The latest technology supporting high-definition video transmission
In recent years, the number of in-vehicle cameras has increased, and the resolution of each camera has also improved. SerDes is increasingly being used to transmit video data from each camera to the ECU with low latency.
In this column, we will explain serializers and deserializers, the latest technology that supports high-definition video transmission.
What is a Serializer/Deserializer?
A serializer/deserializer is an IC that converts video I/F to coaxial or differential signals, and coaxial or differential signals to video I/F.
It can transmit large images over a single coaxial or differential pair, and some models also include copyright protection functions, making it widely used in areas such as multimedia, in-car infotainment, and ADAS.
Video Interface
- MIPI D-PHY
- LVDS
- CMOS RGB
- eDP (Emmbedded Display port)
- HDMI
- DVI
The role of serializers and deserializers
Many video interface standards use multiple data lines to transmit video over short distances. By using SerDes, the data lines are combined into a single differential pair (or coaxial), enabling long-distance transmission.
CABLES CONNECTORS
There are two main types CONNECTORS used in SerDes.
- STP CABLES
Since it transmits signals using a differential pair, it has excellent EMC performance and is often used to connect ECUs together.
- Coax CABLES
It is often used to connect cameras to ECUs because it is superior in terms of cost and CABLES weight, and has advantages in terms of CABLES routing.
Application Examples
SerDes is used in a wide range of areas, but in recent years it has been increasingly adopted in in-vehicle networks.
In the fields of autonomous driving, ADAS, and IVI, SerDes is essential for high-speed, low-latency video transmission, and the number of SerDes used in a single vehicle is also increasing.
The illustration above shows an example of SerDes usage in an ADAS ECU/Cockpit ECU.
This example shows a configuration in which an in-vehicle camera and in-vehicle DISPLAY are connected via multiple SerDes.
Technical explanation
Equalizer
The waveform at the receiving end is attenuated compared to the waveform at the transmitting end.
The reason is that the signal is lost, reflected, and absorbed as it passes through CABLES CONNECTORS.
The technology that restores these attenuated signals is called an equalizer.
Equalizer technology is a technique for correcting or averaging attenuated signals, restoring specific frequency bands, and correcting distortion, making it possible to restore attenuated signals to their original state (or to restore them as closely as possible).
High speed signal/low speed signal
SerDes communicates by superimposing high-speed signals for transmitting video data and low-speed signals for transmitting serial communications such as I2C. By multiplexing signals of different frequencies and encoding them, bidirectional communication is achieved over a single serial CABLES. In connections between a camera and an ECU, for example, video data may be sent from the camera (Ser) to the ECU, while I2C commands and GPIO signals for exposure control may be sent from the ECU (Des) to the camera.
PoC (Power over Coax)
A transmission method called PoC (Power over Coax), which superimposes high-speed signals and POWER SUPPLIES on a coaxial CABLES, is becoming increasingly popular.
This transmission method eliminates the need for a separate POWER SUPPLIES CABLES CONNECTORS as in the past, contributing to lighter vehicle weight and simplified wiring.
As multiple cameras are increasingly being connected to a single vehicle, attention is being drawn to PoC, which does not require a dedicated POWER SUPPLY CABLES CONNECTORS.
In PoC, high frequency signals and DC current are superimposed on a single coaxial CABLES.
A Bias-T circuit is required to prevent high-frequency signals from leaking into POWER SUPPLIES circuit or, conversely, to prevent DC current from flowing into the SerDes IC.
The Bias-T circuit consists of CAPACITORS to block DC components and FILTERS circuit to block high-frequency signals.
