Industrial and automotive applications increasingly rely on high-resolution imaging systems, which must be able to efficiently and reliably provide real-time, high bandwidth video data. Although the GigE Vision standard is widely known and widely used, updated application requirements are prompting people to seek alternative solutions. Gigabit Multimedia Serial Link (GMSL) technology is an alternative technology that supports multiple cameras, enables strict real-time processing, reduces complexity, and has deterministic, low-power characteristics and a compact appearance.
This article briefly introduces the main differences between GigE Vision and GMSL. Then introduce Analog Devices' GMSL solutions and explain how to use these solutions to significantly reduce system complexity, improve reliability, and achieve efficient real-time video transmission.
How camera interface technology affects performance
Different interface technologies provide solutions for extending the distance between camera sensors and host processors to meet the basic requirements of many imaging applications. The GigE Vision camera interface standard based on Gigabit Ethernet (GbE) technology has been widely adopted. GigE Vision cameras typically rely on a signal chain consisting of three main components: an image sensor, a processor, and an Ethernet physical layer (PHY) interface (Figure 1).
Ethernet cameras use processor based signal chain images
Figure 1: The Ethernet camera uses a processor based signal chain to buffer and process image sensor data before transmission. (Image source: Analog Devices, Inc.)
In terms of sensors, the GigE Vision camera can use its internal processor to support customized sensor interface protocols. In terms of transmission, the GigE Vision camera is compatible with various host devices by using standard Ethernet. For example, personal computers and embedded systems typically use GbE ports as standard interfaces. If the GigE Vision camera supports universal drivers (which are typically provided by these systems), it can be used as another plug and play peripheral.
Ethernet based solutions have advantages in single camera applications, but require additional hardware in multi camera applications. Usually, these applications require additional dedicated Ethernet switches or network interface cards (NICs) to handle multiple data streams. Adding these devices to the video data path may affect the throughput and latency between the camera and the host.
In addition, Analog Devices' GMSL technology adopts a point-to-point serial link method, providing an efficient solution for applications that require multiple cameras with minimal latency. GMSL cameras were originally designed for automotive applications, but as an alternative to Ethernet based cameras, GMSL cameras are increasingly being adopted for applications outside of the automotive industry.
In GMSL based applications, multiple compact GMSL cameras can be connected to a single GMSL host without affecting throughput or latency, provided that the on-chip host system (SoC) supports the full bandwidth of all cameras (Figure 2).
GMSL Multi Camera Application Diagram
Figure 2: GMSL multi camera applications can use simple cameras (left), with each camera connected through an independent GMSL link and ultimately converged to a single host (right). (Image source: Analog Devices, Inc.)
Cameras using GMSL typically use simplified signal chains, including image sensors and GMSL serializers. The GMSL sequencer supports two standard sensor interfaces:
The first generation GMSL (GMSL1) device supports parallel low-voltage differential signaling (LVDS) interface.
The second-generation GMSL (GMSL2) and third-generation GMSL (GMSL3) devices support the popular Mobile Industrial Processor Interface (MIPI) standard, allowing GMSL cameras to use a variety of leading image sensors.
In most applications, raw data from image sensors is serialized and sent in its original format over a GMSL link. Due to the absence of processors and other supporting components, the design and manufacturing of GMSL cameras are simpler. They also provide more effective solutions for applications that require compact camera shapes and low power consumption.
The GMSL linked host is usually a customized embedded system that combines one or more hardware deserializers. Running a few lines of code on the host is usually sufficient to access these hardware deserializers and retrieve data. If there is a driver for the image sensor, developers only need to set the appropriate registers to read the video stream from the camera. Analog Devices' GMSL device evaluation kit includes the software required to access these devices and explore their functionality. In order to gain more support for GMSL development, Analog Devices provides an open-source software library for GMSL technology.