What does GMSL stand for?
The abbreviation stands for “Gigabit Multimedia Serial Link” and refers to a technology with which the video signal is transmitted digitally and compression-free via a coaxial cable. Thanks to the Gigabit connection and the high bandwidth, with GMSL, the transmission of digital HD video data for vehicles is possible. As the video signal is sent compression-free, there is no lag. The image is displayed in real-time on the monitor, without time lag. The cooperation of serializers and de-serializers enables the use of different interfaces at each end of the link.
Who are the chip manufacturers for this?
GMSL was developed by Maxim Integrated, a US manufacturer of integrated circuits (ICs). The company’s headquarters are based in San Jose in the State of California. On July 13, 2020, Analog Devices announced that it was going to take over the company for 20.91 billion dollars. The take-over was finalized on August 26, 2021.
What are the applications?
GMSL’s areas of application include Advanced Driver Assistance Systems (or ADAS for short) and infotainment. It can support multiple protocols over the same link, such as video and Ethernet, which simplifies the infrastructure. It can also handle several data streams in combination. This includes the ability to split video streams to cope with the many displays in a car, such as the information cluster (IC) and the central information display (CID).
How was GMSL introduced into cars?
To start with, shielded twisted pair cables were used for GMSL. Therefore, the impedance of the transmission system is 100 Ω. Coaxial cables were used for the first time in 2016 in Daimler’s E-Class. The negative differential inputs and outputs were connected to ground by 50 Ω resistors. Coaxial cables are cheaper and also enable the supply of power to peripheral cameras via Power-over-Coax or PoC for short. In the process, the frequency spectrum is divided. Control signals have a lower data rate and are transmitted in the lower spectrum. Through the 8b/10b or 9b/10b coding on newer versions, the data for the downlink is shifted to the upper part of the spectrum. Modulation takes place with non-return to zero (or NRZ for short), including the GMSL 2 generation. From generation GMSL 3 onwards, PAM4 (pulse-amplitude modulation with four levels) is used. The transmission speed has increased in recent years. The generation with GMSL 3 enables a data rate of 12 Gbit/s. Various serializers and de-serializer chips exist which enable a wide range of video and control interfaces. MIPI CSI-2 (with C-PHY or D-PHY), MIPI DSI, OpenLDI, eDP and HDMI are supported. Not all chips are compatible with each other.
Is LVDS used?
Several chip variants actually exist, some of which are operated with LVDS. However, the SerDes portfolio also includes other variants with CML, HDMI or CMOS/LVCMOS.
Is it possible to compensate for cable influences?
GMSL SerDes technology enables long-distance transmission up to a distance of 15 m. The cameras can be placed 15 meters away from the host processor via the 50 Ω coaxial cable or the 100 Ω shielded twisted pair (STP) to operate at full speed, and still support a high frame rate and less latency. Higher cable attenuation at higher frequencies can also be compensated by using an adaptive line equalizer in GMSL. This cable equalizer has over 12 compensation levels to handle up to 30 m coax and 15 m STP cable lengths. At the other end, the maximum cable length for a standard MIPI® CSI-2 camera connection is 30 cm.