Streaming Linux->Twitch using ffmpeg and ALSA

I stopped using OBS a while ago for a couple reasons–the main one was that it didn’t support my video capture card, but I also had issues with it crashing or lagging behind with no clear indication of what it was doing. I ended up switching to ffmpeg for live streaming, because it’s very easy to tell when ffmpeg is lagging behind. OBS uses ffmpeg internally for video. I don’t especially recommend this setup, but I thought I’d document it in case someone can’t use a nice GUI setup like OBS or similar.

I’m prefer less layers, so I’m still on ALSA. My setup is:

  • I have one computer, running linux. It runs what I’m streaming (typically minecraft), and captures everything, encodes everything, and sends it to twitch
  • Video is captured using libxcb (captures X11 desktop)
  • Audio is captured using ALSA. My mic is captured directly, while the rest of my desktop audio is sent to a loopback device which acts as a second mic.
  • Everything is encoded together into one video stream. The video is a Flash video container with x264 video and AAC audio, because that’s what twitch wants. Hopefully we’ll all switch to AV1 soon.
  • That stream is sent to twitch by ffmpeg
  • There is no way to pause the stream, do scenes, adjust audio, see audio levels, etc while the stream is going. I just have to adjust program volumes independently.

Here’s my .asoundrc:

My ffmpeg build line:

And most imporantly, my ffmpeg command:

Let’s break that monster down a bit. ffmpeg structures its command line into input streams, transformations, and output streams.

ffmpeg input streams

-video_size 1280x720 -framerate 30 -f x11grab -s 1280x720 -r 30 -i :0.0:
Grab 720p video (-video_size 1280x720) at 30fps (-framerate 30) using x11grab/libxcb (-f x11grab), and we also want to output that video at the same resolution and framerate (-s 1280x720 -r 30). We grab :0.0 (-i :0.0)–that’s X language for first X server (you only have one, probably), first display/monitor. And, since we don’t say otherwise, we grab the whole thing, so the monitor better be 720p.

-f alsa -ac 1 -ar 48000 -i hw:1,0:
Using alsa (-f alsa), capture mono (-ac 1, 1 audio channel) at the standard PC sample rate (-ar 48000, audio rate=48000 Hz). The ALSA device is hw:1,0 (-i hw:1,0), my microphone, which happens to be mono.

-f alsa -ac 2 -ar 48000 -i hw:Loopback,1:
Using alsa (-f alsa), capture stereo (-ac 2, 2 audio channels) at the standard PC sample rate (-ar 48000, audio rate=48000 Hz). The ALSA device is hw:Loopback,1. In the ALSA config file .asoundrc given above, you can see we send all computer audio to hw:Loopback,0. Something sent to play on hw:Loopback,0 is made available to record as hw:Loopback,1, that’s just the convention for how snd-aloop devices work.

ffmpeg transforms

-filter_complex '[1:a][1:a]amerge=inputs=2[stereo1] ; [2:a][stereo1]amerge=inputs=2[a]' -ac 2:
All right, this one was a bit of a doozy to figure out. In ffmpeg’s special filter notation, 1:a means “stream #1, audio” (where stream #0 is the first one).

First we take the mic input [1:a][1:a] and convert it from a mono channel to stereo, by just duplicating the sound to both ears (amerge=inputs=2[stereo1]). Then, we combine the stereo mic and the stereo computer audio ([2:a][stereo1]) into a single stereo stream using the default mixer (amerge=inputs=2[a]).

-map '[a]' -map 0:v :
By default, ffmpeg just keeps all the streams around, so we now have one mono, and two stereo streams, and it won’t default to picking the last one. So we manually tell it to keep that last audio stream we made (-map '[a]'), and the video stream from the first input (-map 0:v, the only video stream).

ffmpeg output streams

-f flv -ac 2 -ar 48000:
We want the output format to be Flash video (-f flv) with stereo audio (-ac 2) at 48000Hz (-ar 48000). Why do we want that? Because we’re streaming to Twitch and that’s what Twitch says they want–that’s basically why everything in the output format.

-vcodec libx264 -g 60 -keyint_min 30 -b:v3000k -minrate 3000k -maxrate 3000k -pix_fmt yuv420p -s 1280x720 -preset ultrafast -tune film:
Ah, the magic. Now we do x264 encoding (-vcodec libx264), a modern wonder. A lot of the options here are just what Twitch requests. They want keyframes every 2 seconds (-g 60 -keyint_min 30, where 60=30*2=FPS*2, 30=FPS). They want a constant bitrate (-b:v3000k -minrate 3000k -maxrate 3000k) between 1K-6K/s at the time of writing–I picked 3K because it’s appropriate for 720p video, but you could go with 6K for 1080p. Here are Twitch’s recommendations. The pixel format is standard (-pix_gmt yub720p) and we still don’t want to change the resolution (-s 1280x720). Finally the options you might want to change. You want to set the preset as high as it will go with your computer keeping up–mine sucks (-preset ultrafast, where the options go ultrafast,superfast,veryfast,faster,fast,medium, with a 2-10X jump in CPU power needed for each step). And I’m broadcasting minecraft, which in terms of encoders is close to film (-tune film)–lots of panning, relatively complicated stuff on screen. If you want to re-encode cartoons you want something else.

-c:a libfdk_aac -b:a 160k:
We use AAC (-c:a libfdk_aac). Note that libfdk is many times faster than the default implementation, but it’s not available by default in debian’s ffmpeg for (dumb) license reasons. We use 160k bitrate (-b:a 160k ) audio since I’ve found that’s good, and 96K-160K is Twitch’s allowable range. -strict normal

-strict normal: Just an ffmpeg option. Not interesting.
-bufsize 3000k: One second of buffer with CBR video

rtmp://live-sjc.twitch.tv/app/${TWITCH_KEY}:
The twitch streaming URL. Replace ${TWITCH_KEY} with your actual key, of course.

Sources:

  • jrayhawk on IRC (alsa)
  • ffmpeg wiki and docs (pretty good)
  • ALSA docs (not that good)
  • Twitch documentation, which is pretty good once you can find it
  • mark hills on how to set up snd-aloop

Capturing video on Debian Linux with the Blackmagic Intensity Pro 4K card

Most of this should apply for any linux system, other than the driver install step. Also, I believe most of it applies to DeckLink and Intensity cards as well.

My main source is https://gist.github.com/afriza/879fed4ede539a5a6501e0f046f71463. I’ve re-written for clarity and Debian.

  1. Set up hardware. On the Intensity Pro 4K, I see a black screen on my TV when things are set up correctly (a clear rectangle, not just nothing).
  2. From the Blackmagic site, download “Desktop Video SDK” version 10.11.4 (not the latest). Get the matching “Desktop Video” software for Linux.
  3. Install the drivers. In my case, these were in desktopvideo_11.3a7_amd64.deb.
    After driver install, lsmod | grep blackmagic should show a driver loaded on debian.
    You can check that the PCI card is recognized with lspci | grep Blackmagic (I think this requires the driver but didn’t check)
  4. Update the firmware (optional). sudo BlackmagicFirmwareUpdater status will check for updates available. There were none for me.
  5. Extract the SDK. Move it somewhere easier to type. The relevant folder is Blackmagic DeckLink SDK 10.11.4/Linux/includes. Let’s assume you move that to ~/BM_SDK
  6. Build ffmpeg from source. I’m here copying from my source heavily.
    1. Get the latest ffmpeg source and extract it. Don’t match the debian version–it’s too old to work.
      wget https://ffmpeg.org/releases/ffmpeg-4.2.tar.bz2 && tar xf ffmpeg-*.tar.bz2 && cd ffmpeg-*
    2. Install build deps.
      sudo apt-get install nasm yasm libx264-dev libx265-dev libnuma-dev libvpx-dev libfdk-aac-dev libmp3lame-dev libopus-dev libvorbis-dev libass-dev
    3. Build.
      PKG_CONFIG_PATH="$HOME/ffmpeg_build/lib/pkgconfig" ./configure --prefix="$HOME/ffmpeg_build" --pkg-config-flags="--static" --extra-cflags="-I$HOME/ffmpeg_build/include -I$HOME/ffmpeg_sources/BMD_SDK/include" --extra-ldflags="-L$HOME/ffmpeg_build/lib" --extra-libs="-lpthread -lm" --enable-gpl --enable-libass --enable-libfdk-aac --enable-libfreetype --enable-libmp3lame --enable-libopus --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-nonfree --enable-decklink

      make -j $(nproc)

      sudo cp ffmpeg ffprobe /usr/local/bin/
  7. Use ffmpeg.
    ffmpeg -f decklink -list_devices 1 -i dummy should show your device now. Note the name for below.

    ffmpeg -f decklink -list_formats 1 -i 'Intensity Pro 4K' shows supported formats. Here’s what I see for the Intensity Pro 4K:

Capture some video: ffmpeg -raw_format argb -format_code Hp60 -f decklink -i 'Intensity Pro 4K' test.avi

The format (raw_format and format_code) will vary based on your input settings. In particular, note that-raw_format uyvy422 is the default, which I found did not match my computer output. I was able to switch either the command line or the computer output settings to fix it.

Troubleshooting

  • I’m not running any capture, but passthrough isn’t working. That’s how the Intensity Pro 4K works. Passthrough is not always-on. I’d recommend a splitter if you want this for streaming.
  • ffmpeg won’t compile. Your DeckLink SDK may be too new. Get 10.11.4 instead.
  • I can see a list of formats, but I can’t select one using -format_code. ffmpeg doesn’t recognize the option. Your ffmpeg is too old. Download a newer source.
  • When I look at the video, I see colored bars. The HDMI output turns on during recording. The Intensity Pro 4K outputs this when the resolution, hertz, or color format does not match the input. This also happens if your SDK and driver versions are mismatched.

Sources: