Extracting audio from video using FFMPEG

This tutorial deals with extracting audio from video using GPU accelerated libraries supported by FFMPEG in Ubuntu 16.04.

Introduction

FFmpeg is one of the most famous multimedia frameworks wich is widely used for processeing videos. In order to encode the video, certainly a video encoder must be used. The popular x264 is the one which is widely used however it is not super fast! The lastest NVIDIA GPUs contain a hardware-based video encoder called NVENC which is much faster than traditional ones. In order to be able to utilize this gpu-accelerated encoder, FFmpeg must be installed with NVENC support. The full documentation of FFmpeg integrated with NVIDIA can be fount at here. documentation on NVENC can be found here Moreover The NVENC programming guide can be found here.

In this tutorial the main goal is to show how to extract audio from video with GPU-accelerated libraries in Linux. In this tutorial we do not use the terminal commands directly for employing the FFmpeg with NVENC support. Instead the python interface is being used to run commands in the terminal. This can be done using subprocess python module. This module is employed for execution and dealing external commands, intended to supercede the os.sys module. The trivial method os its usage will be explained in this tutorial. Please refer to this documentation for further details.

The assumption of this tutorial is that the FFmpeg is already installed with NVENC support. The installation guide can be found in FFMPEG WITH NVIDIA ACCELERATION ON UBUNTU LINUX documentation provided by NVIDIA.

Extracting Audio

From now on the assumption is that the .txt file is ready and well-formatted. The python script for processing videos is as below:

import subprocess
import os
import sys

# Pre...
textfile_path = 'absolute/path/to/videos.txt'

# Read the text file
with open(textfile_path) as f:
    content = f.readlines()
# you may also want to remove whitespace characters like `\n` at the end of each line
files_list = [x.strip() for x in content]

# Extract audio from video.
# It already save the video file using the the named defined by output_name.
for file_num, file_path_input in enumerate(files_list, start=1):
    # Get the file name withoutextension
    file_name = os.path.basename(file_path_input)
    if 'mouthcropped' not in file_name:
        raw_file_name = os.path.basename(file_name).split('.')[0]
        file_dir = os.path.dirname(file_path_input)
        file_path_output = file_dir + '/' + raw_file_name + '.wav'
        print('processing file: %s' % file_path_input)

        # subprocess.call(
        #     ['ffmpeg', '-i', file_path_input, '-codec:a', 'libmp3lame', '-qscale:a', '2', file_path_output])
        # print('file %s saved' % file_path_output)
        subprocess.call(
            ['ffmpeg', '-i', file_path_input, '-codec:a', 'pcm_s16le', '-ac', '1', file_path_output])
        print('file %s saved' % file_path_output)

Overall Code Description

The videos.txt file is saved in the absolute path. The code reads the .txt file and stores each line as an item of a list called files_list. The loop processes each file with the subprocess.call command. In each loop the folder of the input file is found and the output file will be stored in the same directory but with different naming convention which is explaned by the comments in the code. Each , in the subprocess.call command in the python is correspondant to an empty space in the terminal. As an example the correspondant shell command is as below:

for i in **/*.mp4; do base=${i%.mp4}; ffmpeg -i $base.mp4 -codec:a pcm_s16le -ac  1 $base.wav; done

FFmpeg Encoder

The command executed by FFmpeg needs to be described. Each of the elements started by - are calling specific operations and the command follows by them execute the desired operation. For example -vcodec indicator will specify the codec to be used by FFmpeg and nvenc which follows by that point to the codec. More details can be found at FFmpeg Filters Documentation. The fllowing Table, summarize the indicators:

#	Attribute	Description
1	-i	path to the input file
2	-codec:a	audio codec
3	-ac	number of audio channels

Code Execution

In order to run the python file we go to the terminal and execute the following:

python /absolute/path/to/python/file

As a consideration, if we are working on any specific virtual environment it has to be activated at first.

Summary

This tutorial demonstrated how to extract audio from a video and specifically using FFmpeg and Nvidia GPU accelerated library called NVENC. The advantage of using python interface is to easily parse the .txt file and looping through all files. Moreover it enables the user with options which are more complex to be directly employed in the terminal environment.