Multicam synchronization without HW connections

[yanxh97]

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Required Info
Camera Model	{ D400 }
Firmware Version	(Open RealSense Viewer --> Click info)
Operating System & Version	{Linux (Ubuntu 14/16/17)
Kernel Version (Linux Only)	(e.g. 4.14.13)
Platform	PC/Raspberry Pi/ NVIDIA Jetson / etc..
SDK Version	{ legacy / 2.. }
Language	{C++ }
Segment	{Robot/Smartphone/VR/AR/others }

Issue Description

I'm working with multi(3) D435s and the synchronization puzzles me. For the task is not that demanding in aspect of synchronization, I would prefer carrying it out without HW sync. Then which timestamp should I check when aligning framesets from different source device? Is there any tricks about the poll_for_frame and wait_for_frame method over this problem?
What I've tried: Suppose the answer to the 1st question is back-end timestamp, I select a master cam and wait_for_frame of it at the start of every loop, and then use poll_for_frame to update the slaves(if it returns false, use the frameset of the former loop/cycle). If the time interval seems too long, I would wait_for_frame of one of the slaves. (for example, the stream is 30Hz and if the frameset I poll comes over 20 ms earlier than master, I would call wait_for_frame, so the slave's frameset comes later but with shorter time interval)

[MartyG-RealSense]

The subject of 'software sync' without hardware is discussed in the link below.

#2148

[yanxh97]

Thanks. @MartyG-RealSense
To be more specific, what function/metadata should I call to get the host timestamp caculated from RS2_FRAME_METADATA_BACKEND_TIMESTAMP - (RS2_FRAME_METADATA_FRAME_TIMESTAMP - RS2_FRAME_METADATA_SENSOR_TIMESTAMP))

[MartyG-RealSense]

My interpretation of the very last comment by @dorodnic in the discussion that I linked to is that the RealSense developers incorporated the formula that you quoted into the SDK from version 2.22.0 onwards as a feature called Global Camera Time.

#3909

[RealSenseCustomerSupport]

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Hi @yanxh97 Do you need further help on this issue?Thanks