "out"
 FileUtils.mkdir_p "out"
 
+# generate the sine wave we'll use as input
 system("ffmpeg -hide_banner -loglevel error -nostats -y -f lavfi -i \"sine=frequency=#{SINE_FREQUENCY}:duration=#{SINE_WAVE_DURATION}\" out/#{SINE_WAVE_FILE_NAME}")
 
+# Generate the commands we'll use to slice the sine wave into segments and the
+# directives we'll use to recombine them later.
 commands_and_directives = (SINE_WAVE_DURATION / TARGET_SEGMENT_DURATION).ceil.to_i.times.map do |i|
  start_time = (i * TARGET_SEGMENT_DURATION * 1000000).round.to_i
  end_time = [((i + 1) * TARGET_SEGMENT_DURATION * 1000000).round, SINE_WAVE_DURATION * 1000000].min.to_i
- cmd, directives = generate_command_and_directives_for_segment(i, start_time, end_time)
+ generate_command_and_directives_for_segment(i, start_time, end_time)
 end
 
 all_directives = commands_and_directives.map { |cmd, directives| directives }.join("\n")
 File.write("out/audio-concat.txt", all_directives)
 
 puts "---"
 
+# Run the commands.
 commands_and_directives.each do |cmd, _|
  puts cmd
  system(cmd)
 end
 
 puts "---"
 
+# Stitch the segments back together.
 concat_cmd = "ffmpeg -hide_banner -loglevel error -nostats -y -f concat -i out/audio-concat.txt -c copy out/stitched.mp4"
 puts concat_cmd
 puts all_directives