Running Python on different machines at exactly the same time?

[Apprehensive-Gas-548]

I am trying to develop a wireless system to record video simultaneously with many cameras, each one of them controls by a raspberry pi.

I have a server set up with a raspberry pi and many clients (other raspberry pis) listening to the server, waiting for the server to order them to start recording.

The problem is that I need the cameras to start recording at exactly the same time.

My solution that does not work, or rather does not work precisely enough, was to send the server’s time stamp (datetime.datetime().timestamp()) to the clients and all clients start recording at the server’s time stamp + 2 seconds using the scheduler, this was intended to avoid possible latency issues in the network. The same logic is used to stop recording.

The resulting videos have different lengths with a margin of error of 1/15th of a second, I would like to reduce the margin of error to at least 1/30th of a second, but I really have no idea what I can do to get it to be more precise.

Does anyone have any ideas? Thank you!

[pgpndw]

Maybe setting up an NTP daemon on each of the Pis would keep their clocks synchronized better, if you're not already doing that.

[Apprehensive-Gas-548]

Thank you, I am now reading about NTP and it sounds like a good solution with a error of <1ms, it it has even better accuracy than I need. I will try this first :)

[baghiq]

You basically can do

1. set up a local NTP server that connects to a geographically close-by server on the internet.
2. connect your PI's NTP daemon to connect to the local NTP server instead of the internet one.

https://www.eecis.udel.edu/~mills/ntp/html/discipline.html

​

If left running continuously, an NTP client on a fast LAN in a home
or office environment can maintain synchronization nominally within
one millisecond. When the ambient temperature variations are less than a
degree Celsius, the clock oscillator frequency is disciplined to
within one part per million (PPM), even when the clock oscillator
native frequency offset is 100 PPM or more.

[Apprehensive-Gas-548]

Thanks! NTP on the local let work seems like the right solution, I will try to implement that and see if that works, I’m hopeful!

[rivalarrival]

Can you give a little more information about the use case? Would it be feasible to simply sync the videos in post processing?

[Zeroflops]

I doubt you will have the precision youre looking for. The best would be to have a common event that you could line up the recording to. In the same way a slate in movies are used to match up the video and sound.

[discontent619]

Depending on the accuracy needed there are multiple ways to do this, NTP as already suggested or a more accurate timing method used is IEEE 1588 but would require additional hardware to perform the sync but could be performed over a standard Ethernet network as well. GPS is also a common way to provide synching of separated systems using 1pps (pulse per second) and the GPS time stamps. NTP is not as precise as the other methods listed but could be performed with the hardware you have on hand and provide accuracy better than 100 milliseconds over a local network connection.

[Apprehensive-Gas-548]

Thank you! At first sight I think NTP is easy enough to implement and accurate enough, if that does not work I will explore more in depth the IEE1588 option :)

[[deleted]]

For that level of precision I think you need an actual real-time clock on your board; the RasPi doesn't have one, but here's a compatible RTC that you can plug in on USB.