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Audio Options

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patrickenfuego edited this page May 9, 2023 · 14 revisions

Introduction

FFEncoder supports the mapping/transcoding of 2 distinct audio streams to the output file. For audio that is transcoded, the primary audio stream is used as it's generally lossless (TrueHD, DTS-HD MA, LPCM, etc.). It is never recommended to transcode from one lossy codec to another; if the primary audio stream is lossy compressed, it is best to stream copy it instead of forcing a transcode.

FFEncoder currently supports the following audio options wih the -Audio/-Audio2 parameters. When selecting a named codec (like EAC3, AC3, etc.) the script will go through the following checks:

  1. If either of the -AudioBitrate parameters are selected, the corresponding stream will be transcoded to the selected codec, regardless if an existing stream is present in the input file
  2. If the -AudioBitrate parameters are not present, the script will search the input file for a matching stream and stream copy it to the output file if found
  3. If no bitrate is specified and no existing stream is found, then the script will transcode to the selected codec at the default bitrates listed below:
Type Values Default Description
Copy copy, c N/A Passes through the primary audio stream without re-encoding
Copy All copyall, ca N/A Passes through all audio streams from the input to the output without re-encoding
AAC aac 512 kb/s Converts the primary audio stream to AAC using ffmpeg's native CBR encoder. Compatible with the -AudioBitrate parameters
FDK AAC fdkaac, faac Variable (VBR) Converts the primary audio stream to AAC using libfdk_aac. Compatible with the -AudioBitrate parameters. See note below for more info
AAC_AT aac_at Variable (VBR) Converts the primary audio stream to AAC using Apple Core Audio Toolbox (MacOS only). Compatible with the -AudioBitrate parameters
AC3 ac3, dd 640 kb/s Dolby Digital. Compatible with the -AudioBitrate parameters
E-AC3 eac3 448 kb/s Dolby Digital Plus. Compatible with the -AudioBitrate parameters
DTS dts Variable (VBR) DTS Core audio. If the source audio is DTS-HD MA, the core is extracted without re-encoding. Warning: ffmpeg's DTS encoder is "experimental". Compatible with the -AudioBitrate parameters
FLAC flac, f Variable (VBR) Converts the primary audio stream to FLAC lossless audio using ffmpeg's native FLAC encoder
Stream # 0-12 N/A Select an audio stream using its stream identifier in ffmpeg/ffprobe. Not compatible with the -Stereo parameters
None none, n N/A No audio streams will be added to the output file

Additional encoding options exist if you have the Dolby Encoding Engine. See below for more info.

Using the libfdk_aac Encoder

FFEncoder includes support for Fraunhofer's libfdk_aac, even though it is not included in a standard ffmpeg executable. Due to a conflict with ffmpeg's GPL, libfdk_aac cannot be distributed with any official ffmpeg binaries, but it can be included when compiling ffmpeg manually from source. For more info, see Dependencies.

One of the benefits of the FDK encoder is that it supports variable bitrate (VBR). When using the -AudioBitrate/-AudioBitrate2 parameters with fdkaac, values 1-5 are used to signal VBR. 1 = lowest quality and 5 = highest quality.

Using the aac_at Encoder

When running FFEncoder on a Mac computer, you gain access to the AudioToolbox AAC encoder (open source port of Apple's high quality encoder) via the aac_at argument. AudioToolbox is, by default, a variable Bitrate (VBR) encoder, but can accept the following values using -AudioBitrate/-AudioBitrate2:

  • -1 - Auto (VBR)
  • 0 - Constant bitrate (CBR)
  • 1 - Long-term Average bitrate (ABR)
  • 2 - Constrained variable bitrate (VBR)
  • 3 - Variable bitrate (VBR)

Downmixing Multi-Channel Audio to Stereo

With FFEncoder, you can downmix either of the two output streams to stereo using the -Stereo/-Stereo2 parameters. The process uses an audio filter that retains the LFE (bass) track in the final mix, which is discarded when using -ac 2 in ffmpeg directly.

When using any combination of copy/c/copyall/ca and -Stereo/-Stereo2, the script will multiplex the primary audio stream out of the container and encode it in a separate thread; this is because ffmpeg cannot stream copy and filter at the same time. See here for a nice explanation. Once the primary encode finishes, the external audio file (now converted to stereo) is multiplexed back into the primary container with the other streams selected.

Using the DEE Encoding Options

DISCLAIMER: The script does not provide a copy of the DEE encoder, only the compiled wrapper program.

Introduction

The DEE Encoder (AKA Dolby Encoding Engine) is an proprietary command-line tool for encoding Dolby audio streams. FFEncoder supports audio encoding with DEE which uses a platform-specific, custom-compiled version of deew to parse the parameters passed to the script; while the script is written in Python, you do not need Python installed to use the compiled wrapper.

Configuration

Before the deew wrapper will work properly, you first need to set the configuration file path as it is no longer included as a sidecar file by default. Open a PowerShell (or bash) window, navigate to the /bin/<os> directory within FFEncoder, and run the following command:

# Force regenerate the configuration file
# Windows
.\deew.exe -gc
# Linux
./deew -gc

This should prompt you for a save path with 2 options:

  1. The default location (OS specific)
  2. The directory where the executable is saved

Choose option 2, as this is where FFEncoder will look for the config file. The config file will be generated afterward in the location you chose. The file settings should look like the following:

# These are required.
# If only name is specified, it will look in your system PATH variable, which includes the current directory on Windows.
# Setup instructions: https://github.com/pcroland/deew#setup-system-path-variable
# If full path is specified, that will be used.
ffmpeg_path = 'ffmpeg'
ffprobe_path = 'ffprobe'
dee_path = 'dee'

# FFEncoder will set this for you - leave it empty
temp_path = ''

# THIS MUST BE DISABLED
logo = 0

# Change this if you want but 50% is a reasonable default
max_instances = '50%'

# Set these to whatever you want
[default_bitrates]
    dd_1_0 = 128
    dd_2_0 = 256
    dd_5_1 = 640
    ddp_1_0 = 128
    ddp_2_0 = 256
    ddp_5_1 = 1024
    ddp_7_1 = 1536

# THESE MUST ALL BE DISABLED
[summary_sections]
    deew_info = false
    binaries = false
    input_info = false
    output_info = false
    other = false

If you don't disable the options marked above, the script will fail as it cannot decode the unicode characters that the rich library prints when embedded in a different script.

Requirements

To use deew, the following 3 things must be available via PATH (the first two are already required by FFEncoder):

  • ffmpeg
  • ffprobe
  • dee

Usage

I made a few changes to the XML templates used in deew based on recommendations from audio engineers. Otherwise, all credit goes to pcroland for writing the code (although I've contributed a bit to the repo).

To encode audio with DEE, use the following arguments for either the Audio or Audio2 parameters:

NOTE: The parameters options dee_ddp_51/dee_eac3_51 are used to force a downmix from 7.1 to 5.1 for an 8-channel audio source. To force a stereo downmix (LtRt), use the Stereo/Stereo2 parameters.

Codec Argument Supported Bitrates
Dolby Digital Plus (E-AC3) 7.1 dee_ddp, dee_eac3 384, 448, 576, 640, 704, 768, 832, 896, 960, 1008, 1024, 1280, 1536, 1664
Dolby Digital Plus (E-AC3) 5.1 dee_ddp, dee_eac3, dee_ddp_51, dee_eac3_51 192, 200, 208, 216, 224, 232, 240, 248, 256, 272, 288, 304, 320, 336, 352, 368, 384, 400, 448, 512, 576, 640, 704, 768, 832, 896, 960, 1008, 1024
Dolby Digital Plus (E-AC3) 2.0 LtRt / 1.0 Mono dee_ddp, dee_eac3 96, 104, 112, 120, 128, 144, 160, 176, 192, 200, 208, 216, 224, 232, 240, 248, 256, 272, 288, 304, 320, 336, 352, 368, 384, 400, 448, 512, 576, 640, 704, 768, 832, 896, 960, 1008, 1024
Dolby Digital (AC3) 5.1 dee_ac3, dee_dd 224, 256, 320, 384, 448, 512, 576, 640
Dolby Digital (AC3) 2.0 LtRt / 1.0 Mono dee_ac3, dee_dd 96, 112, 128, 160, 192, 224, 256, 320, 384, 448, 512, 576, 640
TrueHD dee_thd N/A (Variable Bitrate)

When a DEE option is selected, the audio is encoded in a separate thread to improve speed - when both encodes are completed, the script multiplexes the audio back into the primary container for you. If the video encode finishes before the audio, the script will wait for both to complete before progressing.

Note that if you do not choose a supported bitrate, audio encoding will fail - adding verification logic is time consuming, so just reference the table above and all will be well.

Troubleshooting

If the script is throwing an error when using one of the DEE compatible options, it is most likely due to a rogue configuration file in the default deew file path. To verify this, navigate to the /bin/<os> directory once more and run the following command:

# Print config location and settings
.\deew.exe -c

If the output prints 2 locations (and the printed settings are not correct), delete the file that is not saved in the FFEncoder project directory. This should resolve the problem.

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