Hardware Synchronization for Real Sense


Recently we are working on getting multiple real sense devices synchronized with each other through an external trigger. We found a very helpful white paper online https://realsense.intel.com/wp-content/uploads/sites/63/Multiple_Camera_WhitePaper_rev1.1.pdf. Intel described how multiple real sense devices are supposed to connect with each other. I visited ITP today, with the help of an adjunct professor, Deqing, I learned a lot of (basic) hardware technology.

What we want to do for multiple real sense devices is hardware sync based on the external trigger. See from the image below (Fig 1), once we have the connector plugged in, we can use these 9 pins for different uses. For synchronization, we need to use pin 5 and pin 9.

Fig 1. RealSense with external trigger, connector and related wires.

We need to try this hardware sync solution generatively. First we need to test it with two real sense. Then we increase the number to 3~6 and then we test it with 24 (the final number). For these three steps, the solution are slightly different in hardware design.

From the documentation of Intel Real Sense, we need to toggle the hardware sync feature both in hardware side and software side. In the hardware side, the “pin 5, sync” needs to be connected with the same pin of other real sense, and the same to “pin 9, ground“. In the software side, one camera could be initialized as master and the rest configured as the slave. In that case, the rest of the devices will follow the master device.

Now let’s go to the detail of three steps for different hardware configurations. For the simplest case, we only need to synchronize two devices. So we just need to plugin one end of the first wire to pin 5 of one connector and the other end the wire to another connector. And similarly plugin the second wire to pin 9 of two connectors. Lastly, we need to plugin two connectors to two devices correspondingly.

Step 2, 3~6 cameras. For this configuration, we need to use a passive solution. It is claimed that this solution is working with the cameras whose distances are smaller than 3 meters. From our current setup, the furthest distance between two cameras is around 2 meters. To connect more than two devices, we need to break up the wires we used to plugin to the connectors. Here we’d better use shielded twisted cables to extend the original wires. To extend the wires, we need to break the wires into two pieces. Let’s say we are using one-pair shielded twisted cables, for each pair we have red cable and black cable, see fig 2. Now we need to solder one piece of the wire with one end of the red cable and the other piece of the wire with the other end of the red cable. And then plugin two split wires to pin 5 of two connectors. And apply the same thing with black cable on pin 9. Thus we have an extended version for two devices. Now how could we add one more device? Let’s say we want to connect to the third device in the middle of the cable. So we need to remove the wrapper of the twisted cables in the middle, for both red and black cables. (Caution: avoid contact between these two cables.) Then break the wires into two pieces, solder one end with the twisted cable and plug in the other end to the connector of the third device.

Fig 2. A shielded twisted cable.

Step 3, ~24 cameras. We need to apply active solution now. That means we have to create our own circuit and board to make things happen. Professor Deqing will instruct me on that laterrrr.

For step 2, resistors and capacitors are designed to protect the wires and the device. Similarly, we can use anti-static wrist strap to protect the hardware. And then wrap them with electronic tapes to avoid accidental contact from other sources.

Can’t wait!

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