FFmpeg – the swiss army knife of Internet Streaming – part V

[Index]

PART I – Introduction (revised 02-jul-2012)
PART II – Parameters and recipes (revised 02-jul-2012)
PART III – Encoding in H.264 (revised 02-jul-2012)
PART IV – FFmpeg for streaming (revised 02-jul-2012)
PART V – Advanced usage (revised, 19-oct-2012)
PART VI – Filtering (new, 19-oct-2012)

Introduction

After almost one year from the starting post of this series dedicated to FFmpeg I have found some time to catch-up with this topic and revise/refresh the series. In this year a lot of things happened on the FFmpeg side (and not only), so I have corrected a lot of small errors and changes in syntax of commonly used commands. So this is also a good opportunity for you to refresh your knowledge about FFmpeg and the current state-of-the-art. Above you find the Index of the articles.

The most important changes are around parameters like -vcodec, -b, -ab, -vframes, etc… to avoid misunderstandings now a stream_identifier has been added to specify if the parameter is related to the audio or video track. In case of multiple AV tracks there also an optiona parameter to specify the track number. Take a look at the updates of PART II to have more informations about new syntax and obsolete parameters.

Another important change is realated to libfaac library which is now external. Read point 2 below to know about alternatives.

Last but not least, FFmpeg introduced the possibility to control directly the parameters of x264lib using the -x264opts command. Not for everyone but very useful when you want the control and performance of x264 and all the input and output options of FFmpeg.

Fifth Part – Advanced Usage

This fifth article wants to add more advanced use cases and usages to what was presented and discussed in the previous 4 parts. This article will be enriched in the next weeks and months to include even more advanced examples and use cases that can be solved with a smart use of FFmpeg. Good reading!

1. Optimize multi-pass multi-bitrates encoding

You know that encoding for dynamic streaming techniques (HDS, HLS, Silverlight) requires the renditions to have aligned keyframes and be CBR or capped VBR.
A neat trick to avoid the limit of fixed length GOPs is to assure a consistent alignment of keyframes across renditions reusing the same first pass statfile across renditions.

ffmpeg –i IN –pass 1 –an –vcodec libx264 –r 30 –b 1500k –bufsize 1500k –keyint_min 60 –g 120 –s 1280×720 –vpre slower_fastfirstpass OUT_1500.mp4

This command line is the first pass of the first rendition. The first pass generates a stat file for the second pass.

ffmpeg –i IN –pass 2 –an –vcodec libx264 –r 30 –b 1500k –bufsize 1500k –keyint_min 60 –g 120 –s 1280×720 –vpre slower OUT_1500.mp4

Instead of recreating a first pass stat file for the next renditions, you can use the previous simply launching the second passes of the next renditions

ffmpeg –i IN –pass 2 –an –vcodec libx264 –r 30 –b 1000k –bufsize 1000k –keyint_min 60 –g 120 –s 854×480 –vpre slower O_1000.mp4
ffmpeg –i IN –pass 2 –an –vcodec libx264 –r 30 –b 500k –bufsize 500k –keyint_min 60 –g 120 –s 640×360 –vpre slower O_500.mp4

Since the second pass is less accurate if it use a stat file generated with a too much different resolution and bitrate, may be better to use a rendition in the middle to generate the first pass and not the highest rendition.

2. AAC encoding

libfaac has been extracted from ffmpeg and is now an external library. There are two alternatives yet embedded inside ffmpeg: libvo_aacenc or the standard aac library.

ffmpeg input.mp3 -c:a libvo_aacenc -b:a 96k -ac 2 -ar 44100 output.aac

ffmpeg input.mp3 -c:a aac -strict experimental -b:a 96k -ac 2 -ar 44100 output.aac

I have tested both and it seems to me that libvo is the best alternative. It produces a sufficiently good AAC LC.
In a future article I’ll explore some alternative like encoding audio track externally and remux then with ffmpeg or mp4box.
This is a must go if you need the higher efficiency of HE AAC or HE AAC v2.

3. Joining video

Joining video is strangely a complex task with FFmpeg. A reader suggested this solution (via Steven’s Blog):

ffmpeg -ss 100 -t 10 -i in.mp4 -c copy -bsf h264_mp4toannexb 100.h264

ffmpeg -ss 200 -t 10 -i in.mp4 -c copy -bsf h264_mp4toannexb 200.h264

ffmpeg -i concat:”100.h264|200.h264″ -i in.mp3 -c copy out.mp4

The first two lines generate two h.264 elementary streams. The h264_mp4toannexb option is mandatory to be able to concatenate efficiently elementary streams at binary level.

The third line use the concat option to cancatenate the ES segments to form a new input.
I usually use mp4box for this kind of purpose and not FFmpeg.

4. Use an HLS stream as source 

FFmpeg now supports also Apple HTTP Live Streaming as an input protocol. So it is really simple to acquire or repurpose an HLS streaming, simply specify the path to .m3u8 manifest.

ES: Do you want to stream an existing .m3u8 stream to Flash on the desktop using FMS (now AMS) ? Try this:

ffmpeg -re -i http://server/path/stream.m3u8 -c copy -f flv "rtmp://FMSserver/app/streamName live=1"

5. Record a stream endlessly rotating target file 

Segmenting feature of FFmpeg can also be useful to create an endless recorder with rotating buffer. It can be done using the segment_wrap parameter that wraps around segment index once it reached a limit.

ffmpeg -i rtmp://INPUT -codec copy -f segment -segment_list out.list -segment_time 3600 -segment_wrap 24 out%03d.mp4

The previous commandline records endlessly the INPUT stream in a ring buffer formed by 24 chunk of 1hr video.Conclusionfollow me on twitter to know more about FFmpeg and video related topics (@sonnati).

[Index]

PART I – Introduction (revised 02-jul-2012)
PART II – Parameters and recipes (revised 02-jul-2012)
PART III – Encoding in H.264 (revised 02-jul-2012)
PART IV – FFmpeg for streaming (revised 02-jul-2012)
PART V – Advanced usage (revised, 19-oct-2012)
PART VI – Filtering (new, 19-oct-2012)