USB Controller Link Delay Compensation for Multi-Camera Synchronization
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Solution Overview
Problem
Machine vision systems using multiple cameras face challenges in synchronizing their time bases, leading to significant errors in chronological sequencing of images due to variations in camera time bases.
Innovation Solution
A USB controller mechanism that receives isochronous timestamp packets from a host device, calculates link delays, and adjusts its counter to synchronize camera clocks with microsecond accuracy, using the Precision Time Measurement (PTM) feature in the USB 3.2 specification to generate precise timestamps embedded in image data.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If multiple cameras are used to cover a wide field of vision, then the field of vision is expanded, but the time base synchronization between cameras deteriorates
Solution Approach 1:
A USB host device acts as an intermediary to distribute a common reference clock signal to multiple cameras through USB connections. The host device generates a master timestamp and sends it to each camera along with the video data, enabling all cameras to synchronize their time bases to the same reference without requiring direct inter-camera communication.
Solution Approach 2:
The system implements a feedback mechanism where the host device receives video data from cameras with timestamps, processes the information, and adjusts the reference clock distribution to compensate for measured timing variations. This closed-loop approach continuously optimizes synchronization accuracy across the multi-camera system.
2Ease of operation
If timestamps are used to order video frames from multiple cameras, then chronological sequencing is enabled, but synchronization errors increase due to camera time base variations
Solution Approach 1:
The patent creates a common time reference potential by distributing the same master timestamp from the host device to all cameras. This ensures that all cameras operate from the same time base reference, eliminating potential differences between camera time bases and enabling accurate chronological sequencing of video frames from multiple sources.
3Adaptability or versatility
If camera clocks are synchronized using standard USB connections, then device compatibility is improved, but timestamp precision deteriorates due to hub delays
Solution Approach 1:
The host device performs preliminary measurement of link delays to each camera before actual video capture begins. By measuring the one-way latency from the host to each camera and calculating the round-trip delay, the system pre-compensates for hub-induced delays, ensuring that timestamps remain precise even when passing through USB hubs.
Solution Approach 2:
The system dynamically adjusts timestamp parameters by calculating and applying compensation values based on measured link delays. The host device modifies the reference timestamps sent to each camera by subtracting the measured one-way latency, effectively changing the timestamp parameter to account for transmission delays and maintain microsecond-level precision.
Data Source
AI summary
A USB controller includes: a USB interface that receives an isochronous timestamp packet (ITP) from a host device that includes a current time value from the host device and a delay value associated with each hub through which the ITP passed; controller hardware that initiates a counter, sends a link delay measurement (LDM) link management packet (LMP) request to a nearest upstream hub, stores a first timestamp that corresponds to when the LDM LMP request was sent, receives an LDM LMP response from the nearest upstream hub, and stores a second timestamp that corresponds to when the LDM LMP response was received; and controller software that calculates a link delay between the USB controller and the nearest upstream hub based on the first and second timestamps and delay information included in the LDM LMP response, and adjusts the counter value based on the calculated link delay.