Add custom sample rate support to aac payloader and depayloader

examples/muxer_isom.exs

@@ -30,7 +30,9 @@ defmodule Example do
       })
       |> child(:audio_parser, %Membrane.AAC.Parser{out_encapsulation: :none})
       |> child(:audio_payloader, Membrane.MP4.Payloader.AAC),
-      child(:muxer, Membrane.MP4.Muxer.ISOM)
+      child(:muxer, %Membrane.MP4.Muxer.ISOM{
+        fast_start: true
+      })
       |> child(:sink, %Membrane.File.Sink{location: @output_file}),
       get_child(:audio_payloader) |> get_child(:muxer),
       get_child(:video_payloader) |> get_child(:muxer)

lib/membrane_mp4/depayloader/aac.ex

@@ -62,8 +62,13 @@ defmodule Membrane.MP4.Depayloader.AAC do
     depends_on_core_coder = 0
     extension_flag = 0
 
-    <<aot_id::5, _frequency_id::4, _channel_config_id::4, frame_length_id::1,
-      ^depends_on_core_coder::1, ^extension_flag::1>> = section_5
+    <<aot_id::5, frequency_id::4, section_5_rest::bitstring>> = section_5
+
+    custom_frequency_length = if frequency_id == 15, do: 24, else: 0
+
+    <<_maybe_custom_frequency::integer-size(custom_frequency_length), _channel_config_id::4,
+      frame_length_id::1, ^depends_on_core_coder::1, ^extension_flag::1>> = section_5_rest
+
 
     %{
       profile: AAC.aot_id_to_profile(aot_id),

lib/membrane_mp4/payloader/aac.ex

@@ -7,22 +7,25 @@ defmodule Membrane.MP4.Payloader.AAC do
   """
   use Membrane.Filter
 
-  def_input_pad :input,
+  def_input_pad(:input,
     demand_unit: :buffers,
     accepted_format: %Membrane.AAC{encapsulation: :none}
-
-  def_output_pad :output, accepted_format: Membrane.MP4.Payload
-
-  def_options avg_bit_rate: [
-                spec: non_neg_integer(),
-                default: 0,
-                description: "Average stream bitrate. Should be set to 0 if unknown."
-              ],
-              max_bit_rate: [
-                spec: non_neg_integer(),
-                default: 0,
-                description: "Maximal stream bitrate. Should be set to 0 if unknown."
-              ]
+  )
+
+  def_output_pad(:output, accepted_format: Membrane.MP4.Payload)
+
+  def_options(
+    avg_bit_rate: [
+      spec: non_neg_integer(),
+      default: 0,
+      description: "Average stream bitrate. Should be set to 0 if unknown."
+    ],
+    max_bit_rate: [
+      spec: non_neg_integer(),
+      default: 0,
+      description: "Maximal stream bitrate. Should be set to 0 if unknown."
+    ]
+  )
 
   @impl true
   def handle_stream_format(:input, stream_format, _ctx, state) do
@@ -59,9 +62,11 @@ defmodule Membrane.MP4.Payloader.AAC do
     depends_on_core_coder = 0
     extension_flag = 0
 
+    custom_frequency = if frequency_id == 15, do: <<stream_format.sample_rate::24>>, else: <<>>
+
     section5 =
-      <<aot_id::5, frequency_id::4, channel_config_id::4, frame_length_id::1,
-        depends_on_core_coder::1, extension_flag::1>>
+      <<aot_id::5, frequency_id::4, custom_frequency::binary, channel_config_id::4,
+        frame_length_id::1, depends_on_core_coder::1, extension_flag::1>>
       |> make_esds_section(5)
 
     # 64 = mpeg4-audio

test/membrane_mp4/transmuxing_transpayloading_test.exs

@@ -0,0 +1,52 @@
+defmodule Membrane.MP4.TransmuxingTranspayloadingTest do
+  @moduledoc false
+
+  use ExUnit.Case, async: true
+
+  import Membrane.ChildrenSpec
+  import Membrane.Testing.Assertions
+
+  require Membrane.Pad
+
+  alias Membrane.MP4.Container
+  alias Membrane.Pad
+
+  alias Membrane.Testing.Pipeline
+
+  @tag :tmp_dir
+  test "an AAC track with custom sample rate after demuxing, depayloading, payloading and muxing is identical to the original one",
+       %{tmp_dir: dir} do
+    in_path = "test/fixtures/isom/ref_aac_fast_start_sr_20000.mp4"
+    out_path = Path.join(dir, "ref_aac_fast_start_sr_20000.mp4")
+
+    structure = [
+      child(:file, %Membrane.File.Source{location: in_path})
+      |> child(:demuxer, Membrane.MP4.Demuxer.ISOM)
+      |> via_out(Pad.ref(:output, 1))
+      |> child(:depayloader, Membrane.MP4.Depayloader.AAC)
+      |> child(:payloader, Membrane.MP4.Payloader.AAC)
+      |> child(:muxer, %Membrane.MP4.Muxer.ISOM{
+        chunk_duration: Membrane.Time.seconds(1),
+        fast_start: true
+      })
+      |> child(:sink, %Membrane.File.Sink{location: out_path})
+    ]
+
+    pipeline = Pipeline.start_link_supervised!(structure: structure)
+
+    assert_end_of_stream(pipeline, :sink, :input, 6000)
+    refute_sink_buffer(pipeline, :sink, _buffer, 0)
+
+    assert :ok == Pipeline.terminate(pipeline)
+
+    {in_mp4, <<>>} = File.read!(in_path) |> Container.parse!()
+    {out_mp4, <<>>} = File.read!(out_path) |> Container.parse!()
+
+    assert out_mp4[:moov].children[:mvhd] == in_mp4[:moov].children[:mvhd]
+
+    assert out_mp4[:moov].children[:trak].children[:thkd] ==
+             in_mp4[:moov].children[:trak].children[:thkd]
+
+    assert out_mp4[:mdat] == in_mp4[:mdat]
+  end
+end