HDMI Input Accessory Manual

Table of contents

  1. Overview
  2. Software Setup
  3. Initial Hardware Setup
  4. Hardware Startup Sequence
  5. Software Startup Sequence
    1. Start voxl-v4l2
    2. Start The ffmpeg Docker
    3. Stream Video
    4. Additioanl Params to Tweak
    5. View In QGroundControl
  6. How to Run on Startup


The HDMI Input Accessory allows you to connect a camera with HDMI output to VOXL or VOXL Flight. The VOXL hardware powers the adapter. Using software on VOXL, you can save video to file or stream to a Ground Control Station like QGC.


Software Setup

The following are required to use the VOXL HDMI Input Accessory:

  • System Image 2.5.0 or newer (currently available upon request only)
  • The voxl-v4l2 package (currently available upon request only)
adb push voxl-v4l2_0.0.3.ipk /home/root/ipk

adb shell opkg install /home/root/ipk/voxl-v4l2_0.0.3.ipk

Initial Hardware Setup

  • Ensure VOXL is powered off
  • Obtain the HDMI Input Accessory (VOXL-ACC-HDMI)





  • Connect the HDMI Input Accessory’s J1 to VOXL’s J2 as shown


  • Connect the HDMI cable to the input as shown


  • Connect the HDMI cable to the camera, for example the Sony RX1R II

Hardware Startup Sequence

The following should be done before each use.

  • Power on camera first
  • Power on VOXL
    • The PWR LED illuminates on the HDMI Input Accessory (in the upper left of this picture)


Software Startup Sequence

Start voxl-v4l2

  • Connect to the VOXL shell
  • You can validate hardware connection is OK using a dmesg command:
dmesg | grep 'auvidea'

[    5.695538] auvidea_b102 probe succeeded
  • Start the voxl-v4l2 program, in this case with the -c sony-rx1r option and 10 FPS. This setting can be modified.

    Note: You can run -h to see supported cameras.

voxl-v4l2 -c sony-rx1r -f 10

You should see the following:

 voxl-v4l2 -c sony-rx1r -f 10

Starting voxl-v4l2
sony-rx1r chosen
Frame rate: 10
Using command line frame rate 10
Info: probing for auvidea_b102
Info: device probe succeeded
Probe: sensor_name auvidea_b102
Probe: session_id 2
Info: Could not release csid device
Info: Could not power down the sensor
Waiting for DDC 5V
DDC 5V detected
Waiting for HDMI sync
Sync established!
HDMI pclk 148.080 MHz
HDMI total resolution: 2200 x 1125, active resolution: 1920 x 1080
YCbCr709 (Limited) 1080p YUV422 progressive
buffer processing thread starting

Leave this process running.

Start The ffmpeg Docker

  • After approximately 10 seconds after running the last command, you can start the docker

If you pull the image from the gcr.io repository, run the following to launch the docker container:

docker run -it --rm --privileged --net=host -v /dev/frame0:/opt/frame0 gcr.io/modalai-public/voxl-ffmpeg:v1.0

Or if you built the image yourself:

docker run -it --rm --privileged --net=host -v /dev/frame0:/opt/frame0 voxl-ffmpeg:v1.0
  • You will enter an interactive shell

NOTE: if you don’t run voxl-v4l2 before launching the docker container, the /dev/frame0 will not exist and the system will not work with an error similar to this

Output file is empty, nothing was encoded (check -ss / -t / -frames parameters if used)

Stream Video

Now, streaming brings with it many configurations that may or may not be valid for your use case. Details can be found at https://trac.ffmpeg.org/wiki/Encode/H.264

The following will stream <DESIRED RESOLUTION> to QGroundControl at <QGC IP ADDRESS>

ffmpeg -f rawvideo -pix_fmt uyvy422 -s:v 1920x1080 -i /opt/frame0 -vcodec libx264 -pix_fmt yuv420p -preset ultrafast -tune zerolatency -f mpegts -s:v <DESIRED RESOLUTION> udp://<QGC IP ADDRESS>:4242

For example, 640x480 to

ffmpeg -f rawvideo -pix_fmt uyvy422 -s:v 1920x1080 -i /opt/frame0 -vcodec libx264 -pix_fmt yuv420p -preset ultrafast -tune zerolatency -f mpegts -s:v 640x480 udp://

You’ll see something similar to the following ouptut:

ffmpeg version 4.2.4 Copyright (c) 2000-2020 the FFmpeg developers
  built with gcc 9.2.0 (Alpine 9.2.0)


frame=   85 fps= 10 q=18.0 size=     135kB time=00:00:03.36 bitrate= 330.3kbits/s speed=0.398x

Additioanl Params to Tweak

  • -r - frame rate
  • crf - constant rate factor
  • -minrate and -maxrate - bit rates
  • -bufsize
-crf 10 -minrate 1M -maxrate 2M -bufsize 1M

View In QGroundControl

In this example, we are streaming to QGroundControl. You should configure the video settings with:

  • Video Source: MPEG-TS (h.264) Video Stream
  • UDP Port: 4242

How to Run on Startup

  • Create two new files:



ExecStart=/usr/bin/voxl-v4l2 -c sony-rx1r -f 10



  • NOTE- you need to configure IP address and resolution, see <DESIRED RESOLUTION> and <<QGC IP ADDRESS>
After=docker-start.service docker-autorun.service voxl-v4l2.service
Requires=docker-start.service docker-autorun.service voxl-v4l2.service

ExecStartPre=/bin/sleep 10
ExecStart=/usr/bin/docker run --rm --privileged --net=host -v /dev/frame0:/opt/frame0 gcr.io/modalai-public/voxl-ffmpeg:v1.0 ffmpeg -f rawvideo -pix_fmt uyvy422 -s:v 1920x1080 -i /opt/frame0 -vcodec libx264 -pix_fmt yuv420p -preset ultrafast -tune zerolatency -f mpegts -s:v <DESIRED RESOLUTION, e.g. 640x480> udp://<QGC IP ADDRESS>:4242
  • Copy to VOXL:
adb push voxl-v4l2.service /etc/systemd/system
adb push ffmpeg-docker.service /etc/systemd/system
  • Enable Services to run on bootup:
adb shell systemctl enable voxl-v4l2
adb shell systemctl enable ffmpeg-docker

On the next bootup, the two services will run. To start now, you can use:

adb shell systemctl start voxl-v4l2
adb shell systemctl start ffmpeg-docker

To check status:

systemctl status voxl-v4l2
systemctl status ffmpeg-docker