FFmpeg

Question 1

What is sound and which are its basic Attributes?

Answer 1

Amplitude: loudness
Frequency: a measure of the wave’s vibrations per time unit

Question 2

What is sampling?

Answer 2

Sampling is used to convert the time-varying continuous signal x(t) to a discrete sequence of real numbers x(n). The interval between two successive discrete samples is the sampling period (Ts). We use the sampling frequency (fs = 1/Ts) as the attribute that describes the sampling process.

Question 3

What is quantization?

Answer 3

Quantization is the process of replacing each real number, x(n), of the sequence of samples with an approximation from a finite set of discrete values. In other words, quantization is the process of reducing the infinite number precision of an audio sample to a finite precision as defined by a particular number of bits.

Question 4

How do you convert video (mkv) to audio (mp3)

Answer 4

ffmpeg -i video.mkv audio.mp3

Question 5

Fordownsamplingto 16KHz, converting stereo (2 channels)to mono (1 channel)and converting MP3to WAV (uncompressed audio samples), one needs to use the -ar (audio rate) -ac (audio channel) properties:

Answer 5

ffmpeg -i audio.wav -ar 16000 -ac 1 audio_16K_mono.wav

Question 6

Also, downsampling of an audio file and stereo to mono conversion can be achieved using sox in the following manner:

Answer 6

sox <source_file_ -r <new_sampling_rate> -c 1 <output_file>)</output_file></new_sampling_rate>

Question 7

How can you see the file’s attributes using FFmpeg?

Answer 7

ffmpeg -i audio_16K_mono.wav

Question 8

What will ffmpeg -i audio_16K_mono.wav return

Answer 8

Input #0, wav, from ‘audio_16K_mono.wav’:
Metadata:
encoder : Lavf57.71.100
Duration: 00:03:10.29, bitrate: 256 kb/s
Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 16000 Hz,
mono, s16, 256 kb/s

Question 9

Totriman audio file, e.g. from the 60th to the 80th second (20 seconds new duration):

Answer 9

ffmpeg -i audio.wav -ss 60 -t 20 audio_small.wav

Question 10

What is -i for?

Answer 10

In FFmpeg, the -i flag is used to specify the input file. It tells FFmpeg which file you want to process, whether you’re converting, streaming, or performing any other operation on the media file.

Question 11

What is -ss for?

Answer 11

In FFmpeg, the -ss flag is used to specify the start time for the input or output file. This allows you to seek to a particular point in the media file.

Question 12

How do you seek in the input file?

Answer 12

If you want to start processing the input file from a specific time, use -ss before the -i flag. For example, to start from 1 minute and 30 seconds into the video:

Question 13

How do you seek in the output file?

Answer 13

If you want to start writing the output file from a specific time without affecting the input file processing, place -ss after the -i flag:

Question 14

The placement of -ss can affect the precision and performance of seeking. Using it before the -i flag performs a________ but less accurate seek, while using it after the -i flag is_________but more accurate.

Answer 14

faster, slower

Question 15

Toconcatenatetwo or more audio files one can use the “ffmpeg -f concat” command. Suppose you want to concatenate all files f1.wav, f2.wav and f3.wav to a large file called output.wav. What you need to do is create a text file of the following format (say named ‘list_of_files_to_concat’):

Answer 15

file ‘file1.wav’
file ‘file2.wav’
file ‘file3.wav’

ffmpeg -f concat -i list_of_files_to_concat -c copy output.wav

Question 16

Basic conversion:

Answer 16

ffmpeg -i in.mp4 out.avi

Question 17

To break an audio file into successive chunks (segments) of the (same) specified duration can be done with the “ffmpeg -f segment” option. For example, the following command will break output.wav into 1-second, non-overlapping segments named out00000.wav, out00001.wav, etc.:

Answer 17

ffmpeg -i output.wav -f segment -segment_time 1 -c copy out%05d.wav

Question 18

With regards to channel handling, apart from simple mono to stereo conversion (or stereo to mono) through the -ac property, one may want to switch stereo channels (right to left). The way to achieve this is through the ffmpeg map_channel property:

Answer 18

ffmpeg -i stereo.wav -map_channel 0.0.1 -map_channel 0.0.0 stereo_inverted.wav

Question 19

To create astereo file from two mono files, say left.wav and right.wav:

Answer 19

ffmpeg -i left.wav -i right.wav -filter_complex “[0:a][1:a]join=inputs=2:channel_layout=stereo[a]” -map “[a]” mix_channels.wav

Question 20

On the opposite direction, to split a stereo file into two mono (one for each channel):

Answer 20

ffmpeg -i stereo.wav -map_channel 0.0.0 left.wav -map_channel 0.0.1 right.wav

Question 21

Map_channel can also be used to mute a channel from a stereo signal, e.g. (below the left channel is muted):

Answer 21

ffmpeg -i stereo.wav -map_channel -1 -map_channel 0.0.1 muted.wav

Question 22

Volumeadaptation can also be achieved through ffmpeg, e.g.

Answer 22

ffmpeg -i data/music_44100.wav -filter:a “volume=0.5” data/music_44100_volume_50.wav
ffmpeg -i data/music_44100.wav -filter:a “volume=2.0” data/music_44100_volume_200.wav

Question 23

Volumechange can be achieved withsoxas well in the following way:

Answer 23

sox -v 0.5 data/music_44100.wav data/music_44100_volume_50_sox.wav
sox -v 2.0 data/music_44100.wav data/music_44100_volume_200_sox.wav

Question 24

Remux an Mkv file into MP4

Answer 24

ffmpeg -i in.mkv -c:v copy -c:a copy out.mp4

Question 25

Use the crf (Constant Rate Factor) parameter to control the output quality. The lower crf, the higher the quality (range: 0-51). The default value is 23, and visually lossless compression corresponds to -crf 18. Use the preset parameter to control the speed of the compression process

Answer 25

ffmpeg -i in.mp4 -preset slower -crf 18 out.mp4

Question 26

Trimming without re-encoding:

Answer 26

ffmpeg -ss [start] -i in.mp4 -t [duration] -c copy out.mp4

Question 27

-__ specifies the start time, e.g. 00:01:23.000 or 83 (in seconds)

Answer 27

ss

Question 28

-__specifies the duration of the clip (same format).

Answer 28

t

Question 29

-__copy copies the first video, audio, and subtitle bitstream from the input to the output file without re-encoding them. This won’t harm the quality and make the command run within seconds.

Answer 29

c

Question 30

Trimming with re-encoding:

Answer 30

If you leave out the -c copy option, ffmpeg will automatically re-encode the output video and audio according to the format you chose.

ffmpeg -ss [start] -i in.mp4 -t [duration] -c:v libx264 -c:a aac -strict experimental -b:a 128k out.mp4

Question 31

Mux video and audio from another video. To copy the video from in0.mp4 and audio from in1.mp4:

Answer 31

ffmpeg -i in0.mp4 -i in1.mp4 -c copy -map 0:0 -map 1:1 -shortest out.mp4

Question 32

With -c copy the streams will be stream copied, not ______________, so there will be no quality loss.

Answer 32

re-encoded

Question 33

The ___________ option will cause the output duration to match the duration of the shortest input stream.

Answer 33

-shortest

Question 34

Concat demuxer:

Answer 34

file ‘in1.mp4’
file ‘in2.mp4’
file ‘in3.mp4’
file ‘in4.mp4’

ffmpeg -f concat -i list.txt -c copy out.mp4

Question 35

Delay audio/video by 3.84 seconds:

Answer 35

video: ffmpeg -i in.mp4 -itsoffset 3.84 -i in.mp4 -map 1:v -map 0:a -vcodec copy -acodec copy out.mp4
audio: ffmpeg -i in.mp4 -itsoffset 3.84 -i in.mp4 -map 0:v -map 1:a -vcodec copy -acodec copy out.mp4

Question 36

Burn subtitles

Use the libass library (make sure your ffmpeg install has the library in the configuration –enable-libass).

Answer 36

First convert the subtitles to .ass format:

ffmpeg -i sub.srt sub.ass
Then add them using a video filter:

ffmpeg -i in.mp4 -vf ass=sub.ass out.mp4

Question 37

Extract the frames from a video
To extract all frames from between 1 and 5 seconds, and also between 11 and 15 seconds:

Answer 37

ffmpeg -i in.mp4 -vf select=’between(t,1,5)+between(t,11,15)’ -vsync 0 out%d.png

Question 38

To extract one frame per second only:

Answer 38

ffmpeg -i in.mp4 -fps=1 -vsync 0 out%d.png

Question 39

Rotate 90 clockwise:

Answer 39

ffmpeg -i in.mov -vf “transpose=1” out.mov
For the transpose parameter you can pass:

0 = 90CounterCLockwise and Vertical Flip (default)
1 = 90Clockwise
2 = 90CounterClockwise
3 = 90Clockwise and Vertical Flip
Use -vf “transpose=2,transpose=2” for 180 degrees.

Question 40

Download “Transport Stream” video streams

Answer 40

1. Locate the playlist file, e.g. using Chrome > F12 > Network > Filter: m3u8
2. Download and concatenate the video fragments:
   ffmpeg -i “path_to_playlist.m3u8” -c copy -bsf:a aac_adtstoasc out.mp4
   If you get a “Protocol ‘https not on whitelist ‘file,crypto’!” error, add the protocol_whitelist option:

ffmpeg -protocol_whitelist “file,http,https,tcp,tls” -i “path_to_playlist.m3u8” -c copy -bsf:a aac_adtstoasc out.mp4

Question 41

Mute some of the audio

To replace the first 90 seconds of audio with silence:

Answer 41

ffmpeg -i in.mp4 -vcodec copy -af “volume=enable=’lte(t,90)’:volume=0” out.mp4

Question 41

To replace all audio between 1’20” and 1’30” with silence:

Answer 41

ffmpeg -i in.mp4 -vcodec copy -af “volume=enable=’between(t,80,90)’:volume=0” out.mp4

Question 42

What is Deinterlace?

Answer 42

Deinterlacing is the process of converting interlaced video into a non-interlaced or progressive form

Question 42

Deinterlacing using “yet another deinterlacing filter”.

Answer 42

ffmpeg -i in.mp4 -vf yadif out.mp4

Question 43

Create a video slideshow from images

Answer 43

Parameters: -r marks the image framerate (inverse time of each image); -vf fps=25 marks the true framerate of the output.

ffmpeg -r 1/5 -i img%03d.png -c:v libx264 -vf fps=25 -pix_fmt yuv420p out.mp4

Question 44

Extract images from a video

Answer 44

Extract all frames: ffmpeg -i input.mp4 thumb%04d.jpg -hide_banner

Extract a frame each second: ffmpeg -i input.mp4 -vf fps=1 thumb%04d.jpg -hide_banner

Extract only one frame: ffmpeg -i input.mp4 -ss 00:00:10.000 -vframes 1 thumb.jpg

Question 45

Display the frame number on each frame

Answer 45

ffmpeg -i in.mov -vf “drawtext=fontfile=arial.ttf: text=%{n}: x=(w-tw)/2: y=h-(2*lh): fontcolor=white: box=1: boxcolor=0x00000099: fontsize=72” -y out.mov

Question 46

Metadata: Change the title

Answer 46

ffmpeg -i in.mp4 -map_metadata -1 -metadata title=”My Title” -c:v copy -c:a copy out.mp4