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Thread: HOWTO: Proper Screencasting on Linux

  1. #211
    Join Date
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    Re: HOWTO: Proper Screencasting on Linux

    Quote Originally Posted by coldraven View Post
    if I want medium quality mono sound where do I alter the argument?
    As an output option (anywhere after "-i input" and before "output.foo") you can add "-ac 1". If you apply as an input option you are telling the decoder that the input is i channel, even if it is stereo, which probably isn't something you want to do (or maybe ffmpeg will ignore it).

    Vorbis CBR:
    Code:
    ffmpeg -i input -c:v libvorbis -ac 1 -b:a 32k -c:v libvpx ... output.webm
    Vorbis VBR:
    Code:
    ffmpeg -i input -c:v libvorbis -ac 1 -q:a 3 -c:v libvpx ... output.webm
    See Recommended Vorbis Encoder Settings for an idea of what "-q:a" value to use.

    I'd like to figure out how to use libvpx and then write a proper FFmpeg guide for it (and HTML5 video with ffmpeg)... but hopefully someone will beat me to it.

    See the FFmpeg and AAC Encoding Guide for full details and examples for AAC encoding, and FFmpeg and AAC Encoding Guide for encoding to H.264 video.

    Quote Originally Posted by coldraven View Post
    I'm presuming that it is better to capture directly into the format I want rather than transcoding afterwards.
    The main example in this guide attempts to capture as fast as possible without having to work on compressing things very much so you can achieve your desired framerate (assuming your CPU is the bottleneck). It then suggests re-encoding this temporary, large, lossless output to your desired final output(s). This may also be the best choice for you if you're going to make a mp4 and webm from the same source.

  2. #212
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    Hyperborea
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    Ubuntu

    Re: HOWTO: Proper Screencasting on Linux

    The main example in this guide attempts to capture as fast as possible without having to work on compressing things very much so you can achieve your desired framerate (assuming your CPU is the bottleneck). It then suggests re-encoding this temporary, large, lossless output to your desired final output(s). This may also be the best choice for you if you're going to make a mp4 and webm from the same source.
    Aha! Thanks for that. Progress has been delayed by a visit to the pub
    You will probably hear from me later after I have become totally confused, I need to get my head around all these switches and codecs.
    <goes off to wiki page>

  3. #213
    Join Date
    Aug 2006
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    53
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    Ubuntu 11.04 Natty Narwhal

    Re: HOWTO: Proper Screencasting on Linux

    Hi after reading this tutorial I tried to find gui ffmpeg and found amazing softvare recently released. It is Screencastor. There is also video explaining by some guy how to use it

    Code:
    http://www.youtube.com/watch?v=UQ43XnHM8gI
    I have found official link for screencastor

    Code:
    https://launchpad.net/~hizo/+archive/screencastor
    Happy casting!!
    Last edited by moonliter; February 19th, 2013 at 01:11 PM.

  4. #214
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    Ubuntu 12.10 Quantal Quetzal

    Re: HOWTO: Proper Screencasting on Linux

    Hello again!

    So, any ideas on my (and marvalis', apparently) desync problem?

    Quote Originally Posted by dannyboy79 View Post
    i would suggest turning down your frames per second. give that a try
    As I said - this does not help at all - even if I record in 960x540 @ 15 fps, it desyncs the same way as when I record, for example, in 1920x1080 @ 40 fps. I can see my CPU is mostly idle when recording in low res && low fps, and my HDD is accessed rather rarely. When recording in 1920x1080@40, my CPU still has a lot of headroom. And I can also see my HDD can take it all with no problems. Thus, it's definitely not a performance problem.

    MEANWHILE!!! I tried "-tune zerolatency" option today. It eliminates desycing almost completely! But - it is not usable, unfortunately. Sooner or later, I start seeing things like this spammed in ffmpeg's output:
    Code:
    [matroska @ 0x2b8e420] st:0 PTS: 922148 DTS: 922148 < 922246 invalid, clipping
    And when that starts, I get severe audio corruption - all the sound that is recorded after this starts is virtually unusable (parts full of "clicks", periods of silence, parts with the audio "jumping over" by a couple of secs, and so on).

    Any ideas? Please - I REALLY don't want to be forced to use Windoze just to record things (where I get no such issues whatsoever)
    Why do you wear glasses if you're deaf?
    Code:
    while(1) {
            alone();
    }

  5. #215
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    Feb 2012
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    Re: HOWTO: Proper Screencasting on Linux

    Same problem here. I am doing one hour screencasts. When I start the screencast the audio is in sync. As time progresses the audio/video become more and more out of sync. I suspect all ffmpeg users have this problem, they just don't know it. If you do a five minute screencast you won't see the de-sync issues. After one hour of video it becomes apparent.

  6. #216
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    Mar 2013
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    Re: HOWTO: Proper Screencasting on Linux

    Recording audio seems to be broken with avconv. The problem is that it records audio at a pretty significantly faster rate than video, so in the recording the audio rate is slower than the video rate, causing them to be out of sync, and even causing the audio to have a lower tone. You can even try recording just audio to a .wav and recording just video to an .mp4 at the exact same time in separate windows, and you'll notice that the resulting audio file is significantly longer (9-10% longer in my quick, inaccurate tests). Even the timestamps in the avconv output for audio recording reports that the recording time is increasing faster than realtime. I am currently looking into possible solutions. Are all of the rest of you also using the avconv version from libav-tools? It may be a bug that has been fixed since then. I'll try compiling the most recent version and see if that fixes the issue.

    It's a real shame too, because video recording works so perfectly.

    Edit: VenisLentoje, I just realized you said you were using ffmpeg. Can you tell me what version you're using please? I'm using the avconv from libav-tools, version 8.5. I'm really interested as to whether this is just a setting issue, or if the way that they both record audio is just garbage.
    Last edited by Jacob Mischka; March 17th, 2013 at 11:15 AM.

  7. #217
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    Re: HOWTO: Proper Screencasting on Linux

    This guide is designed for real ffmpeg, not avconv or the fake, bizarro "ffmpeg" from the repositroy. From what I've seen avconv is fairly buggy, but you can compile ffmpeg and see if it makes a difference.

  8. #218
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    Re: HOWTO: Proper Screencasting on Linux

    The two other people that complained about the same issue were using your precious ffmpeg, so unless one of them says their issue is different from mine, that means nothing in this case.

    But I'll try compiling ffmpeg and seeing if that fixes the issue, which I rather doubt. If it does this will be the first time I've seen the two differ at all in actual usage.

    Edit: Done, same thing happens. Audio is stretched, therefore lower toned, and becomes increasingly out of sync as time goes on. It's either a setting issue, a hardware issue (which I doubt), or simply the way they both capture audio is bad.

    Edit: I realize my mention of stretched audio brings async to mind (or whatever equivalent option is in ffmpeg), but using it doesn't help as someone else already mentioned.

    Edit again: I have also used multiple sound cards to no effect, and captured directly from microphone input and output.monitor sources. So it's not our cards, and it's not pulse loopbacks being bad. And it even happens when recording only audio .wavs, so it isn't our cpu not being able to handing the encoding. And of course audio capture with other applications is normal and works as intended.
    Last edited by Jacob Mischka; March 17th, 2013 at 09:44 PM.

  9. #219
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    Re: HOWTO: Proper Screencasting on Linux

    Quote Originally Posted by Jacob Mischka View Post
    The two other people that complained about the same issue were using your precious ffmpeg, so unless one of them says their issue is different from mine, that means nothing in this case.
    My preciousss...

    Quote Originally Posted by Jacob Mischka View Post
    But I'll try compiling ffmpeg and seeing if that fixes the issue, which I rather doubt. If it does this will be the first time I've seen the two differ at all in actual usage.
    I seen the opposite on many occasions. You seem annoyed that I've asked you to try a recent build. The reason for this test is that FFmpeg development is very active and this is always the first step to determine if you are experiencing an already fixed bug. Otherwise you are wasting your time and the time of anyone attempting to help you.

    Now that you got that step out of the way I recommend you ask for assistance at the ffmpeg-user mailing list; developers and other usually knowledgeable users try to answer most questions. Since they are all volunteers, and since there can be many questions per day, asking a well thought out and detailed question will get a better chance of an answer. This means that at a minimum you need to: use a recent ffmpeg build, include your full ffmpeg command, and include your complete ffmpeg console output (or at least the first 50 and last 50 lines). Responses may seem terse, but this is just the effect of trying to be efficient due to a large questions to answering volunteers ratio.

    If you are convinced this is a bug then consider reporting it.

  10. #220
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    Re: HOWTO: Proper Screencasting on Linux

    Not really that you asked me to try a newer build, just that you seemed to cast it off as a solely avconv issue when they are nearly identical and when two people already reported the same issue. I guess I'll submit a bug report. I would have anyway, I was just hoping someone around here would have heard of the issue. Especially since the sole purpose of this thread is about capturing audio along with video. I find it difficult to believe others are having flawless lossless audio recording with these tools, as many people have issues with it. Googling returns nothing, since so many people have different problems with avconv/ffmpeg capturing audio.

    I will try using jack or selecting a specific card with alsa so to bypass pulse and see if that changes anything and report back.
    Last edited by Jacob Mischka; March 18th, 2013 at 08:11 PM.

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