FAQ
What is the minimum hardware I need to try this?
Two Raspberry Pi, two supported WiFi adapters, one Pi cam (Or CSI-HDMI Adapter), and 2 good quality (preferably Industrial micro SD Cards). See Getting Started for a minimal example.
Please read the rest of the documentation and do your homework when you make your purchases! Only specific WiFi cards will work due to the unique requirements of the system.
What is the lowest latency I can achieve?
On a Raspberry Pi, 110ms “glass to glass”. Although most setups are in the 125ms range.
On other boards it can be significantly lower, as low as 40-60ms has been demonstrated with the Jetson Nano and a carefully selected camera. A public release supporting SBC's other than the Raspberry is forthcoming.
What kind of range can I expect?
It depends, 1-3KM is easy to achieve even with low power WiFi cards and the antennas that come with them.
Carefully chosen WiFi cards, antennas, and optionally an antenna tracker should put 20KM+ within reach, and the current record is 75KM on 5.8Ghz frequency with a 30dbi gain panel antenna.
How long does OpenHD take to regain a lost “connection”?
It depends on the kind of camera you are using, if it is a Raspberry Pi camera and the settings are still left on the defaults, the system should recover from serious interference within 300ms at most, more likely the interference will only partially disrupt the video and you will see momentary noise that will clear up rapidly.
When I lose connection will I see a “blue screen”?
OpenHD does not generate a solid blue screen when interference is encountered. If experiencing a loss of signal you will see progressive artifacts and pixilation, finally the image typically freezes. The telemetry and/or RC link is more robust and will often continue to operate well beyond the loss of the video stream.
Does OpenHD interfere with my RC transmitter?
You can use 2 different bands, 5.8Ghz for OpenHD, and 2.4Ghz for RC. You can also send RC control through OpenHD itself, and avoid using 2 different transmitters. There are tradeoffs however, control latency may be slightly higher than a dedicated RC system, and there are currently some limitations on the channel count.
What is a Raspberry Pi?
A computer on a single circuit board. In this case running a derivative of Linux. Don't worry you do not need to know anything about the software side, we have that covered!
Do I need a WiFi adapters for video, another for telemetry, and another for RC control?
No, just one for ground and one for air.
You can optionally use 2x WiFi adapters on the ground side for "diversity", which improves signal reception.
Can I run OpenHD on this other board (Orange Pi, Banana Pi, BeagleBoard)?
In OpenHD 2.0, no. The system is designed around the Raspberry Pi as it is cheap, widely available, and the video encoder/decoder hardware works properly.
In OpenHD 2.1, several other boards will be supported, however there will be some hardware limitations. Most other boards do not have anywhere near the level of stable software support that the Raspberry Pi benefits from. Most other boards also do not have a CSI camera connector, and even those that do may not have properly working video encoder hardware. On those boards, the only option will be IP/USB cameras that have an internal h264 video encoder.
Note that webcams and other USB video devices that do not have an internal h264 video encoder are not usable with OpenHD, the USB interface is not capable of transferring 720p60 or 1080p30/60 video.
Which WiFi adapters do I need?
Can I use this other WiFi adapter that’s not on the list?
Perhaps, but most likely not.
Only a few WiFi adapters are able to work the way OpenHD needs them to, which limits the selection of WiFi adapters you can use.
Why can’t I just use the Raspberry Pi onboard WiFi?
The built-in WiFi chip on the Raspberry Pi does not work the way OpenHD needs it to for video broadcast, however it can still be used for "hotspot" purposes, the you can connect Android and other devices to the ground station over normal WiFi to receive the video signal.
Why am I seeing noise or interference in the video?
There are many possibilities.
You might be experiencing interference from normal WiFi devices. If that is the case, try another frequency or change location. In most cases if you are outside in a suitable location for flying, there are not many normal WiFi devices around.
If you have a long CSI camera cable attached, it may be generating or receiving interference. You can test this by wrapping it in foil or getting a shorter cable. The kind of interference you will see in this case is very specific, it will look like perfectly straight vertical lines.
The AirPi and GroundPi may also be too close together if you are testing on the ground, this can cause the receiving WiFi card to be overloaded.
It is generally necessary to solder power wires and even the USB data wires directly to the WiFi adapters. The Raspberry Pi USB ports are not capable of supplying enough power to cards that require a lot of it, and in some cases the USB connectors can briefly disconnect during flight which may lead to total video loss. if you are still seeing interference or even video freezes, this should be the next thing you address.
Why do I get a low power warning?
In OpenHD 2.0 you should not be seeing a power warning, however you must still carefully check and wire the WiFi cards correctly to avoid power issues.
How many WiFi adapters can I use?
2x on the air side, 3x on the ground with a Pi3b+. If you are going to connect this many WiFi adapters you are strongly encouraged to use a Pi4b instead of a Pi3b+ for stability.
You can use 4x with a Pi4b, but this is overkill.
Where can I buy the hardware I need for OpenHD?
eBay
Amazon
banggood.com
AliExpress
TaoBao (chinese marketplace)
Raspberry Pi are the easiest to find, and you can use any model, but there is no reason to use the old original Raspberry Pi and it is unlikely to work very well.
Is OpenHD legal to use?
Open.HD uses normal WiFi hardware, which is perfectly legal to buy and use.
However, some frequencies such as the 2.3Ghz range, and high power settings, might not be legal in your location.
Open.HD can be disruptive to nearby WiFi networks due to the continuous broadcast, and some of the things that can be changed in the Open.HD settings file can make this worse.
You are responsible for your use of the system, but please ask us for advice if you are concerned about a particular setting or use case.
Can someone else watch my video stream?
Yes, you can receive the video with any GroundPi that is configured to the same frequency as your AirPi.
The system is not encrypted in Open.HD 2.0, but encryption is being added.
Where did my recorded video go?
See the wiki: Ground Recording
Why is the OSD not on the video recording?
In older versions of OpenHD, this was not easily possible due to the way the GPU works in the Raspberry Pi, however it is being added in the new QOpenHD OSD, which can run on the ground station.
What is diversity?
A way to improve the reliability and quality of video reception.
If you connect 2 or more WiFi adapters to the GroundPi, whichever adapter is currently receiving the best signal will be used. This is entirely automatic and occurs in realtime, you do not have to change any settings or monitor anything.
Some OpenHD users connect different kinds of antennas to the 2+ WiFi adapters, such as a directional antenna and a normal omnidirectional antenna. This allows for automatic switching from long range to short range, so that you don't have to keep your directional antenna pointed toward the UAV when it is nearby.
What is FEC/Forward Error Correction?
FEC is a way to add redundancy to the video stream.
Because the latency must be kept to a minimum, there is no opportunity to retransmit parts of the video that were distorted by interference. Instead the system will process the video in a way that allows for some of it to be lost in transmission, without affecting the received video on the GroundPi.
There is a cost to using FEC, the radio bandwidth is a little bit higher (or viewed another way, the maximum video bitrate/quality is a little bit lower). However it is mandatory for safe flight, without it the video would be very easily distorted.
Is latency going to improve in future updates?
Latency is almost entirely a result of the kind of video encoder and decoder hardware being used on the AirPi and GroundPi.
With the Raspberry Pi, there is a lower limit to the minimum latency that can be achieved. This is generally somewhere between 80-90ms, and that requires a particular configuration.
On other boards, latency as low as 40-60ms is possible with a carefully selected camera and video decode/receive hardware.
What do these numbers mean on the OSD display?
Which RC protocols are supported?
The AirPi can send SUMD/Graupner, IBUS/FlySky, SRXL/Multiplex, and Mavlink to the flight controller board.
If you are using Mavlink for both RC and telemetry you will only need to wire 1 pair of UART wires between the AirPi and the flight controller.
Which telemetry protocols are supported?
Mavlink, FrSky, LTM, Vector, and a few others will work with the OSD.
Mavlink supports 2-way communication in OpenHD, which means you can connect a GCS to the ground station and the GCS can change the flight controller settings (for Ardupilot in this case).
What is an AirPi / GroundPi?
This is a shorter way to refer to which Raspberry Pi is transmitting video and which one is transmitting RC and/or uplink telemetry.
They are configured a bit differently depending on their role, and it is critical for each one to know what it is supposed to be doing. At the moment this is determined by looking for a Raspberry Pi camera or a file called /boot/air.txt, if either are found the system knows it is supposed to act as an AirPi and will configure itself accordingly.
How about Circular vs. Linear polarized antennas?
Circular antennas are known to be very effective in suppressing signal reflections (multi-pathing), which degrades the quality of analogue transmission.
Digital systems are not affected by multi-pathing though (and often can take advantage of it). Circular antennas are not as important like they would be with an analog video system.
However, circular antennas still have some advantages compared to linear antennas:
They work independent of angle to each other, almost no polarization losses
Typically circular antennas have a lower gain, and thus higher opening angle than linear antennas
Less susceptible to signal blocking from nearby solid objects
How can I support this project?
Use the system! There are a lot of different features and settings and possible hardware combinations, it can be very difficult to test everything each time we release an update. As a result, we depend on users to tell use when something is wrong, or whether something should be improved.
We also frequently need translators who can read/write both english and another language, as the system supports translation in the OSD.
And of course if you have an idea for something you would like to see use add or change, let us know in Github Issues, on the forum, or on Telegram.
If you want to support the project financially, you can do so via OpenCollective.
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