The diagram below shows two NVIDIA DRIVE platforms with the recording application, each collecting their own data. For each recorder, data is stored with a timestamp that records the time, in milliseconds. Data between the two NVIDIA DRIVE platforms are sycnronized based on timestamps. Multiple sensors allow for higher accuracy, and the exact timestamps ensure precise synchronization between the devices.

Both the TextUI Recording Tool and GUI Recording Tool can support distributed recording with systems reachable by the network. The front-end instance of these tools launches multiple back-end instances on connected Xaviers and synchronizes them over UNIX pipes.

Basic Steps

Step 1: Set up Xavier A & B

On each Xavier:

Verify the Sensors Are Collecting Data in Basic Recording.
Create a DriveWorks Rig Configuration File for the sensor set.
Attach eSATA/USB storage.

Step 2: Collect Rig Files

On the master Xavier (either A or B), collect rig files for both Xavier A and B, and arrange them in the following directory structure:

<rig_directory>
|___10.42.0.28.json
|___10.42.0.29.json

Where each rig file name contains the IP address of that Xavier.

Step 3: Synchronize and then run

Synchronize the Xavier clocks as described in Synchronizing system clocks in these release notes.
On the master Xavier, start the recording application, specifying the rig corresponding configuration file directory:
```
# tools/recorder-tui <rig_directory>
```
Recording is automatically toggled on Xavier B, when toggled on Xavier A.

Sensor Synchronization

NVIDIA DRIVE platforms can record sensor data for various sensor groups on the same Xavier or for various groups on different Xavier processors or NVIDIA DRIVE platforms. The sensor setting in the rig configuration file effects sensor timestamp synchronization. In addition, for recordings on different Xavier processors, other factors affect timestamp synchronization.

Synchronizing for Distributed Recording

By default, the recorder tool does not provide synchronization in a distributed recording setting because PTP is off. But if you synchronize the system clocks with PTP or gPTP, then you can get synchronization within +/- 0.5 frames. If you record when PTP is off, recording timestamps are based on the current Xavier time, which might be not the real time when the recording takes place.

Cross-CSI sync is enabled automatically makes all cameras capture at the same time, HW wise. This will enable the external FSYNC feature of the deserializer and allow the Xavier PWM output to drive the camera sync for multiple camera groups

About synchronization

There are two types of synchronization.

Synchronization of system clocks - Ensures that the timestamp from one Xavier is the same time as another Xavier.

With PTP or gPTP, you can achieve synchronization of system clocks.
Synchronization of camera frames - Ensures that the time of the capture is the same among multiple cameras.

Synchronization of camera frames is not possible. During distributed recording, the Xavier processors control separate deserializers. They are not connected by the same PWM. Even though clocks are synchronized, sensor timing cannot be synchronized. As a result, the actual timing of the image capture cannot be synchronized, meaning that synchronization will be off by at most +/- 0.5 frames.

How gPTP and PTP affect synchronization of system clocks

The NVIDIA DRIVE PDK gPTP application provides the NVIDIA DRIVE platform support for 802.1AS conforming time synchronization messages for hardware time synchronization. gPTP is for multi-Xavier use cases (such as distributed recording), which requires time scales of each Xavier to be synchronized with each other. NVIDIA recommends enabling gPTP at all times.

Synchronizing System Clocks

Use either PTP or gPTP to synchronize between one Xavier as master and 3 other Xaviers as slaves. Depending on the execution timing, you should have +/- 0.5 frames difference between sensor timestamps when measuring the same camera with same framerate.

If PTP is running, have all peers start/stop recording at a predetermined timestamp value. For guidance on using gPTP, search the NVIDIA DRIVE OS 5.1 Development Guide for PTP.

Testing Sensor Synchronization

You cannot compare the timestamp of the first frame to see if PTP sync worked. Recording does not start synchronously on both Xaviers. Instead, you must correlate images and then compare the timestamps on those images.

To test sensor synchronization

Observe with cameras from two Xaviers that are recording the same content, for example a video playing on a monitor.
Find corresponding frames in both video streams, by comparing the frame content.
Compare the timestamp of both frames.
Ensure the timestamps are within +/- 0.5 frames.

Table of Contents