belacoder - live video encoder with dynamic bitrate control and SRT support

This is a gstreamer-based encoder with support for SRT and dynamic bitrate control depending on the network capacity. This means that if needed, the video bitrate is automatically reduced on-the-fly to match the speed of the network connection. The intended application is live video streaming over bonded 4G modems by using it on a single board computer together with a HDMI capture card and strla.

belacoder is developed on an NVIDIA Jetson Nano (Amazon.com / Amazon.co.uk / NVIDIA), and we provide gstreamer pipelines for using its hardware video encoding. However it can also be used on other platforms as long as the correct gstreamer pipeline is provided.

Building

Installing the dependencies on L4T / Ubuntu:

sudo apt-get install build-essential git libgstreamer1.0-dev libgstreamer-plugins-base1.0-dev # let me know if I forgot anything
sudo apt-get install libsrt-dev # only available in Ubuntu 20.04

# alternatively, manually installing SRT on older Ubuntu releases
git clone https://github.com/BELABOX/srt.git
cd srt
./configure --prefix=/usr/local
make -j 4
sudo make install
sudo ldconfig

Building belacoder:

git clone https://github.com/BELABOX/belacoder.git
cd belacoder
make

Usage

Syntax: belacoder PIPELINE_FILE ADDR PORT [options]

Options:
  -d <delay>          Audio-video delay in milliseconds
  -s <streamid>       SRT stream ID
  -l <latency>        SRT latency in milliseconds
  -b <bitrate file>   Bitrate settings file, see below

Bitrate settings file syntax:
MIN BITRATE (bps)
MAX BITRATE (bps)
---
example for 500 Kbps - 60000 Kbps:

    printf "500000\n6000000" > bitrate_file

---
Send SIGHUP to reload the bitrate settings while running.

Where:

• PIPELINE_FILE is a text file containing the gstreamer pipeline to use. See the pipeline directory for ready-made pipelines.
• ADDR is the hostname or IP address of the SRT listener to stream to (only applicable when the gstreamer sink is appsink name=appsink)
• PORT is the port of the SRT listener to stream to (only applicable when the gstreamer sink is appsink name=appsink)
• -d <delay> is the optional delay in milliseconds to add to the audio stream relative to the video (when using the gstreamer pipelines supplied with belacoder)
• -b <bitrate file> is an optional argument for setting the minimum and maximum video bitrate (when using the gstreamer pipelines supplied with belacoder). These settings are reloaded from the file and applied when a SIGHUP signal is received.

Gstreamer pipelines

The gstreamer pipelines are available in the pipeline directory, organised in machine-specific directories (for pipelines using hardware-accelerated features) or generic (for software-only pipelines). The filename format is CODEC_CAPTUREDEV_[RES[FPS]]:

• CODEC is h265 or h264 (for system-specific hw encoders), or x264_superfast / x264_veryfast for x264 software encoding
• CAPTUREDEV is either camlink for Elgato Cam Link 4K (Amazon.com / Amazon.co.uk) or other uncompressed YUY2 capture cards or v4l_mjpeg for low cost USB2.0 MJPEG capture cards (Amazon.com / Amazon.co.uk)
• RES can be blank - capturing at the highest available resolution, 720p, 1080p, 1440p, or 4k_2160p
• FPS can be blank - capturing at the highest available refresh rate, 29.97, or 30 FPS

Note that to encode 4k / 2160p video captured by a camlink you must specifically use h265_camlink_4k_2160p rather than h265_camlink, as the preset-level quality setting of the encoder must be set to a lower value to allow the encoder to maintain 30 FPS in all conditions.

Please check the supplied pipelines for examples. Here are a few unorganised tips & pointers:

• belacoder will work with arbitrary gstreamer pipelines as long as they're valid, however for dynamic bitrate control the video encoder must have name=venc_bps or name=venc_kbps and it must have a bitrate property changeable in the running state; the sink must be appsink name=appsink, which will stream to the SRT IP and port specified as command line arguments
• If a textoverlay element with name=overlay is specificed, then it will be dynamically updated to show the current bitrate
• identity name=a_delay signal-handoffs=TRUE and identity name=v_delay signal-handoffs=TRUE elements can be used to adjust the PTS (presentation timestamp) of the audio and video streams respectively by DELAY milliseconds. Use them to synchronise the audio and video if needed (e.g. audio delay of around 900 for a Gopro Hero7 with stabilisation enabled)
• The Jetson Nano hardware encoders seem biased towards allocating most of the bitrate budget to I-frames, while heavily compressing P-frames, especially on lower bitrates. This can heavily affect image quality when most of the image is moving and this is why we limit the quantization range in our pipelines using qp-range. This range makes a big improvement over the defaults, however in some cases results can probably be further improved with different parameters.