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Is it safe to omit the codec when converting with ffmpeg?

Would like to change audio codec, but keep video settings with ffmpegIs it possible to get ffmpeg to work on Debian with support for amr codec (libopencore-amrnb) ahd h264 encoder (libx264)?Converting AVI encoded file with an IMM5 codecFix audio codec error with ffmpegffmpeg changing Codec ID?Keep input codecsUnable to re-encode audio stream with different bitrate using ffmpegConvert videos to MP4 iPad (h264 and aac), but leave video codec alone if h264 alreadyChanging the codec while keeping the bitrate with ffmpegFFMpeg Video Overlay with Delayed Audio and Video in Background Video

0

I noticed today that I could encode or transcode video and audio streams using ffmpeg without specifying a codec, due to the fact that it seems to imply the video or audio codec being encoded or transcoded to from the extension of the output. This means that:

ffmpeg "input.mp3" -c:a aac "output.m4a"

...can be replaced by:

ffmpeg "input.mp3" "output.m4a"

In addition:

ffmpeg "input.dts" -c:a copy -ac 2 "output.dts"

...can be replaced by:

ffmpeg "input.dts" -ac 2 "output.dts"

Given that always explicitly including the codec is the most popular way to use ffmpeg I've seen - on this site in particular - I wondered whether there were reasons for this to be the case.

Are there any possible pitfalls to consider when using ffmpeg to encode in this way? Is it safe to assume that it would result in exactly the same conversion that explicitly specifying a codec might?

audio video ffmpeg codec transcode

share|improve this question

edited 3 hours ago

Hashim

asked 6 hours ago

HashimHashim

3,09663162

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  You don't need the -ac 2 in your third command: it's being ignored since you are only re-muxing (stream copying). Your fourth command is not the same as the third: it will re-encode while the previous command just re-muxes.
  
  – llogan
  5 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @llogan Great catch. I tested the conversion with a .dts file I had but forgot to check whether the output was actually stereo. So to confirm - any changes made to a stream can only be made if the stream is re-encoded, and a re-encode can only happen if the stream is not copied?
  
  – Hashim
  5 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Yes to both questions
  
  – llogan
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @llogan Interesting. Is there a way to use FFmpeg to use the same codec as the input for the output, while still getting it to re-encode?
  
  – Hashim
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  There is no internal setting for this, but one option is to use ffprobe to get the codec_name, which can roughly correlate to an encoder name, then use if/then statements in your favorite scripting language. IIRC, other users have asked this same question on the Stack Exchange network, so there are probably some examples.
  
  – llogan
  4 hours ago
  
  
  

  
  
  
  

 | 
show 2 more comments

0

I noticed today that I could encode or transcode video and audio streams using ffmpeg without specifying a codec, due to the fact that it seems to imply the video or audio codec being encoded or transcoded to from the extension of the output. This means that:

ffmpeg "input.mp3" -c:a aac "output.m4a"

...can be replaced by:

ffmpeg "input.mp3" "output.m4a"

In addition:

ffmpeg "input.dts" -c:a copy -ac 2 "output.dts"

...can be replaced by:

ffmpeg "input.dts" -ac 2 "output.dts"

Given that always explicitly including the codec is the most popular way to use ffmpeg I've seen - on this site in particular - I wondered whether there were reasons for this to be the case.

Are there any possible pitfalls to consider when using ffmpeg to encode in this way? Is it safe to assume that it would result in exactly the same conversion that explicitly specifying a codec might?

audio video ffmpeg codec transcode

share|improve this question

edited 3 hours ago

Hashim

asked 6 hours ago

HashimHashim

3,09663162

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  You don't need the -ac 2 in your third command: it's being ignored since you are only re-muxing (stream copying). Your fourth command is not the same as the third: it will re-encode while the previous command just re-muxes.
  
  – llogan
  5 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @llogan Great catch. I tested the conversion with a .dts file I had but forgot to check whether the output was actually stereo. So to confirm - any changes made to a stream can only be made if the stream is re-encoded, and a re-encode can only happen if the stream is not copied?
  
  – Hashim
  5 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Yes to both questions
  
  – llogan
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @llogan Interesting. Is there a way to use FFmpeg to use the same codec as the input for the output, while still getting it to re-encode?
  
  – Hashim
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  There is no internal setting for this, but one option is to use ffprobe to get the codec_name, which can roughly correlate to an encoder name, then use if/then statements in your favorite scripting language. IIRC, other users have asked this same question on the Stack Exchange network, so there are probably some examples.
  
  – llogan
  4 hours ago
  
  
  

  
  
  
  

 | 
show 2 more comments

I noticed today that I could encode or transcode video and audio streams using ffmpeg without specifying a codec, due to the fact that it seems to imply the video or audio codec being encoded or transcoded to from the extension of the output. This means that:

ffmpeg "input.mp3" -c:a aac "output.m4a"

...can be replaced by:

ffmpeg "input.mp3" "output.m4a"

In addition:

ffmpeg "input.dts" -c:a copy -ac 2 "output.dts"

...can be replaced by:

ffmpeg "input.dts" -ac 2 "output.dts"

Given that always explicitly including the codec is the most popular way to use ffmpeg I've seen - on this site in particular - I wondered whether there were reasons for this to be the case.

Are there any possible pitfalls to consider when using ffmpeg to encode in this way? Is it safe to assume that it would result in exactly the same conversion that explicitly specifying a codec might?

audio video ffmpeg codec transcode

share|improve this question

edited 3 hours ago

Hashim

asked 6 hours ago

HashimHashim

3,09663162

ffmpeg "input.mp3" -c:a aac "output.m4a"

ffmpeg "input.mp3" "output.m4a"

ffmpeg "input.dts" -c:a copy -ac 2 "output.dts"

ffmpeg "input.dts" -ac 2 "output.dts"

share|improve this question

edited 3 hours ago

Hashim

asked 6 hours ago

HashimHashim

3,09663162

share|improve this question

edited 3 hours ago

Hashim

asked 6 hours ago

HashimHashim

3,09663162

asked 6 hours ago

HashimHashim

3,09663162

asked 6 hours ago

HashimHashim

3,09663162

2 Answers
2

active

oldest

votes

1

It's best to specify the encoder

• 
  

  Some of the default selections are legacy or weird. For example:

  
  
  
  
  

  • flv1 is chosen for FLV output. Most people expect H.264 (libx264).

  
  

  • flac is chosen for OGG/OGA output. Most people expect Vorbis (libvorbis).

  
  
  
  



• The encoder used as the default for an output format can depend on which configure options were used to compile your ffmpeg. So if your ffmpeg does not have --enable-libx264, then for MP4 the encoder mpeg4 (MPEG-4 Part 2 video) is used instead of libx264 (H.264 video). Most people use ffmpeg builds with support for the most popular libraries included, but it happens on occasion.




• It's not much work to be specific.

But sometimes you can be lazy

If you know what encoders are going to be chosen, or if you check and verify, then you can save a few keystrokes if you want. Refer to Stream mapping in the console output to see what encoders ffmpeg uses. Example:

ffmpeg -i input.foo output.mp4

...

Stream mapping:

  Stream #0:0 -> #0:0 (rawvideo (native) -> h264 (libx264))

  Stream #1:0 -> #0:1 (pcm_s16le (native) -> aac (native))

In this case it is using libx264 for video (-c:v libx264), and the native AAC encoder for audio (-c:a aac).

You can view details on a specific muxer to see what codecs will be used by default:

ffmpeg -h muxer=mp4

...

Muxer mp4 [MP4 (MPEG-4 Part 14)]:

    Default video codec: h264.

    Default audio codec: aac.

share|improve this answer

answered 6 hours ago

lloganllogan

25.8k54782

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Great answer, this is the kind of intimate knowledge of FFmpeg I was relying on to get an answer to this question. I noticed that your website says you're an FFmpeg developer. Is there any particular reason for the first point - FFmpeg using legacy/weird encoders - and is there a possibility that they'll change/become more up to date anytime soon?
  
  – Hashim
  5 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Since posting the question, I also noticed that the FFmpeg website uses the simpler syntax on its homepage, so is clearly trying to encourage such use for beginners, and I have to admit the codec syntax and how complicated it looks had, prior to this week, been a barrier to learning how to use ffmpeg for a while.
  
  – Hashim
  5 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Hashim Regarding the legacy encoders, sometimes things are updated (webm now uses libbpx-vp9 & libopus by default for example), but sometimes things languish. I'd be fine with updating some defaults to more modern, sane, or useful values, but I imagine some of the more argumentative developers would balk at some changes, and may suggest it will confuse users (not that the legacy stuff isn't confusing), so some settings stay the same indefinitely.
  
  – llogan
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  That's a real shame. When you use a program like ffmpeg with the pedigree that it has, you'd expect its defaults to be both current and optimised for the majority of people, I hope something changes soon on that front. Thanks regardless for all your help.
  
  – Hashim
  3 hours ago
  

  
  
  
  

add a comment | 

1

Is it safe to assume that it would result in exactly the same
conversion that explicitly specifying a codec might

Not necessarily. It is "safe" in that you will get a file that works. Ffmpeg will pick a preferred codec that works with that container. However using a different file extension MAY choose a different codec. Also the preferred codec could change in the future, or based on how Ffmpeg was compiled.

share|improve this answer

answered 6 hours ago

szatmaryszatmary

1,759711

add a comment | 

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1

It's best to specify the encoder

• 
  

  Some of the default selections are legacy or weird. For example:

  
  
  
  
  

  • flv1 is chosen for FLV output. Most people expect H.264 (libx264).

  
  

  • flac is chosen for OGG/OGA output. Most people expect Vorbis (libvorbis).

  
  
  
  



• The encoder used as the default for an output format can depend on which configure options were used to compile your ffmpeg. So if your ffmpeg does not have --enable-libx264, then for MP4 the encoder mpeg4 (MPEG-4 Part 2 video) is used instead of libx264 (H.264 video). Most people use ffmpeg builds with support for the most popular libraries included, but it happens on occasion.




• It's not much work to be specific.

But sometimes you can be lazy

If you know what encoders are going to be chosen, or if you check and verify, then you can save a few keystrokes if you want. Refer to Stream mapping in the console output to see what encoders ffmpeg uses. Example:

ffmpeg -i input.foo output.mp4

...

Stream mapping:

  Stream #0:0 -> #0:0 (rawvideo (native) -> h264 (libx264))

  Stream #1:0 -> #0:1 (pcm_s16le (native) -> aac (native))

In this case it is using libx264 for video (-c:v libx264), and the native AAC encoder for audio (-c:a aac).

You can view details on a specific muxer to see what codecs will be used by default:

ffmpeg -h muxer=mp4

...

Muxer mp4 [MP4 (MPEG-4 Part 14)]:

    Default video codec: h264.

    Default audio codec: aac.

share|improve this answer

answered 6 hours ago

lloganllogan

25.8k54782

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Great answer, this is the kind of intimate knowledge of FFmpeg I was relying on to get an answer to this question. I noticed that your website says you're an FFmpeg developer. Is there any particular reason for the first point - FFmpeg using legacy/weird encoders - and is there a possibility that they'll change/become more up to date anytime soon?
  
  – Hashim
  5 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Since posting the question, I also noticed that the FFmpeg website uses the simpler syntax on its homepage, so is clearly trying to encourage such use for beginners, and I have to admit the codec syntax and how complicated it looks had, prior to this week, been a barrier to learning how to use ffmpeg for a while.
  
  – Hashim
  5 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Hashim Regarding the legacy encoders, sometimes things are updated (webm now uses libbpx-vp9 & libopus by default for example), but sometimes things languish. I'd be fine with updating some defaults to more modern, sane, or useful values, but I imagine some of the more argumentative developers would balk at some changes, and may suggest it will confuse users (not that the legacy stuff isn't confusing), so some settings stay the same indefinitely.
  
  – llogan
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  That's a real shame. When you use a program like ffmpeg with the pedigree that it has, you'd expect its defaults to be both current and optimised for the majority of people, I hope something changes soon on that front. Thanks regardless for all your help.
  
  – Hashim
  3 hours ago
  

  
  
  
  

add a comment | 

1

It's best to specify the encoder

• 
  

  Some of the default selections are legacy or weird. For example:

  
  
  
  
  

  • flv1 is chosen for FLV output. Most people expect H.264 (libx264).

  
  

  • flac is chosen for OGG/OGA output. Most people expect Vorbis (libvorbis).

  
  
  
  



• The encoder used as the default for an output format can depend on which configure options were used to compile your ffmpeg. So if your ffmpeg does not have --enable-libx264, then for MP4 the encoder mpeg4 (MPEG-4 Part 2 video) is used instead of libx264 (H.264 video). Most people use ffmpeg builds with support for the most popular libraries included, but it happens on occasion.




• It's not much work to be specific.

But sometimes you can be lazy

If you know what encoders are going to be chosen, or if you check and verify, then you can save a few keystrokes if you want. Refer to Stream mapping in the console output to see what encoders ffmpeg uses. Example:

ffmpeg -i input.foo output.mp4

...

Stream mapping:

  Stream #0:0 -> #0:0 (rawvideo (native) -> h264 (libx264))

  Stream #1:0 -> #0:1 (pcm_s16le (native) -> aac (native))

In this case it is using libx264 for video (-c:v libx264), and the native AAC encoder for audio (-c:a aac).

You can view details on a specific muxer to see what codecs will be used by default:

ffmpeg -h muxer=mp4

...

Muxer mp4 [MP4 (MPEG-4 Part 14)]:

    Default video codec: h264.

    Default audio codec: aac.

share|improve this answer

answered 6 hours ago

lloganllogan

25.8k54782

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Great answer, this is the kind of intimate knowledge of FFmpeg I was relying on to get an answer to this question. I noticed that your website says you're an FFmpeg developer. Is there any particular reason for the first point - FFmpeg using legacy/weird encoders - and is there a possibility that they'll change/become more up to date anytime soon?
  
  – Hashim
  5 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Since posting the question, I also noticed that the FFmpeg website uses the simpler syntax on its homepage, so is clearly trying to encourage such use for beginners, and I have to admit the codec syntax and how complicated it looks had, prior to this week, been a barrier to learning how to use ffmpeg for a while.
  
  – Hashim
  5 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Hashim Regarding the legacy encoders, sometimes things are updated (webm now uses libbpx-vp9 & libopus by default for example), but sometimes things languish. I'd be fine with updating some defaults to more modern, sane, or useful values, but I imagine some of the more argumentative developers would balk at some changes, and may suggest it will confuse users (not that the legacy stuff isn't confusing), so some settings stay the same indefinitely.
  
  – llogan
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  That's a real shame. When you use a program like ffmpeg with the pedigree that it has, you'd expect its defaults to be both current and optimised for the majority of people, I hope something changes soon on that front. Thanks regardless for all your help.
  
  – Hashim
  3 hours ago
  

  
  
  
  

add a comment | 

It's best to specify the encoder

• 
  

  Some of the default selections are legacy or weird. For example:

  
  
  
  
  

  • flv1 is chosen for FLV output. Most people expect H.264 (libx264).

  
  

  • flac is chosen for OGG/OGA output. Most people expect Vorbis (libvorbis).

  
  
  
  



• The encoder used as the default for an output format can depend on which configure options were used to compile your ffmpeg. So if your ffmpeg does not have --enable-libx264, then for MP4 the encoder mpeg4 (MPEG-4 Part 2 video) is used instead of libx264 (H.264 video). Most people use ffmpeg builds with support for the most popular libraries included, but it happens on occasion.




• It's not much work to be specific.

But sometimes you can be lazy

If you know what encoders are going to be chosen, or if you check and verify, then you can save a few keystrokes if you want. Refer to Stream mapping in the console output to see what encoders ffmpeg uses. Example:

ffmpeg -i input.foo output.mp4

...

Stream mapping:

  Stream #0:0 -> #0:0 (rawvideo (native) -> h264 (libx264))

  Stream #1:0 -> #0:1 (pcm_s16le (native) -> aac (native))

In this case it is using libx264 for video (-c:v libx264), and the native AAC encoder for audio (-c:a aac).

You can view details on a specific muxer to see what codecs will be used by default:

ffmpeg -h muxer=mp4

...

Muxer mp4 [MP4 (MPEG-4 Part 14)]:

    Default video codec: h264.

    Default audio codec: aac.

share|improve this answer

answered 6 hours ago

lloganllogan

25.8k54782

ffmpeg -i input.foo output.mp4

...

Stream mapping:

  Stream #0:0 -> #0:0 (rawvideo (native) -> h264 (libx264))

  Stream #1:0 -> #0:1 (pcm_s16le (native) -> aac (native))

ffmpeg -h muxer=mp4

...

Muxer mp4 [MP4 (MPEG-4 Part 14)]:

    Default video codec: h264.

    Default audio codec: aac.

share|improve this answer

answered 6 hours ago

lloganllogan

25.8k54782

answered 6 hours ago

lloganllogan

25.8k54782

answered 6 hours ago

lloganllogan

25.8k54782

1

Is it safe to assume that it would result in exactly the same
conversion that explicitly specifying a codec might

Not necessarily. It is "safe" in that you will get a file that works. Ffmpeg will pick a preferred codec that works with that container. However using a different file extension MAY choose a different codec. Also the preferred codec could change in the future, or based on how Ffmpeg was compiled.

share|improve this answer

answered 6 hours ago

szatmaryszatmary

1,759711

add a comment | 

1

Is it safe to assume that it would result in exactly the same
conversion that explicitly specifying a codec might

Not necessarily. It is "safe" in that you will get a file that works. Ffmpeg will pick a preferred codec that works with that container. However using a different file extension MAY choose a different codec. Also the preferred codec could change in the future, or based on how Ffmpeg was compiled.

share|improve this answer

answered 6 hours ago

szatmaryszatmary

1,759711

add a comment | 

Is it safe to assume that it would result in exactly the same
conversion that explicitly specifying a codec might

Not necessarily. It is "safe" in that you will get a file that works. Ffmpeg will pick a preferred codec that works with that container. However using a different file extension MAY choose a different codec. Also the preferred codec could change in the future, or based on how Ffmpeg was compiled.

share|improve this answer

answered 6 hours ago

szatmaryszatmary

1,759711

share|improve this answer

answered 6 hours ago

szatmaryszatmary

1,759711

answered 6 hours ago

szatmaryszatmary

1,759711

answered 6 hours ago

szatmaryszatmary

1,759711