Bus Synchronization (USB3 Camera)

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Introduction

Toshiba Teli's unique "Bus Synchronization Mode" can synchronize exposure time easily in an application with multiple use of USB3 camera BU series.
"Bus Synchronization Mode" provides optimized circumstances for stereo camera, motion capture, multi vision logger etc.

1. Overview

"Bus Synchronization mode" allows you to synchronize the exposure of multiple cameras without using a hardware trigger signal.
In "Bus Synchronization mode", cameras are synchronized with periodical 'Isochronous Timestamp Packet (ITP)' of USB bus.
Isochronous Timestamp Packet (ITP) is used to deliver timestamps from host PC to all USB devices. It is multicast by the hub to all downstream ports.
This article describes the operating conditions and considerations for Bus Synchronization Mode.

Fig.1: 'Bus synchronization' function image

2. Functional Description

2.1. Bus Synchronization mode: FrameSynchronization = Bus

Bus Synchronization mode is a function that uses USB bus timestamp to synchronize the exposure of multiple cameras connected to same bus.
Bus Synchronization mode allows you to synchronize the exposure of multiple cameras without using a hardware trigger signal.

2.2. Internal Synchronization mode: FrameSynchronization = Off

In Internal Synchronization mode, each camera operates with its own individual timing and the exposure is not synchronized.

3. Operating Conditions

3.1. Camera Model

The camera models to be synchronized do not have to be same. It is just required to adjust cameras' frame rate to the slowest model.
For example, if you want to synchronize BU040M (Max. 523 fps) and BU160M (Max. 240fps), adjust both camera's frame rate to less than 240 fps.

3.2. Trigger Mode

Bus sync mode operates in normal shutter mode. (trigger mode = off)
In random trigger shutter mode, the trigger input signal has priority. (Trigger Mode = On)

3.3. Exposure Time

Exposure time should be shorter than frame rate period.
Each camera can have different exposure time. At this time, the end of exposure is synchronized.

Fig.4: Exposure time in bus synchronization mode

3.4. Bus Topology

3.4.1. Single-host controller card with multiple ports

Cameras connected to different port can be synchronized.
The time difference between each port is about 200 to 300 ns. The time jitter is a few tens of ns.

Fig.5: When using a single-host controller card with multiple ports

3.4.2. USB Hubs

You can also use USB Hubs when you want to connect more cameras.
The time delay of USB Hubs is also about 200 to 300 ns per hub.

3.4.3. Multiple-host controller card (e.g. 4-host x 4-port card)

Cameras connected to different port cannot be synchronized because each host has an individual timestamp on 4-host × 4-port card.

Note:
In Intel's native support USB3 Host Controller, each port has same timestamp.
Cameras connected to different port can be synchronized and transfer 400 MByte/s.

4. Considerations

4.1. Bus Bandwidth

All cameras are required to connect to single host controller in Bus Synchronization mode.
The total bus bandwidth should be controlled less than 400 Mbyte/s. (depends on the host controller's performance)
For example, if you want to run three BU406M, Bus Synchronization does not work at default 90 fps setting because the bus bandwidth overflows (1080 Mbyte/s in total).

Fig.8: Example of exceeding bus bandwidth

To solve the bandwidth overflow, the frame rate should be set to 30 fps. The total bandwidth will be reduced to 360 MByte/s. Then, Bus Synchronization will work.